THE POETRY OF ASTRONOMY KNOWLEDGE" LIBRARY. THE POETRY OF ASTRONOMY: A SERIES OF FAMILIAR ESSAYS ON THE HEAVENLY BODIES, REGARDED LESS IN THEIR STRICTLY SCIENTIFIC ASPECT THAN AS SUGGESTING THOUGHTS; iESPZCTINC;) INFINITIES OF TIME AND SPACE, 9F OF VITALITY, AND OF BY RICHARD A. PROCTOR, AUTHOR OF " SCIENCE BYWAYS," " THE BORDERLAND OF SCIENCE," ETC. Profounder, profounder man's spirit must dive : +J } / t S f *- To its aye-rolling orbit no goal can arrive : The heavens that now draw him with sweetness untold Once found for new heavens man spurneth the old. EMERSON. LONDON : WYMAN & SONS, 7 4-?6, GREAT QUEEN STREET, LINCOLN'S-INN FIELDS. All rights reserved. readers of KNOWLEDGE, in which Weekly Paper an effort has been made to bring scientific news and discoveries before the public in a cheap and trustworthy form, have asked me whether I could not publish cheaper editions of some of my own books. In the great majority of cases, the matter does not rest in my own hands ; but having had an opportunity to purchase from Messrs. Smith, Elder, & Co., their rights in my three works, "THE BORDERLAND OF SCIENCE," " SCIENCE BYWAYS," and the. " P.QETRY^ OF ASTRONOMY," / have much pleasure in so far acceding to the above-named wishes, as to publisJi an edition of each of these works at two-thirds of their original price. Owing to the circumstance which led to this, I have thought it desirable to class those three books as the first three volumes of a series to be called the KNOWLEDGE LIBRARY. Most of the volumes of the series are to be republications of Essays by various Contributors to the Magazine from which the series takes its name. RICH D - A. PROCTOR. LONDON, September, 1882. 466701 PBEEACE. MANY think that science cannot truly be called science if clothed in poetic garb, and, on the other hand, others seem to fear that a glory must depart from the face of nature if science scrutinise her mysteries too closely. I believe both these fears to^ba unfounded that science need not be less exact though poetry underlie its teachings; while, beautiful and glorious though the ordinary aspect of nature may be, a deeper poetry, a more solemn significance, a greater beauty, and a nobler glory can be recognised in the aspect of nature when science lifts the veil which hides it from the unaided vision. Nay, I believe that no one who studies aright the teachings of the profoundest students of nature will fail to perceive that our Gralileos, Keplers, and Newtons, our Priestleys, Faradays, and Tyndalls, have been moved in no small degree by poetic instincts, and that their best scientific work has owed as much to their imagination as to their reasoning and perceptive faculties. And, on the other hand, we find abundant evidence in the vi PREFACE. works especially of modern poets that the truths of science are even more impressive than the more direct and obvious teachings of nature. I have had these considerations in view in preparing the essays in the present volume. On the one hand, I have not been deterred by scientific scruples from presenting the poetic aspect of recent astronomical discoveries, nor, on the other, have I feared lest the recognition of the real significance of these discoveries should do aught but enhance our conceptions of the glory and sple'ndour. of the universe. KICHAKD A. PKOCTOR. NEWCASTLE, NEW SOTJTH WALES : August 30, 1880. CONTENTS. PAGE AGE OF THE SUN AND EARTH 1 THE SUN IN HIS GLORY . . . . . . 42 WHEN THE SEA WAS YOUNG 77 Is THE MOON DEAD ? 144 THE MOON'S MYRIAD SMALL CRATERS . . . .182 A NEW CRATER IN THE MOON ..... 215 A FIERY WORLD 244 THE PLANET or WAR . .... 277 LIVING IN DREAD AND TERROR 305 A RING OF WORLDS 333 EARTH-BORN METEORITES 369 THE ARCHITECTURE OF THE UNIVERSE 407 THE POETRY OF ASTRONOMY. AGE OF THE SUN AND EARTH. RapJiael. The sun, as in the ancient days, 'Mong sister stars in rival song, His destined path observes, obeys, And still in thunder rolls along. Gabriel. The vex'd sea foams waves weep and moan, And chide the rocks with insult hoarse, And wave and rock are hurried on, And suns and stars in endless course. GOETHE. WE have learned how small is our domain in space, but as yet we have scarcely been willing to admit that man's duration in time is as utterly minute, and in a sense insignificant. Yet there is scarcely a feature of our recently acquired knowledge about the relations of the earth in space which has not its parallel in known facts respecting time and the earth's relations thereto ; while the mysteries of space, as yet unfathomed and unfa- thomable, have their analogues in the mysteries which a thoughtful mind recognises in relation to time, as well in the remote past as in futurity. We may here- after consider specially in these pages the parallelisms of time and space. At present we note only that the 2 THE POETRY OF ASTRONOMY. subject we have to deal with illustrates strikingly the manner in which the researches of modern science into space relations lead men to consider also the periods of time during which the objects of their research must have existed in the past and are likely to exist in the future. In the infancy of human thought it was a sufficient explanation of the light and heat of the sun to suppose that a bright and hot body circled around the earth (or rather round the place inhabited by the observer), coming into view each day in the east, and passing over by the south towards the west. Eejoicing as a giant to run his course, never varying in his circuit round the earth, the sun was regarded either as himself a being of power, or else as representing the energy of a higher power, which had set this glowing mass in the sky, and had appointed its courses. But while on the one hand the sun was regarded as a smaller body than the earth, so unquestionably the duration of the sun was regarded as of necessity less than that of the earth. For ages this earth had endured, without form and void, cold and dark, before the sun was appointed to gladden her with his beams; and though the future was not so clear to men's minds, yet it was generally supposed that the end of the earth would not come while the sun and the moon endured. The recognition of the vast superiority of the sun over the earth in size was not attained gradually, as some have asserted, but suddenly. The discovery came on men as a revelation. One generation had believed AGE OF THE SUN AND EARTH, 3 in a central earth, all-important in the universe, as well in space as in time. In the lifetime of the next gene- ration the earth had descended from her high position to become one only and by no means the chief of several small bodies circling round the giant orb of the sun. No longer central in space, she could no longer be regarded reasonably as central in time ; in other words, it was no longer reasonable to suppose that her formation, how- ever brought about, her progress, however long-lasting, and her final end, however attained, either marked the beginning, progress, and end of time, or occupied a central position in all time. We do not find that men were as ready to accept this conclusion as they had been (no choice, indeed, being left them) in accepting the earth's non-central position in space. But the inference was undoubtedly the only reasonable and pro- bable one. The earth's history might no doubt occupy a central position in time, precisely as this day on which we write these lines may be exactly midway between the day when life first began on the earth and the day when life here will finally cease. Yet, while either proposition 'might be true, one is not more wildly im- probable than the other. With regard to the sun, which had now come to be recognised as exceeding the earth more than a million- fold in size, it was an equally reasonable inference that his duration also far surpassed that of the earth. Of course the substance of either might reasonably be regarded as existent during all time ; but the fashioning of the mighty orb ruling over a family of which the B 2 4 THE POETRY OF ASTRONOMY. earth was but a small member, might reasonably be supposed to have belonged to a far more remote epoch than that of the earth ; and his continuance as a sun might as reasonably be supposed likely to outlast, not merely by many centuries, but many times, the continu- ance of our earth as the abode of living creatures. Men had no positive evidence, however, on these points, so long as they considered only the dimensions of the sun and earth. It was natural to suppose or rather it would have been natural, for as a matter of fact the supposition was not entertained that as the duration of mankind far surpasses the duration of a nation, and as the duration of a nation far surpasses that of any individual man, so the duration of the solar system, and therefore of the ruler of the system, must far surpass that of any individual planet. But there was only one way (one general way involving many special methods) of determining whether this was actually the case or not; and the researches of men along this special line of research did not begin till long after the importance of the sun in size had been ascertained. We refer to the inquiry into the processes actually taking place in the earth, in the sun, and in the solar system, and into the evidence respecting the continuance and effects of such processes in the past. Men's ideas on some of these points were almost as vague at the beginning of the present century (nay, even much later) as had been the thoughts of the men of old times respecting the proportions of the heavenly bodies and their orbits. We find Sir W. Herechel, for instance, AGE OF THE SUN AND EARTH. 5 adopting and enforcing a theory respecting the sun's condition, and the emission of solar light and heat, which would not account for one week's supply of such sunlight as we actually receive. Still later we find a man like Dr. Whewell, a skilful mathematician and an able physicist, who also, if not strictly speaking an as- tronomer, was well read in astronomy, maintaining in his ' Plurality of Worlds ' the theory that the fixed stars may be mere lights, not mighty masses like our sun a theory which the modern discovery of the conservation of force shows to be utterly inconsistent with the steady emission of enormous quantities of intensely brilliant light during many thousands of years. But now the student of science recognises in the sun's constant radiation of light and heat the existence of a store of energy which must have been in some way garnered up during long past ages. As certainly as the constant deflection of the earth from the direction in which she is moving at any moment indicates the existence of a force residing in the sun towards which body that deflection constantly takes place, so certainly does the emission of light and heat from the sun indicate the action of processes in the past by which the necessary energy has been stored up. We know that the sun cannot be the habitable orb girt round by phosphorescent cloud- masses imagined by Sir W. Herschel, any more than it can resemble the stars, as imagined by Dr. Whewell, in being a mere light without any considerable mass or substance. The working of a steam-engine does not more certainly indicate the consumption of fuel, and 6 THE POETRY OF ASTRONOMY. therefore the prior gathering together of fuel, than does the sun's radiation of light and heat imply the consumption of solar energy, and therefore the prior gathering together of stores of energy. When this was first recognised, students of solar physics were content to inquire how the observed emission of solar light and heat could be accounted for in such a way as to explain the sun's appreciably unvarying size and mass. They perceived that to regard the sun as a mere mass of burning fuel would by no means suffice. We can measure the quantity of heat that the sun constantly emits, because we can measure the amount received by our earth, [which intercepts about one-2,300,000,000th part of all the light and heat emitted by the sun. We thus find that in every second of time the sun emits as much heat as would result from the combustion of 11,600 billions of tons of coal. In passing, it may be convenient to notice that each portion of the sun's surface as large as our earth emits as much heat per second as would result from the combustion of a billion tons of coal a simple and easily remembered relation. Now it is easily calculated from this that if the sun's whole mass consisted of coa and could burn right out to the last ton, maintaining till then the present rate of emission, the supply would not last more than 5,000 years. As the sun has most certainly been emitting light and heat for a far longer period than this, the idea that the solar fire is thus maintained is of course altogether untenable. There are, however, many other reasons for rejecting the idea AGE OF THE SUN AND EARTH. 7 that the sun is composed of burning matter, using the word * burning ' in its proper sense, according to which a piece of coal in a fire is burning, whereas a piece of red-hot iron is not burning, though burning hot. In like manner we find ourselves compelled to reject the belief that the sun may be a body, raised at some remote epoch to an intense heat throughout its entire mass, and gradually cooling. For we find that in the course of a few thousands of years such a mass would cool far more than the sun has cooled (if he has cooled appre- ciably at all), even within the historic period ; and we have evidence that he has poured his heat on the earth during periods compared with which the duration of the human race is but as a second amid centuries, while the duration of historic races is utterly lost by com- parison. This brings us to the consideration of evidence which has only in quite recent times been brought to bear on the question of the sun's age. We know from records left by men of old times that the sun was in their time very much what he is now, though we cannot be altogether certain that he gave out exactly the same amount of light and heat, or even almost exactly the same. Again, from the remains of animals and plants in the earth's crust we can deduce similar inferences. Those animals and plants could not have existed unless the sun had supplied light and heat as at present, though we cannot assert so con- fidently that he supplied the same amount of either. The possible range of variation may have been greater, 8 THE POETRY OF ASTRONOMY. so far as evidence of this kind is concerned, than in the case where we have human records for our guidance. But there is other evidence which, while less exact still as to the actual emission of light and heat, ranges over periods of time far greater than could be directly inferred from the examination of fossil fauna or flora. As yet we are not able to form satisfactory estimates of the periods of time necessary to bring about such and such changes in the various races of plants and animals ; hence, although we may be quite sure that enormous time-intervals must have elapsed before the races whose remains only are found became changed into the races which are their modern representatives, we cannot definitely assign the duration of these time-intervals, or even at present make the roughest approximation to their length. But there are changes depending on the sun's action whose rate of progress we can satisfactorily measure. We know that processes of change are caused on the earth's surface by the downfall of rain and snow, by the action of frost and ice, of winds and waves, by chemical action, by processes of vegetation, and other causes, all depending on solar activity. Greologists no longer assign the existing irregularities of the earth's crust to causes other than those at present at work, or even suppose that, within the range of time over which their researches extend, causes such as these acted much more actively than they do at present. But it may be noted in passing, that, so far as those causes of change are concerned which depend on solar action, it will not greatly affect the argument now to AGE OF THE SUN AND EARTH. 