UNIVERSITY OF CALIFORNIA AT LOS ANGELES LIGHT SCIENCE. I.OXDOX t PRISTBIl HY SPOTTISWOO!iK. AND CO.. NKW-STIIKFT SQtTAHK AXU TAUHAMEM' STIIKUT LIGHT SCIENCE FOE LEISUEE HOUES. Aeries. FAMILIAE ESSAYS ON SCIENTIFIC SUBJECTS, NATURAL PHENOMENA, &c. BY RICHARD A. PROCTOR, B.A. CAMB. JIOXOnARY SECRETARY OP THE ROYAL ASTRONOMICAL SOCIETY ; AUTHOR OF 'THE SUN' 'OTHER WORLDS' 'SATURN* 'ESSAYS OX ASTROXOMY ' ' THE ORBS AROUND US ' ETC. 'Truths of Science waiting to be caught.' TEXXYSON. ftfo LONDON : LONGMAN 8, GREEN, A N D C 0. 1886. All rights reserved. ant PBEFACE. IN this volume of Essays I have included a series of papers relating to the great Solar Eclipses which have been successfully observed since the year 1868, papers written at the time, and describing the anticipations formed as each eclipse approached, and the results actually obtained during the progress of the eclipse. I have removed portions of those papers which are now practically out of date, but I have left enough to afford what I think will be found an interesting history of the progress of eclipse observation during the past fifteen years. The other Essays resemble in general character those in the two former series of * Light Science for Leisure Hours.' KICHD. A. PROCTOR. LONDON : April 1883. CONTENTS. PAOK GREAT SOLAR ECLIPSES 1 Two YEARS LATEB 29 THE ECLIPSE OF 1870 . . . . . . .50 YET A YEAR LATER 74 THE ECLIPSE OF 1871 102 THE ECLIPSE OF 1878 . 131 THE EAETH IN METEORIC SHADOW . . . . . .141 CONDITION OF THE LARGER PLANETS 177 A GREAT SOLAR OUTBURST . . . . . . .199 COMETS 214 THE AUGUST METEORS 220 LOTTERY SCHEMES ......... 225 CONDUCT AND DUTY 232 VIVISECTION . . . . ...... 238 THE AMERICAN TARIFF . . -. ; . . . 241 ORIGIN OF OUR RACE. . . . ... . . . 252 A LATE BOAT RACE . , . ..... . . . 258 ARE WE JEWS ? ' . ; , -. - . . 263 PARADOXES AND PARADOXISTS . . . ..,'.. .269 INFLUENCE OF MARRIAGE ON DEATH AND CRIME . . .281 INCREASE OF THE POPULATION . . . . . 287 COLLISIONS AT SEA DURING FOG 293 THE EYES OF SCIENCE ........ 297 GREAT SUN-SPOTS 303 LIGHT SCIENCE FOR LEISURE HOURS. THIRD SERIES. GREAT SOLAR ECLIPSES. AMONG the total eclipses recorded during historic times, there are some which stand out among the rest on account either of their magnitude or of the historical interest associated with them. I propose to give a brief account of the more remarkable solar eclipses whose records have been preserved. Before doing so, however, it may be well to point out the circumstances on which the magnitude of a solar eclipse depends ; and to explain why it is that so few eclipses occur which deserve to be ranked among great total eclipses. The average apparent dimensions of the sun exceed those of the moon. But both bodies vary in apparent magnitude the moon more than the sun. Perhaps many of my readers will be surprised to learn that we receive fully one-fourth more light from some full moons than from others, owing to the variation of her apparent magnitude. Accordingly, when she is at her largest and the sun at his smallest, she is able to hide III. B 10 2 LIGHT SCIENCE FOR LEISURE HOURS. him wholly from our view, and considerably to overlap his disc all round. But there is another circumstance besides proxi- mity to the earth which affects the moon's apparent dimensions. She appears to grow larger as she rises above the horizon. I am not referring, of course, to the appearance which she presents to the naked eye. Judged in this way she seems to grow smaller as she rises above the horizon. But when she is measured by any trustworthy instrument the reverse is found to be the case. The cause of the peculiarity is not far to seek. We see the moon, not from the centre of her orbit (that is, the earth's centre), but from a point on the earth's surface a point, there- fore, which is four thousand miles nearer to the moon's orbit. Accordingly, if the moon were directly over- head (which never happens in our latitudes), her dis- tance from us would be diminished by four thousand miles, and she would look proportionately larger. The sun is not affected in this way, because four thousand miles is a mere nothing in comparison with the enor- mous distance at which the sun is removed from us. Accordingly, other things being equal, the higher the moon is at the time of a total eclipse, the greater is the eclipse. In order, therefore, that an eclipse may be as great as possible, the sun should be as far as possible from the earth, which happens about the beginning of July ; the moon should be as near as possible to the earth, which happens (roughly speaking) once in every GREAT SOLAR ECLIPSES. 3 lunar month ; and the sun and moon should be almost immediately overhead, which can only happen at mid- day in tropical countries. It will readily be conceived how seldom these conditions can be fulfilled (in com- bination with the other conditions which determine the occurrence of an eclipse at all). In fact it has never yet happened that any very close approach has been made to the simultaneous fulfilment of all the conditions. I proceed to discuss a few of the most remarkable eofipses recorded by ancient historians. It is rather singular that no eclipses are recorded in the Bible. There have been some astronomers who have imagined that the ' going back of the shadow upon the dial of Ahaz ' was caused by a partial eclipse of the sun. But this supposition seems too fanciful to be admitted, even if it were the case that a partial eclipse could have caused the retrogression of the shadow. We are told distinctly that the ' going ba,ck of the shadow ' was a miraculous, not a natural event ; and even if this were not so, or if we might infer that it was the prophet's foreknowledge of an approaching eclipse which constituted the miracle, yet it may readily be shown that no partial or total eclipse could produce the effects described. Such an eclipse un- doubtedly causes an irregularity in the motion of the shadow on a dial; the shadow at first moves more slowly, afterwards more quickly, than it would other- wise do, but it cannot possibly go back. The first important eclipse whose records have- B2 4 LIGHT SCIENCE FOR LEISURE HOURS. reached us is that which occurred in the year 584 B.C. It took place, Herodotus relates, while the Medians and Lydians were engaged in battle. He thus describes the occurrence : ' The war had continued between the two nations with balanced success for five years. In the sixth year of the war another battle took place ; and after both sides had fought with great advantage, and when the engagement was growing warm, the day was suddenly turned into night. This had been foretold to the lonians by Thales the Milesian, who predicted the time of the year in which it would happen. The Lydians and Medes, seeing that the day had given place to night, desisted from combat, and were equally anxious to make peace.' Astronomers and historians had for a long time been in doubt respecting the date of this remarkable eclipse. The astronomical difficulty of the question is connected with an interesting pecu- liarity of lunar motion, into which we need not now enter. Until this peculiarity had been mastered, which has only happened quite recently, Baily's supposition that the eclipse must have occurred in the year 609 B.C. was accepted as the best solution of the difficulty. But Sir G. Airy has now proved beyond a doubt that the eclipse took place on May 28, in the year 584 B.C., the very year assigned to the event by Cicero and Pliny. Xenophon mentions a remarkable eclipse which led to the capture of Larissa by the Persians. During the retreat which was so ably conducted by Xenophon, the Greeks passed 'a large deserted city called Larissa, formerly inhabited by the Medes. Its walls were GREAT SOLAR ECLIPSES. 5 twenty-five feet thick and 100 feet high ; its circum- ference two parasangs ; it was built of burnt brick, on a foundation of stone twenty feet high. When the Persians conquered the Medes, the Persian king besieged this city, but was unable to capture it till a cloud hid the sun wholly from view, when the inha- bitants withdrew in great fear, and the city was captured.' Xenophon mentions that the Greeks, after passing Larissa, reached another deserted city called Mespila. Layard has identified Larissa with the modern Nimroud, where there still exist the very ruins described by Xenophon ; Mespila he identifies with the modern Mosul. Of course it is impossible to doubt that a total eclipse of the sun, and not the mere con- cealment of the sun under a cloud, was the cause of the city's capture. Sir G. Airy has shown that this interesting event occurred on May 19, 556 B.C. Another eclipse has been examined by Sir G. Airy which had given great trouble to historians. This is the eclipse which took place when Xerxes was advanc- ing with his army from Sardis to Abydos. Herodotus relates that just as the army was setting forth the sun suddenly disappeared from its place in the heavens, though there were no clouds, and the sky was perfectly clear ; ' thus,' says he, * the day was turned into night.' Sir George Airy, however, refers this description to the total eclipse of the moon, which took place on March 13, 478 B.C. No total eclipse of the sun appears to be reconcilable with the account of Herodotus, and there- fore it seems reasonable to infer that there is an error 6 LIGHT SCIENCE FOR LEISURE HOURS. of some sort in his narrative. But explaining an eclipse of the sun as an eclipse of the moon is simply preposterous. It is singular how often the occurrence of a total eclipse is connected with the military and naval under- takings of ancient nations. Most of my readers must remember the narrative of the total eclipse which seriously threatened the success of the expedition of the Athenians under Pericles against the Lacedaemo- nians. ' The whole fleet was in readiness, and Pericles on board his own galley, when there happened an eclipse of the sun. The sudden darkness was looked upon as an unfavourable omen, and threw the sailors into the greatest consternation. Pericles, observing that the pilot was much astonished and perplexed, took his cloak, and having covered his eyes with it, asked him if he found anything terrible in that, or considered it as a bad presage? Upon his answer- ing in the negative, Pericles said, "Where is the difference, then, between this and the other, ex- cept that something bigger than my cloak causes the eclipse ? " But perhaps the most interesting of all the problems with which ancient eclipses have supplied our modern astronomers, is that which is connected with what is termed the eclipse of Agathocles. After his defeat by the Carthaginians, Agathocles was besieged by them in Syracuse. But, taking advantage of a relaxation in the vigilance of the blockading fleet, occasioned by the approach of a fleet which had been sent for his relief, he GREAT SOLAR ECLIPSES. 