9 be brought before the reader, whether we consider the activity of such causes to have been widely variable in the past, or to have been appreciably uniform. For it will be seen that the chief difficulty we shall have to encounter resides in the necessity of explaining the total amount of solar radiation in the past. If, in order to shorten the time-intervals indicated by those features of the earth's crust which depend on causes of change due to solar action, we imagine those causes to have once operated far more actively than at present, we necessarily assume that the sun's action was far more intense then than now. We manifestly gain nothing, so far as this special difficulty is concerned, if we have to enhance our conceptions of the solar radiation in the same degree that we reduce our estimate of the time during which his rays have been at work upon the earth. But in reality we are not free to imagine any very noteworthy change in the conditions under which the earth's surface has undergone change during the greater part of the time over which geological researches extend. For there is evidence proving that the progress of changes in the past must have resembled that taking place at the present time. Consider, for instance, the evidence afforded by the various strata which have been deposited at the bottom of the sea. In these strata are the remains of creatures which formerly existed in the sea ; and we find these remains in such a condition in many instances as to prove, beyond all possibility of doubt or question, that, unless those creatures were 10 THE POETRY OF ASTRONOMY. muck more short-lived than their present representa- tives, the average rate of deposition must have closely resembled that now recognised in similar seas. As- Lyell remarks : ' When we see thousands of full-grown shells dispersed everywhere throughout a long series of strata, we cannot doubt that time was required for the multiplication of successive generations; and the evidence of slow accumulation is rendered more striking from the proofs, so often discovered, of fossil bodies having lain for a time on the floor of ocean after death before they were imbedded in sediment. Nothing, for example, is more common than to see fossil oysters in clay, with serpulae, or barnacles (acorn-shells), or corals, and other creatures, attached to the inside of the valves, so that the mollusk was certainly not buried in mud at the moment it died. There must have been an interval during which it was still surrounded with clear water, when the creatures whose remains now adhere to it grew to a mature state.' Nay, there are cases where we have evidence of still slower deposition than could be thus inferred. For we often find that the creature which has attached itself to the shells of defunct mollusks have not only grown to maturity before the shells were covered with deposited matter, but have in their turn died and their hard coverings have been slowly pierced by other creatures, while still the deposit had not covered the shell of the mollusk to half its thickness. It may appear at first sight that evidence about the rate of deposition of mud at the bottom of the sea does AGE OF THE SUN AND EARTH. II not bear very obviously on the question of the sun's radiation of light and heat. But it must be remembered that this deposition of matter measures the rate at which matter has been carried away from the earth's surface above the sea-level; while the rate at which this process or what is called ' sub-aerial denudation' takes place, depends on the downfall of rain and snow, the action of wind and storms, and other causes depend- ing on the energy of the solar rays. We may turn, then, with sufficient confidence to the evidence which the earth's crust affords respecting the time during which the solar radiation has continued. We certainly are not likely to obtain an estimate in excess of the truth, apart altogether from the considera- tion that there may have been, and most probably were, enormous periods of time during which the sun's rays were poured on the earth without producing any effects which can now be recognised, and most probably still more enormous periods before the earth had a crust at all, when the solar radiation was already intensely active. The evidence derived from the earth's crust, however, will be found sufficiently striking, without our entering into the consideration of possibilities relating to pre- ceding eras. ' When we reflect,' says Mr. James Croll, whose researches into this and related subjects are full of interest, ' that with such extreme slowness do these agents ' (rain, snow, ice, running water, &c.) ' perform their work, that we might, if we could, watch their operations from year to year, and from century to 12 THE POETRY OF ASTRONOMY. century, without being able to perceive that they make any sensible impression, we are necessitated to conclude that geological periods must be enormous.' Let us follow Mr. Croll in his consideration of a few of the many facts bearing on this point. (Much that immediately follows here is simply translated into popular language from a very interesting article by Mr. Croll in the ' Quarterly Journal of Science ' for July 1877.) It is well known that many parts of the earth's surface which now show no marked inequalities were formerly the scene of great dislocations (not necessarily produced suddenly), when the surface on one side of the line of dislocation had been depressed hundreds or even thousands of feet below the surface on the other side of that line. On the present surface no signs of these tremendous displacements (whether produced by up- heaval or by sinking, or by both) can now in general be recognised, the inequalities having been all removed by denudation. But to effect this levelling, a mass of rock must have been removed equal in thickness to the extent of the dislocation. If we can ascertain the full depth of the stratum thus removed, and also the average rate at which denudation takes place, we shall have a measure of the length of time required for the levelling process. Only, at the outset, we must remember, first, that an estimate thus formed is likely to fall far short of the truth, even as respects the particular process involved ; and, secondly, that that process is in itself but one in a series of such processes. We learn from a fault, as a AGE OF THE SUN AND EARTH. 13 dislocation of this kind is called, how much more has been denuded on one side than on the other to restore the level; but not how much has also been taken from both sides. Again, where a fault of this kind occurs, the strata which have undergone the process of dislocation are commonly themselves the products of denudation from other surfaces existing, of course, long before the dislocation occurred. And these surfaces in their turn were probably the results of slow processes of deposition of matter denuded from still earlier surfaces. To consider, however, a few examples of extensive faults. Professor Eamsay, describing some of the remark- able faults in North Wales, states that near Snowdon there is a fault where the displacement of the strata amounts to 5,000 feet, and in the Berwyn Hills one of 5,000 feet ; in the Aran range occurs the Bala fault, with a downthrow of 7,000 feet. Between Aran Mowddwy and Careg Aderyn the displacement is be- tween 1 0,000 and 1 1 ,000 feet. ' Here we have evidence,' says Mr. Croll, ' that a mass of rock, varying from one mile to two miles in vertical thickness, must have been denuded in many places from the surface of the country in North Wales.' ' Along the flank of the Grampians a great fault runs from the North Sea at Stonehaven to the estuary of the Clyde, throwing the old red sandstone on end sometimes for a distance of two miles from the line of dislocation.' Professor Greikie concludes that the 14 THE POETRY OF ASTRONOMY. amount of displacement must be in some places not less than 5,000 feet. But perhaps the most remarkable instance known is that of ^the great fault which crosses Scotland from near Dunbar to the Ayrshire coast. On the south side of this fault we find the ancient silurian rocks, north of it the less ancient rocks, the old red sandstone and car- boniferous of North Scotland. 1 The amount of dis- location is in some places fully 15,000 feet, or nearly three miles. Now, it is to be observed that the dis- location is older than the carboniferous era. For originally the silurian rocks south of the fault must have been covered by the prolongation of the old red sandstone, afterwards completely removed by denuda- tion. If the carboniferous strata had then existed, they, lying uppermost, would, of course, have been washed away first. But we find them on the south side of the fault, lying immediately on the old silurian floor, the old red sandstone which originally covered that floor having been entirely removed. Thus the ' enormous thickness of nearly three miles of old red sandstone must have been denuded away during the period which intervened between ' its deposition and the subsequent accumulation of the carboniferous lime- 1 It is absolutely necessary here, and in what follows, to use these technical geological terms. For the subject of our present inquiry it will suffice to say that the carboniferous rocks are later than the old red sandstone (at least in any given geological district), the ojd red sandstone later than the silurian, while the Laurentian rocks, mentioned further on, are older yet than the silurian. Of course the oldest rocks lie lowest. AGE OF THE SUN AND EARTH. 15 stone and the coal measures now lying directly on the silurian rocks ! One other case to indicate the enormous periods required for the formation of some of the features of Scottish scenery. Professor Greikie has shown that ' the Pentlands must at one time have been covered with upwards of a mile in thickness of carboniferous rocks, which have all been removed by denudation.' ' Now,' says Mr. Croll, 4 the Pentlands themselves, it can be proved, existed as hills in much their present form before the carboni- ferous rocks were laid down over them ; and as they are -of lower old red sandstone age, and have been formed by denudation, they ,must consequently have been carved out of the solid rock between the period of the old red sandstone and the beginning of the carboni- ferous age.' But, in order fully to appreciate the vastness of the periods L required for these and kindred changes, it is necessary to recognise the extreme slowness with which such changes proceed. The first calculations directed to the solution of this difficult problem were those made by Manfredi in 1736. In 1802 Playfair took up the inquiry. But the materials available at that time were so imperfect that these earlier calculations were not satisfactory. In 1850 Tylor, from a careful investigation of the evi- dence respecting the quantity of matter brought down by rivers into the sea, deduced the conclusion that 10,000 years would be required to raise the sea level 16 THE POETRY OF ASTRONOMY. by three inches. More recently Mr. Croll, from the latest measurement of the sediment transported by European and American rivers, calculated the rate at which the surface of the land is being denuded. ' The conclusion arrived at in his able memoir,' says Sir Charles Lyell, ' was that the whole terrestrial surface is denuded at the rate of one foot in 6,000 years ; and this opinion was simultaneously enforced by his fellow- labourer, Mr. Greikie.' This was in 1868. It may be well, before considering the bearing of these researches on the subject presently before us the obliteration of the effects of dislocations in the earth's crust to quote the opinion of Sir Charles Lyell on this method of dealing with the general problem of terrestrial denudation. ' It is evident,' he says, ' that when we know the dimensions of the area which is drained, and the annual quantity of earthy matter taken from it and borne into the sea, we can affirm how much on an average has been removed from the general surface in one year ; and there seems no danger of our overrating the mean rate of waste by selecting ' (as Mr. Croll and Greikie had done) * the Mississippi as our example. For that river drains a country equal to more than half the continent of Europe, extends through twenty degrees of latitude, and therefore through regions enjoying a great variety of climate; and some of its tributaries descend from mountains of great height. The Mississippi is also more likely to afford us a fair test of ordinary denudation, because, unlike the St. Lawrence and its tributaries, there are AGE OF THE SUN AND EARTH. I/ no great lakes in which the fluviatile sediment is thrown down and arrested on its way to the sea. In striking a general average we have to remember that there are large deserts in which there is scarcely any rainfall, and tracts which are as rainless as parts of Peru ; and these must not be neglected as counterbalancing others in the tropics where the quantity of rain is in excess. If then, argues Mr. Ofeikie, we assume that the Mis- sissippi is lowering the surface of the great basin which it drains at the rate of one foot in 6,000 years, 10 feet in 60,000 years, 100 feet in 600,000 years, and 1,000 feet in 6,000,000 years, it would not require more than about 4,500,000 years to wear away the whole of the North American continent if its mean height is cor- rectly estimated by Humboldt at 748 feet ; and if the mean height of all the land now above the sea through- out the globe is 1 ,000 feet, as some geographers believe, it would only require 6,000,000 years to subject a mass of rock equal in volume to the whole of the land to the action of sub-aerial denudation. It may be objected that the annual waste is partial, and not equally derived from the general level of the country, inasmuch as plains, watersheds, and level ground at all heights remain comparatively unaltered ; but this, as Mr. Geikie has well pointed out, does not affect our esti- mate of the sum total of denudation. The amount remains the same ; and if we allow too little for the loss from the surface of table-lands, we only increase the proportion of the loss sustained by the sides and bottoms of the valleys, and vice versa' c 1 8 THE POETRY OF ASTRONOMY. We may note, in passing, that, adopting the esti- mated rate of denudation here indicated, the actual time required for the entire submergence of the present continents, if no vulcanian forces were at work to pre- vent submergence, would not necessarily be even ap- proximately represented by the period of 6,000,000 years mentioned above. At the outset the rate of sub- mergence would be greater than the mere rate of denu- dation, since every foot removed from the surface of the continents would cause a rise of about 4| inches in the level of the sea ; so that at first the surface of continents would be lowered on the average not one foot only in 6,000 years, with respect to the sea-level, but 1 foot 4| inches. On the other hand, as the con- tinents became greatly reduced in extent, it is probable that the average annual rate of denudation would be diminished, the portions still remaining above the sea- level being of harder and more durable material than those which had been removed. We need not inquire further, however, into the question here raised, which, though suggested by our subject, does not, strictly speaking, belong to it ; moreover, in nature the process considered cannot take place, the earth's internal forces constantly restoring the balance between land and water by the upheaval of submerged regions. For the purpose of our present inquiry the action of the earth's vulcanian energies need not be con- sidered ; because we are concerned only with the ques- tion how long would be the period of time required for the removal of a stratum so many hundreds or thou- AGE OF THE SUN AND EARTH. 19 sands of feet in thickness. We know, certainly, that, in the special cases we have to deal with, strata of such and such thickness were removed ; and it matters little whether, as the process of removal went on, they were being steadily raised by the earth's subterranean action, or whether the original dislocation was followed by the sudden raising of the strata at one side of the fault and their equally sudden lowering on the other side. How- ever the difference was brought about, it is certain that the raised strata were worn down eventually by the steady action of the same causes which wear down the general surface of the large continents. Having ascer- tained the average rate at which these causes work, we can apply the result to determine how long they would be in producing the observed levelling down of the up- raised strata in faults. There is no reason for supposing that in the remote past the process would go on more quickly than at the present time. And we have seen that even if it did, that would imply a greater activity in the solar energies to which these processes are all in reality due, so that our difficulty would be in no way diminished by any such assumption. The time re- quired would be reduced by a few millions of years perhaps ; but the difficulty we are dealing with is not a question of time at all. We are inquiring now into the amount of the total expenditure of solar energies in past ages ; and the time-intervals indicated by the earth's crust are only of importance in so far as they show how vast that expenditure of energy has been. Doubtless, in considering other questions, the length of c 2 20 THE POETRY OF ASTRONOMY. these time-intervals is a question of great interest, but it does not directly concern us here. Let us, however, follow Mr. Croll in recognising the possibility that, in some of the cases we have to deal with, the rate of denudation may have been greater than the average rate inferred from the consideration of river drainage. To prevent the possibility of over-estimating the periods necessary to effect the observed denudation, let us assume the rate to have been double the average rate, or equal to one foot in 3,000 years. At this rate a thickness of three miles which (at the very least) has been swept away in South Scotland since the old red sandstone period would require 45 million years ! But, older than the old red sandstone rocks, the silurian formations have been denuded in places to depths of thousands of feet before the old red sand- stone strata were deposited. And these ancient forma- tions were themselves deposited in the ocean by the slow denudation of the Cambrian rocks. These in turn had been formed from the earlier Lauren tian strata. And lastly (so far as the researches of geologists at present extend), the Laurentian rocks themselves were built up from the ruins of other rocks which were themselves sedimentary rocks, not the actual primary rocks of our globe. We should almost certainly under- estimate the period required for these processes of denudation preceding the old red sandstone era, if we assumed that it was only equal in length to the period which has elapsed since that era. But making this AGE OF THE SUN AND EARTH. 21 assumption, and assuming also (which is also almost cer- tainly an under-estimate) that the interval which has elapsed since the old red sandstone era is 45 million years, we find a total of 90 million years for the strati- fied rocks. In other words, we find at least 90 million years for the period during which rain has fallen on the earth as at present. During that time, therefore, the sun has poured his rays upon the ocean, raising up their waters by evaporation to be carried by winds (also generated by the sun) over the continents, and there discharged in the form of rain and snow. It may be noticed, in passing, that Sir William Thomson infers, from the observed underground tem- perature of our earth, that the consolidation of the crust cannot have taken place less than 20 million years ago, or the underground temperature would have been greater than it is ; nor more than 400 million years ago, or the underground temperature would have been less than it is. The limits are rather wide ; but a value well within these limits would accord with Mr. Croll's estimate of 90 million years as the interval since the earliest strati- fied rocks were deposited. Now, the difficulty thus raised is this : At present we know of no way in which the sun could have emitted the same amount of heat as at present for anything like this period of 90 million years, without having shrunk to much smaller dimensions than he at present has. It is generally admitted by physicists and astrono- mers that the solar heat has had its origin in the main, almost wholly in fact, in processes of contraction ; and 22 THE POETRY OF ASTRONOMY. that it is maintained by such processes. In other words, the gravitation of the sun's mass has given birth to all, or very nearly all, the heat which the sun has emitted in the past, and will continue to emit till the end of his career as a sun. It was once supposed that meteoric downfall on the sun's surface produced the chief share of the solar heat. The idea has now been generally abandoned for reasons into which we need not here enter. But, practically, it is of no importance whether we consider the sun's heat to have been generated by the downfall of masses on his surface (continually fed by such downfall) or by the gradual contraction of the entire mass now constituting his globe, till it had assumed its present dimensions. This is the accepted form of the gravitation theory of the solar heat. But manifestly, the greatest possible amount of heat which could have been generated in this way would be that produced by the contraction of a great nebulous mass containing all the sun's present substance, from an original extension throughout an infinitely large space to the present dimensions of the sun. It might be supposed, perhaps, that the result of such a process of contraction would be the generation of an infinite amount of heat. But in reality this is not the case, any more than it is the case that a meteoric mass allowed to fall from an infinite distance upon the sun would strike his surface with infinite velocity (after a journey lasting an infinite time which, however, is a mere detail). We know, on the contrary, the rate at which such a mass would strike the sun namely, about AGE OF THE SUN AND EARTH. 2$ 360 miles per second. And precisely as we can calcu- late the velocity of such a mass after being subjected to the sun's pull over an infinitely long journey, so we can calculate the total amount of heat which would result from the contraction of the sun's mass to its present dimensions from a former extension throughout infinite space. We find that it corresponds only to about 20 million years' supply at his present rate of emission. Thus, while the earth seems to tell us that the sun has been pouring his rays upon her at the same rate as at present during at least 90 million years, the sun seems to tell us that he has not been pouring out heat at that rate for more than 20 million years. Even if we reject the earth's evidence, or if we endeavour to show that the rainfalls by which the earth's surface has been again and again denuded were not always due to solar heat, but may have been gene- rated by the earth's own heat, we scarcely find our difficulty removed. For it seems utterly absurd to suppose that the mighty central orb of the solar system only attained its present activity during the compara- tively recent years of the history of our earth, one of the smaller and shorter-lived members of the sun's family. Sir W. Thomson has shown, by the most satis- factory of the three methods he employed to shorten the estimates formed respecting the earth's duration, that more than 20 million years must have elapsed since her crust was formed a time which certainly followed by many millions of years the actual genesis of the earth as a gaseous mass. Many physicists reject 24 THE POETRY OF ASTRONOMY. even the 400 million years given by Thomson as the superior limit, doubting whether the formulae and data he employed could be relied upon as confidently as the various processes of mathematical calculation which he applied to them. But even if we accept his minimum result certainly the very least which science will per- mit us to accept it would still follow that the sun's present emission of light and heat could not have con- tinued throughout the time of our earth's existence as a planet ; if the sun's heat had its origin entirely or chiefly in those processes of contraction combined with meteoric indraught in which astronomers and physicists at present believe, and if ike space into which the sun's mass has contracted is really that which the sun we see appears to occupy. Mr. Croll, who passes over the latter consideration with the remark that if the sun's density increases towards the centre the supply of solar heat might be somewhat greater, suggests, as the true explanation of the difficulty, that the sun may have derived a portion of his energies in another way than merely through the process of contraction. ' In proving that the antiquity of our habitable globe may be far greater than 20 or 30 million years, we prove,' he says, ' that there must have been some other source in addition to gravity from which the sun derived his store of energy.' He goes back to the initial state of things conceived by Laplace in presenting what is usually called the nebular hypothesis of the solar system. According to this, the whole of the solar system was formerly a great AGE OF THE SUN AND EARTH. 2$ gaseous mass ; but whether cold or hot Laplace did not say. As Helmholtz remarks, ' The chemical forces must have been present, and ready to act ; but, as these forces could only come into operation by the most in- timate contact of the different masses, condensation must have taken place before the play of chemical forces began : whether a still further supply of force in the shape of heat was present at the commencement we do not know.' Mr. Croll, who regards the chemical forces as equivalent only to a few thousand years' supply of heat, and, therefore, as comparatively in- significant, thinks we may safely infer that the original nebulous mass was intensely heated, and that in such intense heat we may find the explanation of the problem before us. ' It is evident,' he says, ' that if we admit that the nebulous mass was in a state of incandescence prior to condensation, it will really be difficult to fix any limit either to the age of the sun, or to the amount of heat which it may have originally possessed. The 20 million years' heat obtained by condensation may in such a case be but a small fraction of the total quantity possessed by the mass.' But then the question arises, Whence did the nebu- lous mass derive its heat ? Mr. Croll considers that we may find a satisfactory answer to this question in the assumption that the nebulous mass was formed by the collision of two bodies, each of half the mass of the sun, rushing full tilt upon each other with a velocity of nearly 500 miles per second. Their concussion would generate enough heat to last more than 50 millions of 26 THE POETRY OF ASTRONOMY. years, which we should have to add to the 20 millions of years provided for by the subsequent condensation of the mass. He asks : ' Why may not the sun have been composed of two such bodies ? and why may not the original store of heat possessed by him have all been derived from the concussion of these two bodies ? Two such bodies coming into collision with that velocity would be dissipated into vapour and converted into a nebulous mass by such an inconceivable amount of heat as would thus be generated ; and when condensation on cooling took place, a spherical mass like that of the sun would result.' It will be asked, Mr. Croll says (and certainly it seems likely), 'Where did the two bodies get their velocity ? ' It may as well be asked, he answers, ' Where did they get their existence ? It is just as easy to conceive that they always existed in motion as to conceive that they always existed at rest.' At first sight it might seem a fair rejoinder to this to say that, if we are free to assign these enormous velocities to bodies in space, we must be free also to assign to them other properties such as matter can possess heat, for instance : so that we might solve our problem at once by saying that the nebulous mass was originally supplied with enough heat to last fifty, a hundred, or a thousand millions of years. But there is a difference between motion and heat. Masses of matter might be rushing hither and thither through space for ever, without change, except when collisions occurred ; whereas masses intensely hot must radiate their heat .away. So that AGE OF THE SUN AND EARTH. 