7 quitted Syracuse, and passing over into Africa, waged for four years a successful war against the Carthaginian forces. It is related by Diodorus Siculus that the voyage to Africa occupied six days, and that on the second day of the journey an eclipse occurred, during which the darkness was so great that stars became visible in all directions. There can be no doubt, therefore, that the eclipse was a total one. But it has been found difficult to reconcile this account with the calculated path of the mtron's shadow during the only total eclipse which corresponds with the historical and chronological details of the event. Baily's calculation of the eclipse threw the shadow about 200 miles from the most southerly position which can possibly have been attained by Agathocles on the second day of his journey from Syracuse. The labours of Sir Or. Airy, founded on improved tables of the lunar motions, have been more successful ; and he has shown that the northern limit of the zone of total shadow must have passed some seventy or eighty miles south of Syracuse a distance which might readily have been traversed by Agathocles within the time named. It is related by Philostratus, in his Life of Apol- lonius, that a singular phenomenon preceded and announced the death of the Emperor Domitian. * A certain crown, resembling the Iris, surrounded the sun's disc and hid his light.' I cannot doubt that reference is here made to a total eclipse of the sun, and calculation shows that such an eclipse occurred in the year ninety-five of the present era. 8 LIGHT SCIENCE FOR LEISURE HOURS. I pass to the records of eclipses which have occurred more recently. William of Malmesbury relates that the eclipse of August 2, 1133, presaged the death of Henry I. ' The elements showed their grief,' he says, ' at the passing away of this great king. For on that day the sun hid his resplendent face at the sixth hour, in fearful dark- ness, disturbing men's minds by his eclipse.' Seven years later another remarkable eclipse oc- curred which is thus referred to by the same writer : *Tn Lent, the sun and the moon darkened about noontide, when men were eating; and they lighted candles to eat by. That was the thirteenth day before the calends of April.' (The worthy chronicler might as well have adhered to the more usual method of expressing the date.) 'Men were very much struck with wonder.' ' The darkness became so great,' he says elsewhere, ' that men feared the ancient chaos was about to return, and on going out, they perceived several stars around the sun.' Amongst all the eclipses hitherto mentioned there is only one viz. the eclipse of Thales which is com- parable with that of August 17. And among more recent eclipses there is only one other approaching it in magnitude. This eclipse, which occurred on June 17, 1433, was visible in Scotland, and was long remembered in that country as ' the Black Hour.' It occurred at about three o'clock in the afternoon, and the records preserved respecting it relate that nothing was visible during the height of the totality. Professor GREAT SOLAR ECLIPSES. 9 Grant considers that ' this last remark is a manifest exaggeration.' Be this as it may, there can be no doubt that the eclipse was one of unusual extent, for the mathematician Maclaurin found that ' at the time of its occurrence the sun was only two degrees from apogee, the moon not more than thirteen degrees from perigee.' But neither in this eclipse nor in that of Thales did the totality last so long as during the eclipse of 1868. ^In 1598 another total eclipse occurred which was visible in the British Isles. The day of the eclipse was remembered for a long time afterwards as * Black Saturday.' In a similar way the day of the total eclipse of 1652 was named * Mirk Monday ' by the people of Scotland, and although the eclipse has long since been forgotten, the expression is still used in many parts of that country. It is singular that none of the eclipses I have re- corded had led to any observations of any value to the physical inquirer. Modern eclipses, on the contrary, derive their chief interest from observations of this sort. In the total eclipse of 1706, which was observed at Montpelier, and a variety of other places in Western and Central Europe, the bright stars Aldebaran and Capella, and the planets Venus, Mercury, and Saturn, were visible to the naked eye. 'Bats flew about as they do at dusk. Fowls and pigeons flew hastily to their roosts. Cage-birds were silent, and hid their heads under their wings. Animals at labour in the fields stood still.' Duillier relates that at Geneva IO LIGHT SCIENCE FOR LEISURE HOURS. the Council were compelled to close their delibera- tions, as they could see neither to read nor write. ( In many places people fell prostrate on the ground, and prayed with earnestness, imagining that the Day of Judgment was come. From the tops of the Swiss mountains as many stars were seen as at the time of full moon. A peculiar colour overspread the sky, resembling neither the darkness of night nor the mixed colours of the twilight sky. Even those who were prepared for the spectacle were ap- palled by the solemn gloom which fell upon the face of nature.' Halley speaks in similar terms of the last total eclipse which was visible in London. It took place in the year 1715. ' I forbear,' says Halley, * to mention the chill and damp which attended the darkness of this eclipse, of which most spectators were sensible and equally judges. Nor shall I trouble you with the con- cern that appeared in all sorts of animals, birds, beasts, and fishes, upon the extinction of the sun, since our- selves could hardly behold it without some sense of horror.' The eclipse of May 2, 1733, is remarkable as being the first in which the singular appearances termed the * red prominences ' were observed. * Four spots of a reddish colour were seen near the limb of the moon, but not in immediate contact with it.' The chief interest attending the observation of total eclipses is at present centred on these mysterious protuberances. It has been shown very clearly that they belong to GREAT SOLAR ECLIPSES. 1 1 the sun, but what they may be, or what tremendous processes going on within his atmosphere they may be held to indicate, remains as yet unknown. It is hoped that the long duration of the totality of the approaching eclipse, and the circumstance that it will be possible to observe the eclipse at several points along the shadow's track (it will be remembered that this was written in 1868) will enable astronomers to gain some knowledge respecting the red promi- n^ences. Yet more hopeful is the fact that now, for the first time, the subtle analytical power of the most wonderful instrument of research yet invented the spectroscope will be applied to examine these strange solar excrescences. I pass over several total eclipses to come to the first of those which have been made the object of scientific expeditions. The eclipse of July 8, 1842, which was visible in the north of Italy, and in parts of France, Germany, and Eussia, aroused an intense interest among European astronomers. The leading observers of France, Italy, England, Germany, and Kussia repaired to various suitable stations along the line of central eclipse. M. Arago went to Perpignan, M. Valz to Marseilles, M. Petit to Montpelier; M. Carlini went to Milan, MM. Santini and Conti to Padua ; the Astronomer Royal went to Superga, Baily to Pavia; MM. Schumacher and Littrow awaited the eclipse at Vienna ; and lastly, the Russian observers, O. Struve and Schidlowski, went to Lipesk. All these observers were fortunate in obtaining excellent views 12 LIGHT SCIENCE FOR LEISURE HOURS. of the phenomenon. I shall quote M. Arago's inte- resting description of the occurrence : ' At Perpignan, persons who were seriously unwell alone remained within doors-. As soon as day began to break, the population covered the terraces and battle- ments of the town, as well as all the little eminences in the neighbourhood, in hopes of obtaining a view of the sun as he ascended above the horizon. At the citadel we had under our eyes, besides numerous groups of citizens established on the slopes, a body of soldiers about to be reviewed. The hour of the commencement of the eclipse drew nigh. More than twenty thousand persons, with smoked glasses in their hands, were examining the radiant globe projected upon an azure sky. Although armed with our powerful telescopes, we had hardly begun to discern the small notch on the western limb of the sun, when an immense exclama- tion, formed by the blending together of twenty thou- sand different voices, announced to us that we had anticipated, by only a few seconds, the observation made with the unaided eye by twenty thousand astrono- mers equipped for the occasion, whose first essay this was. A lively curiosity, a spirit of emulation, the desire of not being outdone, had the privilege of giving to the natural vision an unusual power of penetration. During the interval that elapsed between this moment and the almost total disappearance of the sun, we remarked nothing worthy of relation in the counten- ances of so many spectators. But when the sun, reduced to a very narrow filament, began to throw GREAT SOLAR ECLIPSES. 