2J while we can, as Mr. Croll truly says, conceive the existence of bodies in motion for any length of time we please, we cannot conceive the constant existence of an intensely-heated nebulous or other mass. It must lose heat ; whereas the bodies rushing about through space need not lose motion, and certainly would not do so unless they came into collision. Nor is Mr. CrolPs position affected by the argument that neither our own sun nor the other suns which people space are rushing about with anything like these velocities of four, five, or six hundred miles per second. For all the stars are glowing with intense light and heat, and therefore must be regarded as bodies which, like our sun (according to this theory), have been formed by mighty collisions, in which their motion was converted into light and heat. The stars, therefore, are bodies which have already lost the greater part of their original velocities, and the comparatively small velocities left them are precisely what, according to this theory, we might expect. Yet, while an answer may be found to some of the more obvious arguments against this startling theory, it must be admitted that the theory remains surrounded by difficulties of an almost insuperable nature. Without entering into calculations which would be out of place in these pages, we may state that the imagined collisions of bodies rushing hither and thither, even with the enormous velocities suggested, tlirough stellar space, would resemble in frequency, or rather in paucity, collisions between bullets in an engagement 28 THE POETRY OF ASTRONOMY. between two very widely scattered parties of skirmishers. At a rate of 500 miles per second (possessed by eacti), two bodies as far apart as our own sun and his nearest neighbour among the stars, would meet each other (if their motion were suitably directed) in about 700 years. Supposing a million stars, scattered as stars are now scattered, were to rush in a flight to meet a million stars similarly scattered, at the rate just mentioned, a million years or so would elapse before the two flights had rushed through each other, and the chances would be many millions to one against even a single collision occurring. Such bodies would have to be strewn far more densely through space than the stars are to make it probable that among several millions of them one collision would occur in a million years. As the supply of light and heat resulting from each collision would not last more than 50 or 60 millions of years on the average, only fifty or sixty stars would be visible at any given moment among all those millions of bodies. So that for each star shining in that region of space (and the same reasoning applies to the whole of the stellar universe) there would be millions of dark bodies. Of these a certain proportion, probably very small, would consist of orbs which had undergone collision, had shone for 50, 60, or say 100 millions of years, and were now dead suns. The rest of the dark bodies, out- numbering the visible stars millions of times, would be bodies which had not yet encountered others after the fashion which the theory requires. These would be dangerous fellows. They might at any time come into AGE OF THE SUN AND EARTH. 29 collision either with each other, making new suns, or with suns already glowing, making these suns glow a great deal more brightly, and destroying the inhabi- tants of any worlds circling around them. More- over we ought, in the course of comparatively short periods, to see such new stars suddenly begin their ex- istence as vaporous masses glowing with an intensely bright light. Now, nothing in the least corresponding to the process of sun-formation required by this theory has ever been observed. The so-called new stars are not at all what the theory requires. They have shone with intense brightness for a few months at the out- side, and have then died out ; but according to the theory we require stars which shall burn with steady fires for many millions of years. Now, we might reasonably expect that for some short time following its first for- mation, a new sun would shine much more brightly than afterwards. Mr. Croll, indeed, supposes otherwise, his line of argument as to new stars (presently to be noticed) assuming that after a collision a star would immediately begin the steady emission of light and heat at about the rate at which it would continue to emit them afterwards : but a collision in which the supply of heat and light for 100 millions of years was generated in a moment would unquestionably produce also a great temporary outburst. Those new stars, however, which astronomers have been able to observe since the spectroscope was invented, have not behaved in the required manner. (As a friend of ours is apt to say when observation does not accord with theory, 30 THE POETRY OF ASTRONOMY. 4 They didn't know, poor things, what they were expected to do.') One was found to be a star already recorded in star-maps, and has faded to its original lustre ; the other, after shining for awhile as a bright star, has faded into a faint nebula or star-cloud. Mr. Croll reasons thus as to the probable number of new stars which would be formed according to his theory : ' The formation of a sun by collision is an event that would not be likely to escape observation if it occurred within the limits of visibility in space. But such an event must be of very rare occurrence, or the number of stars visible would be far greater than it is. The number of stars registered down to the seventh magnitude inclusive is, according to Herschel, some- where between 12,000 and 15,000, and this is all that can possibly be seen by the naked eye. Now, if we suppose each of them to shine like our sun for (say) 100 million years, then one formed in every 7,000 or 8,000 years would maintain the present number un- diminished. But this is the number included in both hemispheres, so that the occurrence of an event of such unparalleled splendour and magnificence as the formation of a star, or rather nebula for this would be the form first assumed is what can only be expected to be seen in our hemisphere once in about 15,000 years. The absence of any historical record of such an event having ever occurred can, therefore, be no evi- dence whatever against the theory.' If, however, as may most reasonably be assumed, the formation of a sun in this way would be in the first instance accoin- AGE OF THE SUN AM) EARTH. 31 panied by a most tremendous outburst of light and heat, far exceeding that which the body in its ultimate condition as a sun would emit, then we should be able to recognise the formation of any such sun within the region of space over which our telescopes range ; and in that region of space there are more like a hundred million than twelve or fifteen thousand stars. An out- burst ought to be recognised, on the average, about once a year ; and certainly new suns are not entering on the first stage of their existence at this rate. Moreover, apart from what we have mentioned above as to the duration of so-called new stars, what is known about one at least of the two new stars which have appeared during the last twelve years, by no means accords with what we should expect if the outburst were caused by the collision of two other suns, or of two dark masses rushing along at the rate of four or five hundred miles per second. One of them was found not to be a new star at all. It was simply a tenth- magnitude star which had suddenly acquired the brightness of a second-magnitude star. It rapidly lost its new lustre, returning to the brightness which it had had before the outburst. The other the new star which appeared in the constellation Cygnus in November 1876 did behave in a manner reconcilable in some degree with Mr. CrolPs theory. For, while no star had been known to exist where this star suddenly appeared, the new star, after shining for awhile with light resembling in character that emitted by other stars, gradually, as it lost its light, assumed a nebulous 32 THE POETRY OF ASTRONOMY. character, and is at this moment shining with light of precisely the same kind as is emitted by the gaseous masses called (from their appearance) planetary nebulae. It would be rather rash, however, to assume that here was a case where two orbs rushing through space had encountered full tilt, and after a certain time, during which the heat excited by their collision had been re- ducing their substance to vapour, the entire mass had become a nebula. If we are to suppose that dark, hard masses produce suns by their collision, we enormously increase the chances against collision, because we enormously reduce the dimensions of the bodies supposed to be travelling through space. Re- turning to our illustration from a battle-field, it is as though each bullet were reduced in size to the thou- sandth part of the smallest of small shot. There is, moreover, this inherent difficulty in the theory thus presented, that if the heat resulting from collision vapourises the entire mass, making a mighty nebula out of which in the course of many millions of years a solar system is to form, by far the greater part of this heat will be radiated away into space while the nebula is passing through the mere beginning of the process of contraction, and ages before a single member of the future solar system has assumed the form of a habitable world. The total amount of energy corres- ponding to the collision, if it could all be kept in stock, so to speak, till the time that the , members of the system were fully fashioned, might suffice for as many millions of years as we find that our earth has actually AGE OF THE SUN AND EARTH. 33 been exposed to the rays of the sun. But there is no conceivable way in which the supply could thus be reserved till it was wanted. While a nebulous mass was contracting, it would be expending most of the heat equivalent to each successive stage of contraction. Of course, as regards the contraction due to cooling that is, to the emission of heat every part of such con- traction would be exactly compensated by loss of heat. But the contraction due to gravitation, the only part of the process of contraction by which heat would in any sense be generated, would cause from the beginning a steady emission of heat ; and whether the total rate of such emission were greater or less in the earlier stages than now, it is certain that the duration of those earlier stages would enormously exceed say, rather, would exceed many hundreds of times the period which has elapsed since first rain fell upon this earth, or winds blew over its surface. 1 It appears to us that the true explanation of the difficulty (the first full recognition of which we owe to Mr. Croll) must be sought elsewhere. Apart from the fatal objection considered in the last paragraph, a 1 This paragraph was already written when we received from Professor Kirkwood of Bloomington, Indiana, U.S. (one of the most ingenious and original astronomers of the day), a paper in which he presents the same general argument. The conclusion at which he arrives is that much the greater part of the supply of heat 'must have been radiated into space before the planets were separated from the solar mass, and consequently that the amount of geological time cannot to any great extent have exceeded the limits indicated by the researches of Sir W. Thomson.' The latter inference, as will be seen, does net appear to us to be made out ; but the former seems unquestionably correct. D 34 THE POETRY OF ASTRONOMY. theory involving the genesis of all the millions of ex- istent stars from accidental collisions among millions of millions (for fewer would not suffice) of dark masses, constantly rushing through space at the rate of many hundreds of miles per second, is not one which can find acceptance among those who are acquainted with the actual present position of stellar research. But the difficulty indicated by Mr. Croll remains to be en- countered. Somewhere the premisses must be wrong which lead to an erroneous conclusion. Now, we are not disposed to question the validity of the reasoning which Mr. Croll and other geologists have based on the condition of the earth's crust. The only way of diminishing our estimate of the time- interval necessary for the stratification of the earth is to assume (as we find Professor Kirkwood does) that in former ages the stratification proceeded more rapidly than at present. But, as we have already seen, this amounts really to the assumption that in former ages the sun exerted a more powerful action upon the earth than at present ; and we are in no way helped, because it is the totality of the sun's action on the earth with which we are in reality alone concerned. We revert, then, to the original proposition of the difficulty, to see whether there may not be any other way of escape. It appears that if the sun has con- tracted into his present dimensions from a nebula originally extending far beyond the orbit of Neptune, the supply of solar heat would not have lasted any- thing near the time during which we know, from the AGE OF THE SUN AND EARTH. 35 study of our own earth, that the supply has lasted. We have assumed all along that the sun's dimensions are those which the sun actually presents to the eye. May not our mistake lie here ? May not the sun or, rather, the chief portion of his mass have contracted in reality to far smaller dimensions than he appears to possess ? Not many years ago, a question of this sort would have appeared altogether fanciful. But facts have been ascertained in the last few years which have greatly altered our ideas respecting the sun. It is quite certain that the sun we see is not the whole sun. It is, in a sense, a mere accident that we see the sun as he actually appears. If our eyesight were of a somewhat different quality, we should see the sierra which surrounds the entire globe of the sun to a depth of five or six thousand miles ; thus we should see a much larger sun. With a yet further change of visual power we should recognise the inner corona, and the sun would appear yet larger. And we can quite readily conceive the possibility of the outer corona being discerned; in which case the sun would not merely appear larger, but many times larger than he is at present. It would, indeed, be possible to see the sun thus enormously enlarged without any change in our visual powers, if our standpoint were somewhat altered and (a slight but necessary detail) if we could exist under the new conditions. From the moon's surface, an observer possessing visual powers such as ours, and capable of existing without air or water, would see all those solar appendages which are con- D 2 36 THE POETRY OF ASTRONOMY. cealed from our view (except during total solar eclipse) by a veil of sunlit air. Now, precisely as it is conceivable that by a change in our visual powers or in the conditions under which we observe the sun, we might see him occupying (as he really does, for the corona is a part of him) a region of space many times larger than that occupied by the sun we see, so it is conceivable that the sun we see occupies a region of space many times larger than that occupied by the true mass of the sun. In the same sense in which we say now that the sun's volume is that indicated by the visible surface of the sun, be- cause the mass of all which lies outside that surface is as nothing compared with the mass which lies within it, it may well be that the true globe of the sun lies far within the glowing surface we see, the entire mass of matter lying outside such much smaller true globe being insignificant compared with the mass forming that globe. This is not a mere hypothesis, devised to meet the difficulty indicated by Mr. Croll. That it does meet that difficulty will be obvious if we consider that the difficulty depends entirely on the observed present largeness of the sun's diameter. If the diameter were one-half its supposed length, the estimated duration of the emission of heat would be doubled ; if the diameter were one-third its supposed length, the duration of the emission would be trebled ; and so on. The density of the solar globe would be increased in much greater degree. With a diameter reduced one-half, the density AGE OF THE SUN AND EARTH. 