13 upon the horizon only a very feeble light, a sort of uneasiness seized upon all ; every person felt a desire to communicate his impressions to those around him. Hence arose a deep murmur, resembling that sent forth by the distant ocean after a tempest. The hum of voices increased in intensity as the solar crescent grew more slender ; at length the crescent disappeared, darkness suddenly succeeded light, and an absolute silence marked this phase of the eclipse, with as great precision as did the pendulum of our astronomical cfock. The phenomenon in its magnificence had triumphed over the petulance of youth, over the levity which certain persons assume as a sign of superiority, over the noisy indifference of which soldiers usually make profession. A profound stillness also reigned in the air ; the birds had ceased to sing. After an interval of solemn expectation, which lasted about two minutes, transports of joy, shouts of enthusiastic applause, saluted with the same accord, the same spontaneous feeling, the first reappearance of the rays of the sun. To a condition of melancholy, produced by sentiments of an indefinable nature, there succeeded a lively and intelligible feeling of satisfaction, which no one sought to escape from or moderate the impulses of; to the majority of the public the phenomenon had arrived at its term. The other phases of the eclipse had few attentive spectators, beyond the persons specially devoted to astronomical pursuits.' M. Arago quotes also a beautiful anecdote in illus- tration of the peculiar influence produced by the total 14 LIGHT SCIENCE FOR LEISURE HOURS. \ eclipse of the sun's light, and of the joy which springs unbidden to the heart at the return of his beams. A little girl was watching her flock when the sun began to be darkened. As it gradually lost its light she became more and more distressed, and when at length it disappeared altogether her terror was so great that she began to weep and to cry out for help. ' Her tears were still flowing when the sun sent forth his first ray. Reassured by his light, the child signed herself with the cross, exclaiming, in the patois of the province, " 0, beou Souleou ! " (oh, beau soleil !) ' Remarkable effects were produced on birds and animals by the sudden darkness. Bats and owls came out from their retreats ; domestic fowls went to roost ; and swallows were seized with so great a terror that in some places they were caught in the streets. A herd of cattle grazing in the fields near Montpellier ' formed themselves into a circle, their heads directed outwards, as if to resist an attack.' Horses and oxen employed in the fields ceased from their labours when the sun was totally eclipsed, and lay down, neither whip nor spur availing to induce them to resume their work until the sun's light returned. On the other hand, M. Arago states that ' the horses employed in the diligences continued to pursue their courses without seeming to be in the slightest degree affected by the phenomenon.' During this eclipse, also, it was noticed that several plants closed their leaves. The close accordance between the calculations of mathematicians and the observed circumstances of the GREAT SOLAR ECLIPSES. 15 eclipse excited great attention, and led scientific as well as unlearned men to contemplate with admiration the perfection and regularity of the movements of the celestial bodies. ' All the accounts respecting this eclipse,' says Signer Piola, ; contain reflections on the perfection of that great machine of the universe, whose movements are so regular that the astronomer is enabled, long beforehand, to predict their effects with unfailing precision; and from contemplating the machine, it was natural to ascend to the Supreme Apfirlcer. While this idea swells in the mind there is another which at the same time shrinks into insigni- ficance that suggested by contemplating the position of man in the midst of creation. The magnificence of the scale upon which the phenomena of the eclipse, whether atmospheric or celestial, took place, was patent to every spectator. The extensive coloration of an unusual hue that was visible ; the rapid changes which occurred ; above all, the obscurity which settled over nature like the funereal pall thrown over a dead body, and whose subsequent withdrawal in an instant operated like a resurrection ; all this produced on the mind a mixture of profound and indefinable impressions which it will be pleasing to hold long in remembrance.' And in conclusion I would remark how unworthy of the philosopher and student of nature is that spirit which leads men to look with less admiration on natural phenomena that have received their interpreta- tion from the labours of scientific men. No mystery 1 6 LIGHT SCIENCE FOR LEISURE HOURS. of nature has ever yet been unveiled without disclosing what is yet more mysterious. Copernicus revealed the secret of the solar system, to leave undetected the laws which harmonise the planetary motions. It was Kepler's boast that he had revealed these laws, but he left men to admire without understanding their per- fection and harmony. Then Newton upraised the veil and disclosed to our admiration the noble law of gravi- tation which sways all systems through the universe. But we have more now to perplex us, more to reveal to us the insignificance of our powers, more to make us recognise an infinitely wise Creator, than had the simple Chaldsean shepherds, who Watched from the centres of their sleeping flocks Those radiant Mercuries, that seemed to move, Carrying through aether, in perpetual round, Decrees and resolutions of the gods. If our higher knowledge of the mysteries of nature should lead us to recognise less clearly the author of those mysteries, it would have been better to have never gained that higher knowledge. Our words and works should be worthy of our new light. Let knowledge grow from more to more, But more of reverence in us dwell ; That mind and soul according well, May make one music as before, But vaster. We are fools and slight ; We mock Thee when we do not fear : But help thy foolish ones to bear Help thy vain worlds to bear thy light. The next great eclipse is that of July 28, 1851, which was visible in Sweden ; the other is the eclipse GEE AT SOLAR ECLIPSES. 17 of July 18, 1860, which was visible in Spain, and led to the interesting * Himalaya expedition.' The totality lasted nearly twice as long in the eclipse of 1851 as in that of 1842. Sir G. Airy, who had witnessed the earlier eclipse, was one of a dis- tinguished company which left England for Sweden to observe the eclipse of 1851. *I have no means of ascertaining,' he writes, l whether the darkness really was. greater in the eclipse of 1842. I am inclined to think that in the wonderful, and I may say appalling, obscurity, I saw the grey granite hills, within sight of Hvalas, more distinctly than the darker country surrounding the Superga. But whether because, in 1851, the sky was much less clouded than in 1842 (so that the transition was from a more luminous state of sky to a darkness nearly equal in both cases), or from whatever cause, the suddenness of the darkness in 1851 appeared to be much more striking than in 1842. My friends who were on the upper rock, to which the path was very good, had great difficulty in descending. A candle had been lighted in a lantern about a quarter of an hour before the totality ; and M. Hasselgren was unable to read the minutes of the chronometer's face without having the lantern held close to the chrono- meter.' During this eclipse the red prominences were seen with remarkable distinctness. Airy at Grottenburg, Hind and Dawes at Ecevelsburg, Lassell at the Trollhatten Falls, and other observers, took drawings of these remarkable appearances ; and the agreement ni. C 1 8 LIGHT SCIENCE FOR LEISURE HOURS. between the drawings is such as to leave no doubt of the care with which those observers examined and recorded what they saw. Bound one part of the black limb of the moon there was seen a serrated band of rose-pink light, in another place a pyramidal red mountain, in a third a curved streak of red light formed like a Turkish scimetar, and in a fourth a red detached cloud, which Airy and Lassell picture as nearly circular in form, while Hind and Dawes represent it as triangular. No doubt could exist that these objects belonged to the sun and not to the moon, since the moon was seen to traverse them; insomuch that on the side towards which she was moving their altitude diminished, while on the opposite side they grew larger until the appearance of the sun's disc in this neighbourhood obliterated them through excess of light. The observers were especially struck by the perfect distinctness with which these remarkable appearances were exhibited. 'I had heard them described as but faint phenomena,' says Lassell. ' My surprise and astonishment may therefore be well imagined when the view presented itself to my eyes which I am about to describe. In the middle of the (telescopic) field was the body of the moon, rendered visible enough by the light of the corona attended by the apparent pro- jections. These prominences were of the most brilliant lake colour a splendid pink quite denned and hard. They appeared to me to be not quiescent : but the moon passing over them, and therefore exhibiting them in different phase, might convey an GREAT SOLAR ECLIPSES. 19 idea of motion. They were evidently to my senses belonging to the sun, and not at all to the moon ; for, especially on the western side of the sun, I observed that the moon passed over them, revealing successive portions of them as it advanced. In conformity with this observation also, I observed only the summit of one on the eastern side, though my friends, observing in adjoining rooms, had seen at least two ; the time occupied by me in observing with the naked eye not having allowed me to repair again to the telescope until the moon had covered one and three-fourths of the other. . . . The first burst of light from the emergent sun was exactly in the place of the chief western flame, which it instantly extinguished.' When we consider the actual dimensions of these prominences we are enabled to form some conception of the importance of the problem which they present to astronomers and physicists. The scimetar-shaped protuberance was estimated to extend fully one-twelfth part of the sun's diameter from his surface. His diameter is known to be eight hundred and fifty thousand miles, so that the height of this singular object was fully seventy thousand miles, or nearly three times the circumference of our globe. Consider, again, the long sierra extending around nearly a quarter of the sun's circumference. This sierra was about twenty- five thousand miles high. Now many of my readers have doubtless seen the ranges of the Alps as they appear, when seen from some distant point in clear weather, and they know how imposing is the aspect c2 20 LIGHT SCIENCE FOR LEISURE HOURS. of these gigantic land-masses. Yet the highest peaks of the Alps are little more than fifteen thousand feet above the sea-level. Imagine, then, the magnificence of glowing masses twenty-five thousand miles above the mean level of the sun's surface. During the eclipse of 1860, the red prominences again attracted a great deal of attention among astro- nomers. It will be remembered that many leading English astronomers, amongst whom the Astronomer Koyal again figured, took part in the celebrated Hima- laya expedition. MM. Leverrier and Groldschmidt of Paris, the Padre Secchi of Home, and a host of astro- nomical celebrities, took part in observing the various phenomena, astronomical, physical, and meteorological, which attended the totality of this important eclipse. It is interesting, in the first place, to compare Sir Greorge Airy's impressions as to the general effect of the totality with those which he formed during the two former eclipses. It is not often that the same observer and that observer so skilful and eminent has the opportunity of contrasting together three total eclipses of the sun. In fact, I doubt very much whether any similar case is on record. Hence, a peculiar value attaches to Sir Gr. Airy's remarks. ' On the progress of the eclipse,' he says, ' I have nothing to remark, except that I thought the singular darken- ing of the landscape, whose character is peculiar to an eclipse, to be sadder than usual. The cause of this peculiar character I conceive to be the diminution of light in the higher strata of the air. When the sun is GREAT SOLAR ECLIPSES. 21 heavily clouded, still the upper atmosphere is brilliantly illuminated, and the diffused light which comes from it is agreeable to the eye. But when the sun is partially eclipsed, the illumination of the atmosphere for many miles round is also diminished, and the eye is oppressed by the absence of the light which usually comes from it. ... I had a wax-candle lighted in a lantern, as I have had at preceding total eclipses. Correcting the appreciations of my eye by reference to this, I found that the darkness of the approaching totality was much less striking than in the eclipses of 1842 and 1851. In my anxiety to lose nothing at the telescope, I did not see the approach of the dark shadow through the air ; but, from what I afterwards saw of its retreat, I am sure it must have been very awful.' ' About the middle of the totality I ceased my measures for awhile, in order to view the prospect with the naked eye. The general light appeared to me much greater than in the eclipses of 1842 and 1851 (one cloudy, the other hazy) perhaps ten times as great ; I believe I could have read a chronometer at the distance of twelve inches ; nevertheless, it was not easy to walk where the ground was in the least uneven, and much attention to the footing was necessary. The outlines of the mountains were clear, but all distances were totally lost ; they were, in fact, in an undivided mass of black to within a small distance of the spectator. Above these, to the height perhaps of six or eight degrees, and especially remarkable on the north side, was a brilliant yellow, or orange, sky, without any trace 22 LIGHT SCIENCE FOR LEISURE HOURS. of the lovely blush which I saw in 1851. Higher still the sky was moderately dark, but not so dark as in former eclipses.' Mr. Airy noted a remarkable circumstance in con- nection with the red prominences. They were not of the same colour as in 1842 and 1851. The quality of the colour was exactly the same ' full-blush red (or nearly lake) but it was diluted with white' (an evidence of higher temperature), ' and more diluted at the roots of the prominences close to the moon's limb than in the most elevated points.' Of course the red prominences do not necessarily reach from the sun's surface, as a mountain from the surface of the earth. Masses suspended in the solar atmosphere would appear as prominences resembling mountains, unless they happened to be of comparatively moderate extent, and were seen in such a position that the space between them and the sun's surface became perceptible. Those serrated ridges, therefore, that we see may belong to the upper surfaces of masses sus- pended high above the true surface of the sun. Before proceeding, however, to inquire a little into the probable constitution of these marvellous objects, it will be well to give a brief description of what was seen by Continental observers during the last great eclipse. Leverrier says that the first object which he saw in the telescopic field of view when totality had commenced, was * an isolated cloud, entirely separated from the moon's limb by a space equal to its own size.' He adds, that the colour of the cloud was a fine rose, GREAT SOLAR ECLIPSES. 2$ tinged with violet, and almost white in some parts through exceeding brilliancy. Near this cloud were two others, one above the other, the upper being the smaller ; these were very equally illuminated. Else- where he saw two elevated prominences close to each other, and in another part a protuberance resembling a tooth. Keturning to the point where he had seen two clouds, he found them unaltered in figure. He now directed his attention to the part of the moon's limb behind which the sun was about to appear. Here he saV a long ridge of reddish purple colour, having a serrated outline. M. Groldschmidt describes the appearance of one of the rose-coloured prominences in the following terms : * The most imposing, as well as complicated of the prominences, which I will call the chandelier, was grand beyond description. It rose up from the limb, appearing like slender tongues of fire, and of a rose colour, its edges purple and transparent, allowing the interior of the prominence to be seen ; in fact, I could see distinctly that the protuberance was hollow. Shortly before the end of the totality I saw escape from the rose-coloured and transparent sheaves of light a slight display in the shape of a fan, which gave to the protuberance a real resemblance to a chandelier. Its base, which at the commencement of the totality was noticed to be very decidedly on the black limb of the moon, became slightly less attached, and the whole took an appearance more ethereal or vapourish.' M. Goldschmidt observed that the small jets of light 24 LIGHT SCIENCE FOR LEISURE HOURS. disappeared as soon as the sun's rays became visible, but the prominence itself remained distinctly visible nearly five minutes after the reappearance of the sun. The rest of M. Groldschmidt's account corresponds closely with what is described by other observers. I may remark that his opinion respecting the hollowness of his * chandelier prominence ' seems founded on very insufficient evidence. The transparence of the outer parts of the prominence is a proof rather that the central parts were denser than that the prominence was hollow. But all that M. Groldschmidt says that he observed may be accepted with the fullest confidence, though no other observer has described similar appear- ances ; for there has seldom lived so acute and skilful an observer as this astronomer. He was well known to fame as the discoverer of no less than thirteen asteroids, and numbers of nebulae and variable stars. The Padre Secchi, of the Collegio Romano, remarks of one protuberance, that the point was * rather slender and curved, resembling a flame somewhat agitated.' He remarked that as the moon passed across the solar disc so many luminous points appeared on the following edge of the black disc that he was embarrassed which to choose for observation and measurement. The pro- minences increased in size as the moon glided forwards, and he ' saw, with surprise, an almost continuous arc of purple light instantaneously formed, composed of small protuberances, in that part of the lunar disc where the reappearance of the sun was expected.' He remarks that his observations have convinced him GREAT SOLAR ECLIPSES. 