37 would be increased eightfold; while if the diameter were reduced to one-third its present (seeming) length, the density would be increased twenty-seven times. Now, whether it be permissible to assume that the sun's globe could have a mean density many times greater than that usually assigned to it, there can be no manner of doubt that this supposed mean density is very much less than the known conditions under which the sun's mass exists would lead us to expect. The mean density of the sun is only one-fourth the mean density of our earth, while the pressures existing in the sun's interior are thousands of times greater than those inside our earth. True, the sun's temperature is enormous, and thus an expansive power exists throughout the sun's mass which would readily overcome such contractile forces as exist within the earth's frame. But the pressures produced within the sun by gravity are so tremendous that the elastic forces of the gaseous ma- terials of the sun's globe must be quite incompetent to resist the contractile tendency. The proof that this is so is found in the constant emission of solar heat, which represents in reality the yielding of the solar mass to the influence of its own gravitating energies. We approach here the consideration of relations such as we are entirely unable to understand or even conceive. No experimental researches we can make can throw any trustworthy light on the condition of the sun's interior, where pressures far surpassing any we are familiar with contend with temperatures equally surpassing the fiercest heat known to us on earth. It is probable that 38 THE POETRY OF ASTRONOMY. the entire mass of the sun, whatever its real extension, is gaseous ; for the heat of all the materials of that mass is greater than the critical temperature of the densest elements that temperature at which no pressure, how- ever great, would liquefy or solidify them. If at this tremendous temperature and at the enormous pressures to which they are exposed the constituents of the solar globe were perfect gases, there would be no limit to the density they would attain in the sun's interior. But we have every reason to believe that after a certain density had been attained under pressure, these gases would no longer behave as perfect gases, their density increasing with pressure. And we find it difficult to imagine that gaseous matter could under any pressure, however great, acquire a density exceeding many times that of the elements we chiefly see in the solid form. Yet it would be unsafe to assume any limits to the density which might be attained under constantly increasing pressure by matter maintained always at so tremendous a tem- perature that it was prevented from becoming liquid or solid. If we inquire what seems suggested by the actual available evidence respecting the sun's condition, inside that glowing globular surface which conceals from us all that lies within, we find reason to believe that the sun's interior is thus enormously compressed. It can readily be shown that if the sun's mass is not thus compressed, then, rotating at the observed rate, his globe should be flattened to an extent which should be recognisable by the best methods of modern measurement. The flatten- AGE OF THE SUN AND EARTH. 39 ing, be it understood, would still be very small. It might even escape observation, so small would it be ; but the probability is that it would have been detected. On the other hand, if the sun's interior is exceedingly dense, then the flattening of his globe would certainly not be observable. Since, as a matter of fact, no flatten- ing has been observed, the probability is that the sun is enormously compressed near the centre. It must be admitted that this part of the evidence is not very strong ; but, such as it is, it bears in the direction indicated Strangely enough, we derive from a different orb the strongest evidence on this particular point. Jupiter's mean density is the same as the sun's, if we take the visible disc of Jupiter as indicating the true size of the planet. Now it has been shown by Mr. George Darwin (from a careful comparison of the motions of Jupiter's moons with those calculated on the assumption that Jupiter's mass is not greatly compressed at the centre) that Jupiter must be very much denser at the centre than near the visible surface of his globe. This agrees with all that is known respecting that planet. We have pointed out, on former occasions, in these pages, how utterly impossible it is to explain the phe- nomena presented by the giant planet, on the assump- tion that the disc we see and measure is the true globe of Jupiter. Mr. Darwin's reasoning proves in another way that this globe lies far within the apparent outline of the planet, which in reality represents probably the region where lie the feathery clouds forming his outer- 40 THE POETRY OF ASTRONOMY. most cloud-layer. Within it lie other cloud-layers, and an atmosphere of exceeding depth. Nay, it is probable that the greater part, if not the entire mass, of each of the planets Jupiter and Saturn exists at so intense a heat (though the cloud-envelopes we see are not intensely hot), that solidification and liquefaction are impossible at any pressure, however great. In this case the density of the internal parts of these planets, as of the internal parts of the sun, would be due to the vast- ness of the pressures exerted upon the nuclear regions. Without insisting on this, let it simply be noted that in the case of Jupiter and Saturn it has been to all intents and purposes demonstrated that the condensa- tion of the planet's mass is very much greater than we should infer from the apparent dimensions of the planet's globe. Since these planets are probably intermediate in condition, as they are in size, between our earth and the sun, we find another reason for inferring that the nuclear parts of the sun are exceedingly dense. If so, the difficulty which Mr. Croll has sought to deal with by imagining that not our sun only, but every sun peopling space, has been produced by the collision of formerly dark masses rushing hither and thither with inconceivable velocities, would no longer exist. One circumstance, however, remains to be noticed. We have endeavoured to explain the apparent age of our earth's strata by an assumption which in reality implies that the sun is a great deal older than he had been supposed to be. Not merely does our hypothesis require that he should be regarded as a great deal older, AGE OF THE SUN AND EARTH. 41 but, as it has not directly enhanced our estimate of his possible total duration, it assumes in fact that he is many millions of years nearer to his end as a living sun (so to speak) than has been commonly supposed. The process of contraction, on which his vitality as a sun depends, has gone on much farther, if our theory be sound, than if we suppose the globe of the sun, as we see it, to be of uniform or nearly uniform density throughout. But it does not seem to us that the estimate of the sun's duration which would result from our theory, would fall short of that which astronomers had formed on the hypothesis that the sun is of uniform density. (We call our view a theory, because it is based on observed facts ; the usual view an hypothesis, because no one has ever ventured to assert that any facts indicate its correctness.) On the contrary, according to the usual view, astronomers had recognised a certain limit- ing density not very far removed from the present sup- posed density of the sun, beyond which the process of contraction could not probably compress his globe. According to the theory we have brought forward in explanation of observed facts, the elements composing a mass at so high a temperature and subject to such enormous pressure as the sun's may attain even in the gaseous form a density far greater than has hitherto been considered possible. Enormously though we sup- pose the process of contraction to have gone beyond the extent heretofore believed in, we no longer recognise as close at hand any limit beyond which that process 42 THE POETRY OF ASTRONOMY. cannot pass. For our own part, in fine, while we con- sider it quite possible that the nucleus of the sun may be so tremendously compressed as to correspond to a past emission of solar heat for many hundreds of millions of years, we see no reason to believe that the process of contraction may not continue with the same emission of heat as at present for hundreds of millions of years to come. It appears to us as absurd to measure the probable amount of solar energy, either already exerted in the past or available for the fu- ture, by considerations based on the behaviour of the elements at the temperatures and pressures we can obtain experimentally, as it was of old times to estimate the proportions of the heavenly bodies on the assump- tion that the earth is the all-important body which they were made to serve, or as it is in our own time to esti- mate the duration of the heavenly orbs by the minute time-intervals corresponding to the various stages of our earth's relatively insignificant existence. THE SUN IN HIS GLORY. ANOTHER step has recently been taken towards a more exact knowledge of the nature and condition of that mighty orb which rules and lights and warms the earth and all the family of planets. The total eclipse of the year 1878, the last which for several years is likely to be observed by scientific men, has not passed without THE SUN IN HIS GLORY. 43 adding notably to our knowledge respecting the sun. Other opportunities for observation, and other methods of research, have also been employed of late with con-, siderable success. The occasion then seems a fitting one for presenting a brief and simple statement of the present position of solar research. It is strange to consider how wonderfully our ideas respecting the sun have changed during the last quarter of a century. Twenty years or so ago, the sun was regarded by many as what Sir W. Herschel had said that possibly the sun might be a dark orb surrounded by various envelopes of lustrous and heat-emitting clouds. According to this view, the sun might even be regarded as possibly a fit abode for living creatures. Others held a different view, the ancient but on the whole more probable opinion that the sun is a great mass of intensely hot matter. The sun-spots were known, and had been carefully watched and studied. Indeed the best series of observations ever made on the sun were either completed, or very nearly so, in the year 1858. But though much was known about the spots very little was understood. As to the physical constitution of the sun, nothing was known about it, and no one had any hope at that time that aught could be learned exactly on that point, though some few con- sidered it barely possible that inferences of greater or less probability might be suggested by various lines of research then entered upon. At present all this is altered we know the sun to be infinitely more complex in structure, infinitely more 44 THE POETRY OF ASTRONOMY. wonderful in physical condition, than it was formerly thought to be. We have learned what its substance consists of, in what condition that substance exists, or rather through what varying conditions it passes. We have found the sun to be something utterly unlike the orb we see, for we have learned that even as the glowing veil of air hides by day the chief glories of the universe, so it hides the largest (though not the most massive) part even of that one sun among hundreds of millions of suns which can at that time be seen. We have also learned more exactly to measure and weigh the mighty orb which rules the motions of our earth and her fellow- planets. The sun, as seen in the sky, is a globe of fire some eight^hundred and sixty-thousand miles in diameter, and lying at a distance from us amounting to about ninety-two and a third millions of miles. It affords a startling conception of this tremendous distance to con- sider that a ball fired at the sun from the mouth of an Armstrong gun, and travelling with undiminished speed directly towards him, would only reach him in about thirteen years. If the sound could travel sunwards at the same rate as in air, the sound of the explosion would reach the sun almost half a year later. An American student of science, Professor Mendenhall of Columbus, has given a striking, though fanciful, illustration of the sun's distance. ' If a baby had (which is not customary) an arm ninety-three million miles long, and on the first day of its existence touched the sun, then, accord- ing to the best estimates of the rate at which feeling THE SUN IN HIS GLORY. 45 travels, the baby might grow to manhood and the man attain to extreme old age, without ever feeling the pain of the burn. In fact, one hundred and thirty-two years would be required to convey along that monstrous arm the sensation of burning which had affected the finger tips. But, in reality, the most striking thought in connection with the sun's distance is that light, though travelling over a distance nearly equal to eight times the circumference of the earth in a single second, takes nearly nine minutes in reaching us from the sun, In passing, a word or two may be said respecting changes which have recently been made in our estimates of the sun's distance. Many who may have remembered that a distance of more than ninety-three and a third million miles was announced in Parliament last year, as the result of the British transit expeditions, may perhaps look with doubt on the distance of ninety-two and a third million miles, which I have just mentioned. It may be well to say, then, that the official astronomers responsible for working out the transit observations have come round to this smaller distance. I ventured to express in the ' Times,' even when our chief official astronomer had stated his belief that no considerable change would be made in his result, my own opinion that the lesser distance would be eventually adopted by others if not by him. But I did not expect to find my opinion so quickly confirmed as it has been. Step by step each two or three hundred thousand miles long our official astronomers have reduced their estimate ; until finally (at least I suppose so) they announce 46 THE POETRY OF ASTRONOMY. ninety-two million four hundred thousand miles as the most probable result of the British transit observations. They admit, as I had also ventured to point out, that any value within a million miles or so on either side of this distance can be reconciled with the observations. But ninety-two and a third million miles is the most probable value ; and as six or seven different and far superior series of observations had pointed to the same distance, we may unhesitatingly accept that as within two or three hundred thousand miles of the true dis- tance separating us from the mighty mass of the sun. With a diameter exceeding our earth's one hundred and nine times, the sun has a surface exceeding the earth's eleven thousand eight hundred times, and a volume exceeding hers about one million two hundred and seventy thousand times. In mass or quantity of inatter he does not so greatly exceed the earth. Still it would take about three hundred and thirty thousand globes like the earth to make up the quantity of matter which exists in the sun. All the planets together do not amount in mass to a seven hundred and fortieth part of the sun's mass. The enormous quantity of matter he possesses gives the sun tremendous power, though his actual action on our earth is not so great as many imagine, for his energy is enormously reduced by distance. A child could hold a ton of matter against the pull of the sun at the earth's distance. But if this earth of ours, re- taining its present size, contained as much matter as the sun, the strongest man (supposing he himself not THE SUN IN HIS GLORY. 