25 * that the protuberances are connected with the sun, and that it is absurd to assert the contrary.' It appears to me that very little doubt can exist as to the general character of the red prominences, though we are very far from asserting that their exact consti- tution can be readily determined. In the first place, it is tolerably clear that they are not fixed in position. No motion has, indeed, been observed in them during the short time that they have continued visible in total eclipses. But we know that thfe whole of the sun's surface is in a state of continual agitation. The spots break out, vary in form, expand, contract, expand again, whirl around their nuclei, are suddenly spanned by sharply defined bridges of light, and after many such changes vanish altogether. All this while the region around the spots shows obvious traces of a continual flux and reflux of matter. Then, again, there are the periodic variations in the frequency of spots, and of the faculae and maculae which accom- pany them. And although there are only two bands on the sun's surface (corresponding in position to the temperate zones upon the earth's surface) on which these changes take place, yet we have distinct evidence that the great eleven-year period affects the whole surface of the sun. For at the time when spots are least frequent the sun's disc presents sometimes for several months an appearance never observed at any other time. Instead of appearing darker round the edge of the disc, it is seen perfectly uniform in tint over its whole surface. This variability in the appear- 26 LIGHT SCIENCE FOR LEISURE HOURS. ance of the sun's surface is inconsistent with the exist- ence of masses of matter, fixed in position (or even permanent in character, but unfixed in position) over extensive solar regions. We have seen the probability that exists that the red prominences are detached from the sun's surface. We know, thirdly, that they must exist at an in- conceivably high temperature. Lastly, the spectroscope has proved that the sun's light reaches us after passing through an extensive solar atmosphere, consisting of the vapours of many of our best-known metals. The vapour of iron, for instance, forms a part of the sun's atmosphere much in the same way as aqueous vapour appears as a constituent of our own air. It seems to me reasonable to conclude from these considerations that the objects called the red pro- minences are, in reality, solar clouds ; only instead of consisting, as our terrestrial clouds do, of visible aqueous vapour (that is, of minute globules of water), they consist of the visible vapours of the various metals which exist in the solar atmosphere. In other words, they are clouds formed by the condensation of the metallic vapours into liquid globules. [This was written a few days before evidence was obtained, showing that the prominences are gaseous that is, even more mobile and tenuous than I had inferred. The reasoning was sound, but proved more than was insisted on.] Leverrier was led by his observation of the eclipse GREAT SOLAR ECLIPSES. 2 7 of 1860 to associate the solar spots with the red pro- minences in a manner closely according with the view we have here put forward. * Observation proves,' he says, "that the rose-coloured matter is accumulated occasionally on certain points in quantities more con- siderable than in others, and as the light of the corre- sponding part of the sun may possibly be found more or less extinguished, we arrive at a natural explanation of the spots on the sun's surface. These spots will exhibit the most varied forms and appearances, subject to ^tKe most rapid changes, in a similar manner to what has been already observed, provided they are produced by clouds. They will change their position on the surface of the sun like clouds on the surface of the earth.' I trust that the great eclipse which is approach- ing will not pass without adding largely to our know- ledge of solar physics. Everything seems favourable the regularity of the Indian climate ; the long range of inhabited country traversed by the shadow; the careful preparation which has been made for spectro- scopic observation, and for taking photographic views of the phenomena presented during the totality. All these circumstances, and the exceptional character of the eclipse itself, combine to afford promise of inter- esting and important discoveries. Since the above was written I have heard of several other expeditions which have been sent out to view this important eclipse. The Prussian Government has 28 LIGHT SCIENCE FOR LEISURE HOURS. sent an expedition to Aden, almost the nearest point at which the eclipse will be visible as a total one. The French Government sends out M. Janssen, the eminent observer, at the head of a well-appointed expedition. The Pope sends out Father Secchi. Mr. Pogson, the superintendent of the Grovernment Observatory at Madras, will also take part in the work of observing the eclipse. He has been supplied by Mr. Huggins, the eminent spectroscopist, with instruments for analysing the light from the corona and the coloured prominences. Certainly the eclipse will be well watched unless the weather should unfortunately prove unfavourable. Nor will observers at home be altogether idle. The careful survey of the sun's disc for several days before and after the great eclipse will doubtless be carefully attended to by the eminent students of solar physics who have charge of the Kew Observatory. Thus it will be possible to determine what spots, if any, were on or near the boundary of the disc at the moment of totality; and the suspected association between the spots and the coloured prominences will be put to a satisfactory test. Cornhitt Magazine : August 1868. TWO YEARS LATER. 29 TWO YEARS LATER. Two years ago, astronomers were looking anxiously forward to an event which they justly thought likely to prove an epoch in the history of solar research. The great eclipse of August 1868 was not only re- markable on account of the great extent of the black shadow cast upon the earth by the moon, but also as the* first total eclipse during which the powers of the most wonderful instrument of research ever invented by man were to be applied to the phenomena visible at such a time. The coloured prominences which had so long perplexed astronomers could hardly fail, it was considered, to reveal their secret under the searching scrutiny of the spectroscope. What the light-gathering powers of the telescope had failed to explain, the light- sifting qualities of the spectroscope might be expected to interpret and that almost at a glance precisely as they had resolved r so many other questions of interest. Every one knows how abundantly these expectations were .fulfilled. Not at one station only in India were the observers successful in mastering the secret of the coloured prominences, but, by a wonderful piece of good fortune, every single observer who had made arrangements to direct the spectroscope to the solar prominences succeeded in answering the question which had so long perplexed astronomers. From 30 LIGHT SCIENCE FOR LEISURE HOURS. Lieutenant-Colonel Tennant at one Indian station, and from Lieutenant Herschel at another, from Kayet and Janssen, and from the Prussian astronomers who observed the eclipse at Aden, came the same answer the prominences are masses of glowing vapour. A few bright-coloured lines had in an instant taught the great lesson astronomers had been so long waiting for. Had the coloured prominences been mountains, as some had supposed, the spectroscope would have shown the rainbow-tinted streak which speaks of the solid nature of a source of light. Had they been clouds suspended in the solar atmosphere, there would have been seen the rainbow streak crossed by dark lines, corresponding to that structure. But, because they consist of glowing gas, the rainbow-tinted background was wanting ; and only a few bright-coloured lines, corresponding to the particular gases present in these mighty flames, were seen along the spectral range. Then followed one of those strange coincidences which the history of science has so often presented. Janssen, one of the observers of the eclipse, was struck by the thought that since the light from the promi- nences is thus gathered up concentrated, so to speak into a few bright lines, it might be possible to see those lines even when the sun is not eclipsed. It is easy to see why this might k be possible. The promi- nences shine faintly when compared with the solar disc ; and so, if we use darkening glasses in observing the latter, we obliterate their light altogether. Nay, even if we absolutely get rid of the direct sunlight, TWO YEARS LATER. 31 yet we cannot see the prominences. Sir George Airy tried the experiment long since. He placed a card, out of which a circular aperture had been cut, in such a way that the image of the sun, formed by a powerful telescope, would have been visible in the place whence the card-circle had been removed the image just filling that space. Under the actual arrangement, however, the light passed through the aperture, and was received into a black bag, where it was quenched. Now, it might have been supposed thatr by this ingenious method the image of the prominences would have been rendered visible all round the circular aperture. But the glare from the illuminated air was much more than sufficient to obliterate all traces of them. And so it might seem that no means we could adopt would render the prominences visible. But it occurred to Janssen that since the spectroscope turns the solar light into a long streak which can be made as dim as we please by increasing its length while the same instrument turns the prominence-light into a few bright lines which are unchangeable in brightness it might be possible to see these lines after sufficiently reducing the light of the rainbow-tinted solar streak. He tried this the day after the eclipse, and found that it was so : he could distinctly see the prominences-lines even when the sun was shining with full splendour. Janssen sent news of this discovery to Europe, and on a certain day, nearly two months after the eclipse, the letter announcing the discovery was placed in the 32 LIGHT SCIENCE FOR LEISURE HOURS. hands of the President of the Imperial Academy at Paris. Five minutes before, however, the president had read a communication from Mr. Warren De La Hue, announcing that an English observer had lighted independently upon the same discovery. Let me briefly indicate how this had come about, premising that what we have yet to learn from future eclipses is so intimately associated with the history of what we have already learned, that it would be im- possible rightly to present the hopes of astronomers respecting the eclipse of next December without con- sidering the progress of past research. Mr. Huggins, the eminent spectroscopist, had in 1866 examined the light of a star which blazed out suddenly in the constellation of the Northern Crown. He had found that this star owed its great increase of lustre to an outburst of hydrogen flames ; for he could distinctly see the bright lines belonging to the spectrum of glowing hydrogen, superposed on the rainbow-tinted streak crossed by dark lines, forming the ordinary spectrum of a star. It occurred to Mr. Lockyer that if the spectrum of a glowing gas can thus be recognised in the case of a distant star, we might be able to detect masses of glowing gas on our sun, which is relatively so near to us; so that, if the prominences are of this nature as many astronomers even at that time thought pro- bable we might be able to see their spectral lines even when the sun is not eclipsed. He directed the attention of the Koyal Society to this method of obser- TWO YEARS LATER. 33 vation, and urged them to grant a sum for the construction of a suitable spectroscope. With the usual generosity of the Koyal Society, such a sum was placed at Mr. Lockyer's disposal. The problem of making a spectroscope which would adequately lengthen out the solar spectrum was successfully solved by Mr. Browning, the eminent scientific optician ; and, finally, some two months after the eclipse of 1868 Mr. Lockyer tried the powers of the instrument thus placed in his possession. As already mentioned, the bright-coloured linfeTof the prominences were distinctly seen with the new spectroscope ; and although Janssen's similar observations had been made nearly two months earlier, no question rests on the independent nature of Mr. Lockyer's observation. Indeed, so successfully had Mr. Browning mastered the optical difficulties of the problem, that no doubt whatever can exist that Mr. Lockyer would have been successful, altogether inde- pendently of the information afforded him, in the ac- tual case, by the eclipse observations of August 1868. But now let us see the position in which spectro- scopists stood. The new mode of observing the pro- minences presented no special difficulties at least, what difficulties there were referred to the optician rather than the astronomer, (riven a telescope of adequate power, armed with a spectroscope spreading out sufficiently the rainbow-tinted streak which forms the solar spectrum, and it became at once possible for any tolerably well-trained observer to make a series of such researches as, twenty years ago, no man of science III. D 34 LIGHT SCIENCE FOR LEISURE HOURS. would probably have believed to be possible. The visi- bility of a certain set of bright lines would demonstrate not only the existence of a prominence of a particular height at one part of the solar disc, but the nature of the gases of which that prominence was constituted. Nay, so far as the existence of a prominence was concerned, one line alone would suffice for the observer's purpose. But now new results of extreme importance began to be obtained. One of the first of these was the confirmation of a theory which had been put forward by Father Secchi several years ago. This eminent observer, making use of a particular mode of viewing the sun, had detected signs of the existence all over the sun's surface of a layer of the same coloured matter which forms the prominences. Combining these indications with the observations he had made during the total eclipse of 1860, he asserted with great, but not unjustified, confi- dence his belief in the actual existence of this envelope. * The observation of eclipses,' he remarks, ' furnishes indisputable evidence that the sun is really surrounded by a layer of this red matter, of which we commonly see no more than the elevated points.' Now the new mode of research was admirably suited to test the views of Father Secchi. In search- ing around the solar disc, Mr. Lockyer could only here and there find traces of the existence of prominences ; but all round the disc he found short bright lines close to the disc's edge, indicating beyond all question that Father Secchi had been right, and that there really exists all over the bright surface of the sun a gaseous TWO YEARS LATER. 35 envelope, corresponding, though not absolutely iden- tical in structure, with the prominences. Mr. Lockyer called this envelope the chromosphere (for chromato- sphere), and it would seem no undeserved compliment to the acuteness of one of the most indefatigable of modern astronomers that this envelope should be known in future by the name of Secchi's sierra. 1 I have said that this envelope is not actually identical in structure with the prominences. It is, in factj more complicated. Only certain gaseous ele- ments of the sierra seem capable of rising to the enormous height attained by the prominences. In these great masses the principal element is hydrogen, but in the sierra many elements are commonly recog- nised such as sodium, magnesium, barium, iron, &c. while under favourable circumstances the bright lines in the sierra are so numerous as to indicate the pre- sence of quite a large proportion of the elements which exist in the sun's substance. But then other modes of research with the spectro- scope came into operation. Mr. Lockyer entered into an alliance with Dr. Frankland, one of the most eminent physicists of the day, and very quickly the wisdom and advantage of this course were manifested. It is one of the most promising characteristics of spectroscopic research, that work done in the labora- tory becomes available to tell us of the structure of orbs many millions of miles away from us. Dr. Frankland 1 I "write thus with perfect knowledge that other astronomers had jet earlier suspected the existence of the chromosphere. D 2 36 LIGHT SCIENCE FOR LEISURE HOURS. soon obtained results which supplied most important information respecting the solar constitution. It had been shown long since by Pliicker and Hittorf, that the bright lines which form the spectrum of hydrogen and other gases vary in appearance according to the circumstances of pressure, temperature, and so on, under which the gases give out their light. Here was at once a powerful means of inquiring into the condi- tion of the gases forming the solar envelope. Dr. Frankland was able to conGrm the researches of Pliicker and Hittorf, and further he went far towards proving that pressure is the chief circumstance affecting the appearance of the bright lines of hydrogen and other gases. He showed, too, that with a great dimi- nution of pressure some of the bright lines disappear. Now, turning to the sun, Mr. Lockyer was able through these researches to form at least a probable opinion respecting the pressure at which the hydrogen forming the solar prominences in reality exists. He found that the bright lines are so narrow as to indicate a singularly low pressure considering the enormous force with which the sun attracts his atmospheric envelope a pressure actually less, indeed, than that of our own atmosphere. He could trace also the elevations at which some of the lines of the various elements forming the solar atmosphere vanish the others remaining and so, by comparison with Dr. Frankland's elaborate researches, could tell at least with some probability what, is the actual pressure at different heights in the solar atmosphere. It was, in- Tiro YEARS LATER. 