47 crushed flat and thin as gold-leaf by his own weight) would not be able to lift the quantity of matter in one of our half-ounce weights. It would press downwards with as much energy as one hundred and sixty-five thousand ounces, or nearly five tons of matter, on the earth as she at present is. A small mass such as this raised only to a height of a single inch and let fall would strike the earth with three times the velocity of the swiftest express train. At the surface of the sun himself his attractive energy is not nearly so great, because his size is so much greater, and his surface so much farther away from the centre. Still a man of average weight, if placed at the sun's surface, and sup- posed not to be in a moment converted into thinnest vapour, would be pressed down as with the weight of twenty-six other men on his shoulders, and crushed completely flat. Such is the sun's mass, the quality in virtue of which he bears sway over the members of his family. Mercury has to travel at the rate of nearly twenty-nine and a half miles in a second to get centrifugal tendency enough to retain his distance from the sun. Venus, farther away, requires only a velocity of about twenty- one and a half miles a second ; our earth only eighteen and a half miles ; Mars, fourteen and three-ninths ; Jupiter about eight ; Saturn six ; Uranus four and a fifth ; and lastly, at the outskirts of the system, so far as it is yet known, we find Neptune able to retain his distance against the enormously reduced solar attrac- tion, by virtue of the centrifugal tendency resulting 48 THE POETRY OF ASTRONOMY. from a velocity of barely three and a third miles a second. An as yet altogether unexplained circumstance is to be noticed with regard to the power of the sun's attraction, and indeed of gravity generally. The action of this force is exerted instantly, or, to speak more strictly, the time occupied in transmitting the action of gravity over the greatest distances in the solar sys- tem is inappreciable. Gravity cannot take so much as a second in acting over the distance separating Neptune from the sun. I cannot conveniently explain here how this is proved. (Elsewhere I have shown this, but the subject is too difficult for treatment here.) It is, however, as certain as aught within the domain of scientific research that gravity acts in this instan- taneous manner. That it should do so is one of the greatest, if not absolutely the greatest, of scientific mysteries. If the sun only ruled the motion of our earth, we should profit little from his existence. It is because by thus ruling her movements he retains her always where she can receive the necessary amount of light and heat from him, that his attractive energy is impor- tant to us. If his heat, and the light which results from it, should fail him, our earth would still continue to travel round him as at present, but she would no longer be the abode of life. There is a fine description, in Byron's ' Darkness,' of the horrors which would follow the extinction of the sun, and though the de- scription (as Sir J. Herschel long since pointed out) is THE SUN IN HIS GLORY. 49 not scientifically accurate, it is perhaps more suggestive than a less poetic but more exact account would be : ' I had a dream,' he says, which was not all a dream, The bright sun was extinguished, and the stars Did wander darkling in the external space, llayless and pathless ; and the icy earth Swung blind and blackening in the moonless air ; Morn came and went and came and brought no day. the thrones And palaces of crowned kings, the huts The habitations of all things that dwell, Were burnt for beacons ; cities were consumed, And men were gathered round their blazing homes, To look once more into each other's face ; Happy were those who dwelt within the eye Of the volcanoes, and their mountain-torch. The world was void, The populous and powerful was a lump, Seasonless, herbless, treeless, manless, lifeless, A lump of death a chaos of hard clay. Ships sailorless lay rotting on the sea, And their masts fell down piecemeal ; as they dropped, They slept on the abyss without a surge The waves were dead ; the tides were in their grave, The moon, their mistress, had expired before ; The winds were withered in the stagnant air, And the clouds perished ; Darkness had no need Of aid from them she was the Universe 1 In reality the long miseries described so powerfully by Byron in the passages omitted from the above quota- tion, would not trouble the people of this earth if ever ' the bright sun was extinguished.' In less than a day every drop of moisture in the air would be pre- cipitated. (Herschel allows two days, but I cannot see how even a day could pass without this change being completely wrought.) And then in less than another $0 THE POETRY OF ASTRONOMY. day, all the heat remaining to the black earth would be radiated away into space, and a cold in comparison with which the cold of the bitterest Arctic winter would be as the warmth of a summer's day, would take possession of the entire earth ; no living thing could possibly survive to the end of the third day, if we can call that interval a day which would pass unmeasured by the light of either sun or moon. Among all the discoveries of modern science few are more surprising than those relating to the fires of the great central orb. When we consider merely the quantity of heat which is distributed moment by moment to the worlds around the sun, and in still greater abundance to surrounding space, we are ready almost to believe that the desolation described by Byron may be no such remote danger after all. In each second of time the sun distributes as much heat as would be produced by the consumption of eleven thousand eight hundred millions of millions of tons of coal. (If our earth's surface glowed with the same heat, she could give out as much heat as would result from the burning of almost exactly a billion tons.) But it is easily calculated that if the sun consisted entirely of coal, burning at this rate, he would burn out in less than five thousand years. It is not then by burning that he gives out heat. Again, if he were simply glowing with inherent heat and radiating that heat into space, he would lose so large a portion of his heat in five thousand years that he would be quite unfit to serve as our sun. Whence, then, is THE SUN IN HIS GLORY. 51 the supply maintained? and (stranger question still) whence has it been derived ? The answer is a startling one. The solar heat is derived from the gravitation of the sun's mass, leading to his steady contraction in volume. Meteoric down- fall may supply, and doubtless does supply, a part of the sun's heat. But that downfall is in reality a part of the same process of contraction. The meteors as yet ungathered, which nevertheless are one day to be gathered in by the sun, must be regarded as belonging to the sun, just as clouds floating in our air belong in reality to our earth. When the sun gathers in such meteors he receives a certain accession of heat ; but his volume has in corresponding degree diminished, and we know that while the contraction which he can undergo in this way, by gathering in the outlying meteoric portions of his substance, is but small, the contraction he can undergo by the shrinking inwards of his present substance is enormous ; were it otherwise we could not expect a long continuance of his present emission of light and heat. But there seems scarcely any limit to the contraction he can undergo in the future. In the past, the sun has undergone contraction far greater in amount, but less important when regarded in its heat-generating effect. For the more the sun contracts, the more effective becomes each part of the process of contraction. It is, however, certain from the study of our earth's crust, and the evidence it affords of long past ages during which the sun has poured B 2 52 THE POETRY OF ASTRONOMY. light and heat upon the earth, that the past contraction of the sun must have been competent to produce a supply of heat such as he now emits, for a period of one hundred million years. Now here a strange difficulty presents itself. If the sun's mass had originally occupied infinite space and had contracted till it oc- cupied as it now seems to do a spherical space eight hundred and sixty thousand miles in diameter, the entire supply of heat corresponding to that process of contraction would not have amounted to more than would maintain the sun's present emission for about twenty millions of years. Some have been led to believe, on this account, that the evidence given by the earth's crust must be erroneous. Others who perceive that the above-named period of a hundred millions of years cannot exceed and is probaby in reality far short of the truth, have adopted the startling theory that a large proportion of the sun's heat was derived from the collision of two suns, each travelling, with enormous velocity through space, combining to form his present mass. It appears to me that while this theory must be regarded as altogether untenable and while the. evidence given by the earth's crust must be accepted as incontrovertible, we may accept, or rather we are almost forced to accept, another theory, which suggests very strange thoughts as to the actual condition of our sun. We get rid of our difficulty at once if we adopt the theory that the central part of the sun is very dense compared with the outer part, if we assume in fact that nearly all the real mass of the sun is THE SUN IN HIS GLORY. 53 comprised within a nucleus as small perhaps compared with the globe we see, as that globe is compared with the volume of the real sun in his glory, as total eclipses reveal that glory to our view. There are other reasons, which could not properly be considered here for regarding this view as probable. If the difficulty I have mentioned above can be removed in no other way and I confess I see no other solution which can be regarded as even plausible the belief in a mightily contracted solar nucleus will become a scientific necessity. Before passing from the consideration of the sun's heat there is one thought which may well for a moment detain us. The emission of solar heat is altogether the most important process, represents altogether the mightiest energies, with which we are acquainted from actual observation. The shining of the stars at night speaks indeed' of energies compared with which the energy of our sun is but as one unit among hundreds of millions. But we have no means of actually measuring the total heat emission of any star, far less of determin- ing the totality of stellar heat and light. Solar heat we can measure ; and we know of no process which can be for a moment compared with the sun's activity. Indeed, almost every other process or form of energy with which we are acquainted is a direct product of the solar energies. Now it is a strange, one may almost say a fearful thought, that the products of this tre- mendous and constant activity, are, according to our way of viewing the matter, almost utterly wasted. 54 THE POETRY OF ASTRONOMY. Only one ray out of two thousand millions emitted by the sun falls on the earth ; and not more than one ray out of two thousand and thirty millions falls on any of the planets. All the rest are poured into the star depths, and serve no useful purpose of which we have (at present at least) any cognisance whatever! Com- pared with this, no instance of apparent waste among the myriads of such instances which nature presents to our view, seems worthy of a thought. Here is the great source of all the forms of energy existing on the earth, apparently wasted in the proportion of two hundred and thirty million lost parts to one part utilised. What the telescope has to tell us of the sun is doubtless tolerably well known to all who will read these pages. They have heard how Galileo, Fabricius, and Scheiner detected spots on the surface of the orb which had for so many ages been regarded as free from stain or blemish, an emblem of celestial purity. From the study of these the law of the sun's rotation was gradu- ally determined a law by no means so simple as some imagine. It is one of the strangest of all the facts known about the sun that the spots near his equator are carried round in less than twenty-five days, while those farthest from that circle only complete their circuits in some twenty-eight days. It is very difficult to picture the condition of an orb whose equatorial regions thus gain three days' rotation on regions in mid latitude in a single turning, or one complete rotation in nine or ten turnings. I believe that, when we come to regard the real mass of the sun as lying far within THE SUN IN HIS GLORY. 