37 fact, much as though an observer on the sun could see our barometric columns standing at a height of thirty inches at the sea level, and at lesser and lesser heights at greater and greater elevations. Precisely as such an observer, supposing him to be acquainted with the nature of the barometric column, could tell the circum- stances of pressure at different heights in our atmo- sphere, so Mr. Lockyer, knowing the gases which form the chromosphere, and informed by Frankland's re- searches of the interpretation of the vanishing of bright lines, could tell the variations of pressure at different heights in the solar atmosphere. I have said, however, that the result was not absolutely certain. It is easy to see why this is. Temperature has an undoubted effect upon the bright lines belonging to the gaseous spectra, and it is obvious that the heat throughout the solar atmosphere must far surpass any which our chemists can artificially produce in their laboratory experiments. So that it must still remain open to some question whether we can reason quite so confidently respecting the condition of things in the sun's neighbourhood, as we might if such peculiar relations did not necessarily exist there. But at present it seems at least a probable inference that the gases forming the prominences are not sub- jected to very great pressure. And this brings me to the consideration of the phenomena which will un- doubtedly occupy the chief attention of observers during the approaching total eclipse of the sun. When the sun is quite concealed from view by the interposing 38 LIGHT SCIENCE FOR LEISURE HOURS. moon, there springs suddenly into view a crown of glory all round the moon's black disc, which has for centuries perplexed astronomers. This appearance the solar corona, as it is called had been supposed until quite recently to be a solar atmosphere, other interpretations having one by one been abandoned. But then, the corona extends in appearance to a distance at least half as great as the moon's apparent diameter from the lunar disc. So that if in reality it is due to a solar atmosphere, that atmosphere must be at least half the sun's diameter in altitude that is, upwards of four hundred thousand miles high. Now our own atmosphere is probably not more than one hundred miles high, so that the solar atmosphere assuming the view we are considering to be correct would be no less than four thousand times as high as ours ; and on this account alone the pressure at its base would enormously exceed the pressure of the air we breathe. But this is not all. The pressure of our air is due wholly to the earth's attraction, and would be increased or diminished if the earth's attractive force were increased or diminished. Now the sun exerts an attractive force so vastly exceeding that exerted by the earth, that if a man could be placed at the sun's surface remaining uninjured by all other circumstances' he would be crushed flat by his own weight. We can see, therefore, that the atmosphere of the sun would have its pressure enormously increased through this cause also. Combining the two causes, it is not too much to say that the pressure at the sun's TWO YEARS LATER. 39 surface, under such an atmosphere as we have been supposing, would suffice to liquefy if not to solidify the most subtle gases we are acquainted with. There cannot be any question therefore that the spectroscopic observation of the sun has sufficed to throw very great doubt indeed upon the theory that the corona is due to a solar atmosphere. Or rather, we may fairly say that that theory has been distinctly shown by Dr. Frankland's laboratory researches to be untenable. *But then there remains the difficulty of explaining what the corona really is. We know that it cannot be a lunar atmosphere, because a number of very exact observations have shown, beyond all possibility of question, that the moon has no atmosphere of appre- ciable extent, far less such an atmosphere as would account for the corona. Again, the theory which was put forward by De Lisle in the seventeenth century, that the corona is caused by the diffraction of the sun's rays as they pass by the moon, has been disproved by the inquiries of Sir David Brewster. There is indeed another theory, which has strangely enough been exhumed quite recently. Ac- cording to this theory the solar corona is simply a phenomenon belonging to our own atmosphere. The theory was first mentioned though only to be sum- marily rejected by Halley, and touched on somewhat contemptuously by other astronomers. It explains the corona as due to the illumination of the upper regions of the air by the sun's rays. We know that if we hide the sun with a globe or disc of any sort, a strong light 40 LIGHT SCIENCE FOR LEISURE HOURS. is seen all round the interposed object. And it might seem that since the moon is but a globe somewhat larger than our experimental one, and somewhat farther off, we ought to see a similar light all round the black disc of the eclipsing moon. But a little consideration will show the fallacy of this reasoning. When we hold a globe so as just to hide the sun, we do not throw into shadow those upper regions of air from which the atmospheric glare really comes. But when the moon conceals the sun during total eclipse, she causes an enormous shadow to fall right through the whole depth of the air. This shadow, even at its narrowest, that is, where it reaches the earth, has been in many total eclipses fully one hundred miles wide ; and as the part of the air capable of reflecting solar light to an appreciable extent is shown by the twilight- arch to be but forty or fifty miles high at the outside, we see that, in the case of such eclipses, the moon's shadow in the air is of a vast drum-shaped figure, at least twice as wide as it is high. It is most obvious, then, that to an eye placed at the centre of the vast base of this drum-shaped shadow, no light can possibly come from the air for a wide range all round the place of the eclipsed sun. Imagine a shadow hiding nearly all England and fifty miles high ; then to an eye placed, say at Hereford, the upper surface of the shadow would cover an enormous extent of sky, while the eclipsed sun, at the apparent centre of that surface, would be but as a relatively minute circle. TWO YEARS LATER. 41 It need hardly be said that considerations so ob- vious have not escaped the attention of astronomers. I have said that Halley rejected the atmospheric glare theory, and that other astronomers have spoken of it with but little respect. In quite recent times, com- petent astronomers, who have had occasion to examine it, have in like manner rejected it. Dr. Harkness, who witnessed the American eclipse, and was led by the study of the corona as actually seen to inquire into, the physical nature of the phenomenon, remarks respecting the theory that ' the moon's shadow, at the point where it enters the earth's atmosphere, usually has a diameter of one hundred miles or more, and if it were possible for an observer placed within that shadow to see the illumination of the atmosphere outside of it, the appearance presented would be that of a halo having an interior diameter much greater than the size of the moon.' Dr. Curtis, also a skilful mathematician after exhibiting precisely the same line of reasoning, remarks that 'it is geometrically impossible for an observer near the centre of the shadow to see any portions of our atmosphere which lie beyond the cone of darkness which portions alone could, of course, under any circumstances be illuminated in apparent contiguity with the moon's disc.' Thus we are brought back to the theory that the corona really is a solar phenomenon, while yet we are precluded from supposing that it is a solar atmosphere. What, then, can it be ? Now astronomers hope for much, and for very 42 LIGHT SCIENCE FOR LEISURE HOURS. useful information respecting the corona, during the progress of the eclipse of December 1870 ; and the question may suggest itself, how far it is wise to discuss the subject of the corona now, when in a few months we may be in so much more advantageous a position for theorising respecting it. This is a consideration well worth dwelling upon ; yet the arguments by which we are to deal with it are sufficiently simple. If we have already exhausted all means of inquiry applicable to what we already know respecting the corona, then our proper course is to wait. It would undoubtedly be absurd in that case to attempt to evolve from the depths of our moral consciousness a theory respecting this wonderful and mysterious phenomenon. But in truth there is little fear of our thus over- riding observation. In these days observation pro- gresses with such amazing rapidity that reasoning is left far in the rear. It is not too much to assert that if all observation were from this moment to cease, the students of science would find abundant employment for a decade of years, at least, to come, in examining and utilising the observations which have been already made. Let us, then, look round our storehouse, and see if we may not at once, and almost at random, light upon a few observations which may serve to help us in interpreting the wonders of the corona. A few months ago Captain Noble was looking at the planet Venus when almost directly between us and the TWO YEARS LATER. 43 sun. She was so placed with respect to the sun that,, had his globe been eclipsed, she would have been seen near the edge of the coronal light. Of course her un- illumined side was turned towards us. It appeared darker than the background on which it was projected. Whence came that light which illumined the back- ground ? Or rather, what light was it which the globe of Venus concealed ? Something beyond Venus un- doubtedly else how could she conceal it ? Obviously, then, there is some light where we see the corona when the sun is eclipsed, and that light comes from a region farther off than Venus is when nearest to us. Here is another evidence to strengthen our conviction that the corona is an extra-terrestrial phenomenon, supposing any faint doubts to remain after the evidence already adduced. We have gone to Venus, seemingly so little associated with the corona, for evidence respecting that phenomenon. Turn we now to other objects which seem at first sight even less likely to give us any in- formation. Consider thoughtfully the meteor which flashes across the dark background of the sky at night. We know now quite certainly that every falling star has travelled before reaching our atmosphere along an orbit of enormous dimensions. It has been proved further respecting all the meteors whose real path has been determined, that their orbits are very eccentric, inso- much that though they cross the earth's path other- wise, of course, we should never see them they pass 44 LIGHT SCIENCE FOR LEISURE HOURS. out to distances exceeding in some cases those at which Uranus and Neptune pursue their wide careers. We may conclude, then, of far the greater number of the meteors the earth encounters, that their paths, having their most distant portions so much farther from the sun than our earth, must have their nearest part to the sun much closer to the sun than the earth is. So that if meteors were as large as planets, it would necessarily happen that many meteors belong- ing to systems which the earth encounters would be seen, at certain seasons, shining close by the sun. But, as a matter of fact, meteors are individually far too minute to be thus seen ; nor could all the meteor systems traversed by the earth become visible by the combined lustre of their components. It is clear, however, that the meteor systems tra- versed by the earth can be but a few among the meteor systems actually existing, and having paths carrying their components nearer to the sun than the earth is. Taking at random any such path, the chance that the earth's path would cross it is indefinitely small ; so that clearly an indefinite number of such systems must exist in order that the earth might have a fair chance of encountering a single one. And since she actually encounters hundreds, it will be seen how enormous must be the real number of systems actually existing. This mode of reasoning, though in strict accordance with recognised and certain principles, may not seem convincing at first sight. But, in reality, it will be found that we quite commonly, and as it were uncon- TWO YEARS LATER. 45 sciously, follow the guidance of the principle in ordinary life. We are out walking, suppose, and a drop of rain falls upon us ; now, there is no absolute reason why a single drop falling from the sky should not light upon us ; yet so certain are we that the odds against such an event are enormous, that we conclude at once that rain is falling over a wide space all around us. Or again, suppose we meet some day five or six persons dressed in a peculiar costume, and not forming one party we conclude at once that there is to he some gathering of such persons on the day in question. And so in a thousand instances which will occur to every one. I conclude, then, with the utmost confidence that for every meteor system encountered by the earth there must be thousands which she does not en- counter. And these multitudinous systems, illuminated as they must be by the sun's rays, might very reasonably be expected to become visible under favourable circum- stances as, for example, when the sun is eclipsed. Nay, knowing that the meteors travel in paths re- sembling those of comets, and in some cases associated in the most intimate manner with the paths of known comets we may conclude that large numbers of meteors- pass as close to the sun as some comets have been ob- served to do, or even nearer, for observed comets form but a small proportion of the total number of such objects. Now, Sir John Herschel has shown that the comet of 1843 passed so close to the sun that it must have been subjected to a heat exceeding three and a 46 LIGHT SCIENCE FOR LEISURE HOURS. half times that obtained by means of Parker's great lens, which melted such refractory substances as corne- lian, agate, and rock-crystal. Meteors so close as this to the sun would be so intensely heated that their in- herent light would be even more brilliant than that which they would be capable of reflecting. Many would even be vaporised as they rushed past the point of their nearest approach to the solar orb. We see then that, quite apart from the information which a solar eclipse affords us, we really have just reason for pro- nouncing with considerable confidence that something very like what the corona appears to be must exist in the sun's neighbourhood. Now it is well worth noticing that if we suppose the corona really to be caused by the illuminated meteoric systems, we get rid at once of that difficulty which spectroscopic analysis opposed to the theory that the corona is a solar atmosphere. These swiftly rushing meteors would no more tend to increase the pressure at the base of the solar atmosphere than the moon, circling as she does round the earth, tends to increase our own atmospheric pressure. It happens, too, that such evidence as has hitherto been given by the spectroscope respecting the actual constitution of the corona corresponds very satisfactorily with the conclusions above deduced. I do not enter here into a very particular account of that evidence first, because the observations made by different astro- nomers have not yet been brought into complete accordance ; and secondly, because it is confidently TWO YEARS LATER. 47 hoped that the approaching eclipse observations will make abundantly clear that which is at present some- what confused. But the very diversity of results corresponds with the diverse character of the light which comes according to the above results from the meteoric systems near the sun. Meteors simply reflecting solar light would exhibit the rainbow-tinted streak crossed by multitudinous dark lines, which forms the solar spectrum ; meteors incandescent through intensity of heat would exhibit a rainbow-tinted spectrum without dark lines; and lastly, meteors vaporised by heat would exhibit a spectrum of bright lines. The combination of such spectra in varying proportions would quite satisfactorily account for the results hitherto observed by spectroscopists. It is, however, worth noticing that electrical discharges exerted by the sun's action, and taking place between the meteors, would even more completely account for observed results ; and perhaps it is only because elec- tricity has come to be regarded as a sort of refuge for the scientific destitute, that men of science have been hitherto unwilling to resort to such an explanation of what has been observed. But astronomers hope that during the eclipse of next December the spectroscope will be applied much more effectually than has yet been done to the scrutiny of the solar corona. Photography, too, it is hoped, will be so applied as to exhibit the corona, and not merely, as hitherto, the pink prominences and the more brilliant part of the glare around the sun. Then 48 LIGHT SCIENCE FOR LEISURE HOURS. the polariscope is to be applied, though for my own part I have very little faith in the possibility that this instrument can give intelligible and reliable results respecting such a phenomenon as the corona would appear to be. If its light really is of the mixed nature I have described, it can scarcely be but that the polariscopic teachings will be discordant and practically unmeaning. Last, but not least, a large array of observers have devoted themselves to the scrutiny of the general features of the eclipse. I wish very strongly to in- dicate the opinion that much remains to be done in this way. It seems to me that in all the eclipses hitherto observed, attention has been somewhat too exclusively directed to the eclipsed sun and its imme- diate neighbourhood. I write this in the full know- ledge of the meteorological and other observations which have been made during eclipses. The class of observation which, as it seems to me, has been insuffi- ciently attended to, includes the special study of the varying illumination of the sky, not near the sun, but at all orders of distance from him. Remembering- O that we know the actual figure, dimensions, and position, from second to second, of that vast shadow- cone which the moon projects upon our atmosphere, we can interpret in a very satisfactory manner the apparent changes of illumination in different regions of the sky. From such observations, properly made and studied, more could be learned, I do not hesitate to say, respecting the height of the air, the quality of its TWO YEARS LA.TER. 49 upper and unattainable regions, and other like subjects, than from all the methods by which as yet men of science have attempted to master these stubborn problems. It must not be forgotten, however, that the eclipse will last as far as totality is concerned for a very brief time. For about two minutes the exact time cannot be known until the observing stations are decided upon the sun will be totally hidden from view ; and whatever new information is to be obtained respecting the constitution of the corona, and about other subjects which will occupy the attention of observers, must be gleaned in that short interval of time. Those at home must not be surprised or disap- pointed, therefore, if the results actually obtained should seem at first sight disproportioned to the expense and trouble involved by the expeditions, or to the number of persons who will take part in the work of observation. Science must be content in such cases to expend a large amount of time and trouble where yet the prospect of remuneration will be but small. All the more credit to those who are ready to join in an enterprise so arduous, and presenting so many chances of failure. in. 5