55 the limits of the globe we see and measure, this diffi- culty will be in a great degree removed. If we adopt this opinion, however, the spots could no longer be regarded as belonging to the true mass of the sun. They would be phenomena affecting the globular region around his true mass, but not necessarily affecting his nuclear regions at all. We know, of course, that enormous quantities of matter must occupy the surrounding region which is limited by the visible solar surface. We are even able to determine the elements of which that matter consists. But enormous though the absolute quantity of this surrounding matter may be, it is probably exceedingly small rela- tively. Thus we may find an explanation of the rapid changes which affect the solar spots, and also the pecu- liarities of appearance which the spots present during the various stages of their formation, development, and dissolution. The singular striation of the half-tinted fringe surrounding the dark central parts of spots, might perhaps be found rather to resemble the striation of our* own terrestrial and auroral curtains, than to result from an actual material striation of the sun's substance. The changes affecting sun-spots would be, like those affecting the auroral streamers, changes mainly re- sulting from a change of condition, not from any real change in the position of the matter forming the streamers. However, we must not here enter into the considera- tion of theories or speculations, which could only be properly dealt with at much greater length and with 56 THE POETRY OF ASTRONOMY. much closer reasoning than would be suitable for these pages. Only it may be mentioned, in passing, that the theory of the auroral nature of all visible solar phenomena will be found to have its bearing on phenomena visible during eclipses of the sun, as well as on those which the telescope discloses in the solar globe as seen in the sky. The alternate increase and diminution of the spots in number in a period averaging about eleven years and a ninth remains still among the unexplained results of solar research. We have theories in abundance to show how it might be brought about, none which seems to explain how it is actually produced. The motion of 'Jupiter in his orbit has been regarded as in some way associated with the sun-spot period ; but since for many Jovian periods in succession sun-spots have appeared in greatest number when Jupiter was nearest to the sun, and for many such periods the greatest number when Jupiter was farthest from the sun (to say nothing of long intermediate intervals during which the greatest number of sun-spots appeared when Jupiter was nearly at his mean distance), this association must be regarded as altogether imaginary, or rather it would be more correctly described as altogether unimaginable. Other relations supposed to exist between the sun-spots and the planetary motions seem almost equally open to ex- ception. In fact, we can only say at present that the sun-spots wax and wane in number in a period rather exceeding eleven years, with certain subordinate periods, as also some longer but less clearly recognised periods THE SUN IN HIS GLORY. $? of variation : we can assert nothing positively respecting cause and causes of such periodic changes. Nay, when we examine the records of solar observation since the days of Galileo, we find reason even to ques- tion whether the period of eleven years and a ninth is the true one, or only approximately true during the present century. For in Wolff's list of years in which the sun has been most spotted and least spotted since Galileo's time, we find some intervals of more than twenty years instead of about eleven years from maxi- mum to maximum or from minimum to minimum of sun-spot frequency. Similar doubts seem to hang over the relation once believed to exist between sun-spots and terrestrial magnetism. This relation has always been denied by the present astronomer royal, who states that the Greenwich magnetic records afford no evidence in its favour. But it is now regarded by many astronomers as disproved in a more general way. Faye, for instance, the eminent French mathematician, asks how two periods can be regarded as associated by the very persons who maintain most strenuously the trust- worthiness of researches assigning ten years as the value of one period and eleven years as the value of the other. There seems no room on the one hand for doubting the accuracy of Wolff's estimate of the sun- spot period at eleven years and a ninth. On the other there seems no reason for questioning the value of about ten years which Sabine and Lamont have assigned to the period in which the earth's magnetism oscillates 58 THE POETRY OF ASTRONOMY. in energy. Suppose, now, that at any given epoch the time when sun-spots are most numerous agrees with the time when terrestrial magnetic disturbances are greatest; then sixty years after that epoch, six times ten years having passed, terrestrial magnetic disturbances will again have attained their greatest value ; but as five sun-spot periods and a half will have passed, sun-spots will be fewest in number. The time of greatest magnetic disturbance will therefore agree with the time of least solar disturbance a relation the exact opposite of that which had prevailed sixty years before. Sixty years later the original state of things will be renewed. Sixty years later still, it will be again reversed, and so on continually. In other words, the two periods are in no way connected together. I fear we must regard the supposed connection between the sun-spot period and magnetic disturbances, the occurrence of great auroras, and so forth, as having its origin, like so many other relations which the pro- gress of science has caused to be forgotten, in mere coincidence. It so chanced that the sun-spot period was first fully recognised at a time when the time of many sun-spots agreed with the time of greatest mag- netic disturbance for several sun-spot periods in suc- cession. If the sun-spot period had been recognised sixty years earlier or sixty years later, the probability is that the time of fewest sun-spots would have been associated with the time of greatest magnetic disturb- ance. And if the sun-spot period had been recognised thirty years earlier or later, no relation at all would THE SUN IN HIS GLOEY. 59 have been suspected, for the time of most sun-spots would then have come midway between the time of greatest and the time of least magnetic disturbance. This reasoning, like Faye's, is based on the as- sumption that both the sun-spot period and the period of the magnetic oscillatory changes have been deter- mined either correctly or nearly so. If they have not been, then it still remains possible that the supposed association between the two periods may have a real existence. The determination of the actual substances, or many of them, which form the sun's mass must be re- garded as among the greatest triumphs of science. In some respects it even surpasses in interest the recog- nition of the law of universal gravity. Without entering into details which would here be out of place, the way in which the sun's material con- stitution was determined may be thus described. By means of the instrument called the spectroscope, it was found possible to separate the rays which form the sun's light into their several colours. The red rays are brought to one place, the orange rays set next, the yellow next, then the green, the blue, the indigo, and lastly the violet. Not only are the colours thus dis- tributed, but they are arranged according to their several tints, the red merging by indefinite gradations into the orange, the orange into the yellow, and so on. Now if we imagine a number of threads of different colours thus arranged, we see that the finer the threads and the greater their number, the more perfect would 60 THE POETRY OF ASTRONOMY. the gradation be. We can readily conceive that though threads of all the tints of red, orange, yellow, &c., might not be present in the collection, it might yet happen that the entire space occupied by the array would be covered, simply because the breadth of the individual threads might enable them to cover more than the space really due to their re- spective tints. But if for coarse threads fine threads were substituted, for fine threads mere filaments, for these still finer filaments, and lastly such filaments as twisted in hundreds would form but a thread like that of the spider's web, we see that millions of tints might be represented in a rainbow-tinted streak a few inches in length, and yet the fine filaments composing it might barely touch each other, allowing no gaps to be seen. Such an array of tints (the threads lying square to the length of the rainbow-tinted streak) would fairly illus- trate the separation of colours in a pure spectrum. But when sunlight is thus dealt with, when the count- less millions of tints really forming it are brought to their proper position in the spectrum, it is found that thousands of tints are missing. It is as though from the array of fine filaments just described, forming a complete rainbow-tinted streak, hiding a dark back- ground, hundreds of red filaments (not close together, but of distinct tints) were withdrawn, hundreds of orange filaments, hundreds of yellow filaments, and so forth, until instead of a perfect rainbow-tinted streak there remained a rainbow-tinted streak crossed (athwart its length) by multitudes of fine dark lines, representing THE SUN IN HIS GLORY. 6l the places where filaments had been withdrawn. Such is the solar spectrum. In other words, sunlight con- tains rays of all the colours of the rainbow, but of the millions of millions of tints included in these colours hundreds of thousands are missing from among the solar rays. Now it was found that every element when in the vaporous form and glowing with intensity of heat, has the power of emitting rays of certain special tints pecu- liar to itself. At first no connection was perceived between this discovery and the existence of dark lines in the solar spectrum. But at length Kirchhoff dis- covered that each element has the power of absorbing rays of the same tints which it emits. When a vapour is interposed between the eye and a mass of glowing solid or liquid matter, the vapour allows all rays except those peculiar to itself to pass freely. But it absorbs the rays which it is capable of emitting. If it is cooler than the glowing solid or liquid matter it cannot make up by its own emission for the rays which it absorbs ; thus when the light which reaches the eye is analysed with the spectroscope, these tints are found to be defi- cient. If it is hotter it more than makes up for the loss of these rays, and under analysis with the spectro- scope these tints are found to be in excess. Lastly, if it is at the same temperature as the glowing solid or liquid source of light, its emission just makes up for its absorption, and the spectroscope affords no trace of any effect produced by the interposition of the vapour. The application of this great discovery of Kirch- 62 THE POETRY OF ASTRONOMY. hofPs to the interpretation of the solar dark lines is obvious. Every dark line or missing tint is due to the absorptive action of some vapour in the sun's atmosphere. Any family of such lines which can be shown to be identical with a family of lines which some given gaseous element emits when in a given condition, proves that that element in that condition is present in the sun's atmosphere at a cooler temperature than the mass which it surrounds. If any set of bright lines be seen in the solar spectrum that is, if a certain family of tints appear in excess, and can be similarly identified with the special tints of a known element we infer, or rather we know, that that element is present in the sun's atmosphere at a higher temperature than the general mass of the sun. 1 1 The subject of spectroscopic analysis is so difficult to explain, and so great a variety exists in the receptivity of different minds for different explanations, that I feel tempted to quote here an entirely new and original explanation given by Professor Newcomb in his fine work on Popular Astronomy : ' Suppose Nature should loan us an immense collection of many millions of gold pieces out of which we were to select those which would serve us for money, and return her the remainder. The English rummage through the pile, and pick out all the pieces which are of the proper weight for sovereigns and half-sovereigns; the French pick out those which will make five, ten, twenty, or fifty franc pieces ; the Americans the one, five, ten, and twenty dollar pieces, and so on. After all the suitable pieces are thus selected let the remaining mass be spread out on the ground according to the respective weights of the pieces, the smallest pieces being placed in a row, the next weight in an adjoining row, and so on. We shall then find a number of rows missing ; one which the French have taken out for five-franc pieces, close to it another which the Americans have taken for dollars ; afterwards a row which has gone for half- sovereigns, and so on. By thus arranging the pieces one would be able to tell THE SUN IN HIS GLORY. 63 By this method it has been shown that the sun's vaporous envelope contains iron, lead, copper, hydro- gen, sodium, magnesium, cobalt, nickel, and a number of other elements, with which we are more or less familiar. The presence of oxygen and nitrogen is also strongly suspected. Indeed, the experiments of Dr. Draper, of New York, seem to demonstrate the existence of oxygen if not of nitrogen : but doubts have been raised in this country as to the validity of the evidence which he has obtained, and for the present it will be safer to regard the matter as undecided. Albeit we can entertain little doubt that these gases and all the ter- restrial elements are present in large quantities in the sun ; the point which remains undetermined is whether we have yet obtained valid evidence on the subject. The a priori probability that these elements exist in the sun, is so great that without any evidence we might feel tolerably certain that they are present. what nations had culled over the pile, if he only knew of what weight each one made its coins. The gaps in the places where the sovereigns and half-sovereigns belonged would indicate the English, that in the dollars and eagles the Americans, and so on. If, now, we reflect how utterly hopeless it would appear, from the mere examination of the miscellaneous pile of pieces which had been left, to ascertain what people had been selecting coins from it, and how easy the problem would appear when once some genius should make the proposed arrangement of the pieces in rows, we shall see in what the fundamental idea of spectrum analysis consists. The formation of the spectrum is the separation and arrangement of the light which comes from an object on the same system by which we have supposed the gold pieces to be arranged. The gaps we see in the spectrum tell the tale of the atmosphere through which the light has passed, as in the case of the coins they would tell what nations had sorted over the pile.' 64 THE POETRY OF ASTRONOMY. But the sun we see is only a small part (so far as volume is concerned) of the true sun. So great is his splendour, and so brilliantly in consequence does he illuminate our atmosphere, that when he seems to be shining in full glory in the heavens he is, in reality, shorn of a large portion of his true glory, so far at leas* as the splendour of a heavenly body may be regarded as depending on volume. Immediately outside the globe, we see, comes an amazingly complex atmosphere, some three or four hundred miles in depth. We may say of this atmo- sphere that it has never been seen even during eclipses. Its existence has been demonstrated, but it is too shallow and remains in view too short a time to be actually per- ceived with the most powerful telescope yet directed towards it. Its existence has been demonstrated by spectroscopic analysis, as follows : When the moon passing over the sun's disc in total eclipse just hides the last fine thread of sunlight, there remains still in view for two or three seconds whatever vaporous matter may lie just above the solar globe. Now during this short interval of time the light received by the observer comes from this vaporous envelope. If analysed with the spectroscope, then it must show the tints belonging to the vapours which form the envelope. And if the envelope contains all or most of the vapours whose absorptive action causes the solar dark lines, then, since these vapou rs are for the moment shining alone, we ought to have a spectrum showing just as many bright lines as the solar spectrum shows THE SUN IN HIS GLORY. 65 dark lines. To use an illustration employed in an article in the ' Times ' which has been attributed to myself, 4 suppose that athwart a strip of dark paper millions of fine coloured threads were laid so as to form a rainbow-tinted streak (as described above), and that while all those corresponding to the missing tints of the solar spectrum were gummed down to the dark paper, these were left ungummed ; ' then ' if a similar strip of dark paper, having a slightly adhesive surface, were superposed on the rainbow-tinted array of threads and gently pressed down, so that when lifted again all the ungummed threads adhered to it, the first strip, thus robbed of twenty or thirty thousand tints, would fairly picture the solar spectrum ; while the second strip, showing these twenty or thirty thousand threads, would fairly picture the spectrum of the solar atmosphere. One would be a rainbow-tinted streak crossed at right angles by numbers of dark lines ; the other would be a dark strip crossed by a rainbow-tinted array of bright lines at right angles to its length.' The latter beautiful appearance was presented to Prof. C. A. Young (of Dartmouth College, Hanover, N. H.) when during the total eclipse of December 1870 he examined the light received from the portion of the sun invisible under ordinary conditions, lying just outside the part of his invisible disc which last disappeared. The observation, which was questioned by some less experienced in physical research, has since been several times repeated, and always with the same result. We learn, then, that outside the visible globe of the sun there is an atmo- F 66 THE POETRY OF ASTRONOMY. spheric shell, which cannot be less than two hundred or more than five hundred miles in depth, in which are present all or nearly all the vapours whose absorptive action produces the solar spots. The vapours of iron, copper, and lead, for instance, are present in enormous but in varying quantities in that fiery atmosphere, just as in our own atmosphere the vapour of water is always present, but not always to the same extent. Glowing hydrogen is there as a fixed constituent just as oxygen and nitrogen are fixed constituents of our own air. Whether glowing oxygen and nitrogen are present in it as non-luminous (because cool) oxygen and nitrogen are present in ours, remains yet to be determined. I had partly hoped that some evidence might have been obtained on this point during the recent eclipse ; but it does not appear that any attempt was made to identify the bright lines of oxygen or nitrogen on that occasion. Above the complex atmosphere, to a height varying from five or six thousand to ten thousand miles, is the sierra, an envelope of glowing hydrogen, into which, from time to time, other vapours are poured (apparently) from the sun's interior, especially from the neighbour- hood of great spots. These sometimes extend to enor- mous heights, forming the objects called prominences. I have said that vapours are apparently poured forth ; for it is by no means certain that, when in any particular region of the sierra, the glowing vapour of sodium, magnesium, or other elements, makes its appearance ; it has necessarily just flowed into that region. Quite THE SUN IN HIS GLORY. 6? probably there is merely a change of condition in vapour already present, the vapour not arriving when it makes its appearance, but then beginning to glow. Indeed, the motions which seem to take place in the sierra and in the coloured prominences are so tremendous that it is almost a relief to recognise some way of explaining the observed phenomena which would free us from the necessity of believing in such rapid transfer of matter. Cornu has recently advanced the doctrine that the changes which have been supposed to indicate the occurrence of eruptions in the sierra, and the enormous prominences which have been regarded as phenomena of eruption, are in reality due simply to electric currents, by which vast masses of gas are caused to glow with intense lustre. He points out that the rapid extension and sudden contraction or disappearance of prominences would thus be explained, without recourse to the improbable hypothesis of jets of gas having velocities of hundreds of miles a second. It must, however, be mentioned that this explana- tion would not account for the spectroscopic evidence of motions of enormous velocity towards or from the observer. If this evidence is valid that is. if the spectral lines really have shifted their position, as some spectroscopists assert there must have been a bodily transference of the glowing gas to which they belong, in the direction of the line of sight. The change of condition, suggested by Cornu, would in reality corre- spond to a greater range of vibration in the molecules of the gas. The spectroscopic observation implies an F 2 68 THE POETRY OF ASTRONOMY. apparent change in the wave-lengths, which is a very different thing, and cannot be brought about (that is, even the appearance of such a change cannot be brought about) by the change which Cornu imagines. This point is so important that I shall endeavour to illustrate it in such a way as to make the considerations on which it depends clear to the reader. Suppose smooth water, perfectly clear, to be invisible to a par- ticular observer, but that so soon as the water is ruffled he can see it. Then, if he observed at a distance the appearance of ruffled water, where a moment before he had seen none, he might imagine that there had been an inrush of water there, taking place with enormous rapidity. But if he reflected that a mere change in the condition of the water would account for its becom- ing thus suddenly visible, he would probably accept as the most probable inference the belief that a change of this kind had taken place and that there had been no influx of water. Suppose, however, that he makes an observation of an entirely different sort. Suppose he can recognise the repeated waves, and count them as they cross a certain point in his field of view. Suppose it to be known to him that if the water is undergoing no bodily transference the waves must cross that point at a certain definite rate whether the ruffling of the water is slight or great, and that, as a matter of fact, he finds they are crossing that point at a greater or less rate. Then he cannot account for this peculiarity by assuming any change to have taken place in the extent to which the water is ruffled. He must infer, if his THE SUN IN HIS GLORY. 69 observation is trustworthy, that there is a current then transferring the water bodily, waves and all, athwart the field of view, in the same direction as the waves travel if they pass the fixed point more quickly, in the opposite direction if they pass that point more slowly, than before. Now it is such evidence as this that the spectroscope gives, or has been said to give, respecting the vapours forming the sierra, and the coloured pro- minences. When a spectral line is shifted, the meaning is that the waves are following each other either more quickly or less quickly, according to the direction of the change. But no change in the temperature of the vapour would cause this to happen, in whatever way this change was brought about. We can only explain an observed change in the position of a spectral line by concluding that there is at the moment a bodily trans- ference of the glowing vapour to which the line belongs, taking place from or towards the eye with enormous rapidity. On the whole I cannot think Cornu has succeeded in overthrowing the evidence on which the inference has been based that these motions take place in and near the sun, at the rate of many miles in every second of time. The coloured prominences may be regarded as occu- pying a region some hundred thousand miles in depth outside the region occupied by the sierra. It is true that a region of this enormous depth is not at any time filled with the glowing prominences, and probably over the equatorial and polar regions prominences seldom attain so great a height as a hundred thousand miles, ?0 THE POETRY OF ASTRONOMY. while in the spot-zones they often range far higher. Yet in the general sense in which indeed we can alone speak of the height of the complex atmosphere and of the sierra, we may say that a region of about a hundred thousand miles in height, all over the sun, is that in which the vast coloured prominences are formed and exist. Two noteworthy discoveries have been made within the last ten years respecting the red prominences. In the first place those over the spot-zones are found to be markedly different in character from those over the sun's equatorial and polar regions. Whereas these, though often of vast dimensions, are usually cloudlike and comparatively quiescent, those over the spot-zones often present an appearance as of being caused by mighty eruptions from the sun's interior. On one occa- sion, a great mass of prominence matter one hundred thousand miles in length, observed by Professor Young, appeared to have been torn into shreds by a mighty eruption from below, and Young watched the upward motion (or apparent motion) of these shreds (threads of gas from three hundred to one thousand miles in length) till the uppermost had reached a height of more than two hundred thousand miles from the sun's surface. Whether they really travelled to that height from their former position, some forty or fifty thousand miles above the sun's surface, at the enormous rate indicated by their apparent motion, a rate of more than a hundred miles per second, is open to doubt for reasons which I have already indicated. But it is certain, that when THE SUN IN HIS GLORY. 71 Professor Young watched the gradual fading out of the luminous threads at their highest range above the sun's surface, there was glowing hydrogen at least one hundred thousand miles farther from the sun than hydrogen had ever been traced before in that condition. Hydrogen relatively far cooler appears, from observations made during total eclipses of the sun, to exist at greater heights. But never before or since has hydrogen plowed in that way at so great a distance from the sun's surface. In whatever way we explain this phenomenon we cannot doubt that it indicated the existence of a very unusual disturbance in that part of the sun which lay immediately under the apparently uprushing filaments. My own opinion is that probably not hydrogen, but masses of solid, liquid, or very highly compressed gaseous matter, was suddenly flung forth on that occa- sion, and that the long filaments indicated regions where these masses had swiftly rushed through the relatively cool hydrogen surrounding the sun. But we have not yet reached the outermost solar envelope. Besides the red prominences, there can be seen, during total eclipse of the sun, the solar corona somewhat resembling the glory round the heads of saints in pictures. This is a solar appendage, or (we may more truly say perhaps) a part of the sun, which has attracted more attention from astronomers than almost any other during the last ten or twelve years. It was shown first in 1868, that the corona consists in great part of matter which reflects the light of the sun. For the light of the corona examined with the 72 THE POETRY OF ASTRONOMY. spectroscope was found to contain all the colours of the rainbow ; and though the dark lines or missing tints of the solar light were not then recognised, the circum- stance was very readily explained when the difficulty of the observation (at that time) was duly taken into account. If it had then been proved that the light of the corona is not in the main reflected sun-light, but consists in great part of such rays as come from glowing solid or liquid matter, the inference would have been that the substance forming the corona was raised to a white heat by the sun's rays. Although this did not seem altogether improbable, yet there are difficulties in the way of such an explanation. The scattered meteors travelling around the sun, and occupying (in this scattered way) the whole region of space where the corona appears, although they would be exposed to in- tense heat a heat so intense indeed that bodies on our earth would become white hot if exposed to it would be free to radiate so readily into space, whatever heat they received, that they would probably never become white hot right through. As the moon, having no atmosphere, or only a very rare atmosphere, becomes as hot as boiling water on the side turned towards the sun, but is colder than ice on the side turned away from him, so would meteors surrounding the sun be intensely hot on the side turned towards him, but comparatively cool on the side turned from him. Thus probably the average heat of each such meteoric mass would not be greater than that of red-hot iron. Nevertheless the intense heat to which the substance forming the sun's THE SUN IN HIS GLORY. 73 corona is unquestionably exposed must be remembered in considering the nature of this appendage. It is so great that the earth during a total eclipse receives an appreciable amount of heat from the corona, as Edison succeeded in showing with his wonderful heat-measur- ing instrument, the tasimeter, during the eclipse of last July. In 1869, astronomers found that a portion of the light of the corona comes from glowing gas, for on the rainbow-tinted background of the corona's spectrum a bright green line was seen. It was at first supposed to be identical with a line belonging to iron, but this has been shown by more exact observations to be a mistake. The element whose presence in the corona is indicated by this bright line has not yet been ascertained. All we know respecting it is that it is gaseous. It does not follow, however, that the sun is surrounded by a gaseous envelope to the height of some half-million or so of miles. Quite possibly scattered portions of this gas, whatever it may be, are either formed in the sun's neighbourhood by his intense heat poured on solid masses travelling there, or are flung forth from his in- terior. Or possibly the luminosity to which this bright line is due has its origin in electric discharges between solid masses travelling near the sun, not in an absolute vacuum but in an exceedingly tenuous atmosphere. During the eclipse of December 1870 the corona was for the first time successfully photographed by Willard, of Philadelphia, at Xerez in Spain, and by Brothers, of Manchester, at Syracuse. The photograph 74 THE POETRY OF ASTRONOMY. obtained by Brothers is one of the best we have, even to the present time. It is specially interesting on account of the enormous extension of the corona on one side. In December 1871 six excellent photographs were obtained by Mr. Davis, who superintended the photo- graphic operations of an observing party sent by Lord Lindsay to Baicull on the western shore of Mysore. Col. Tennant, at Ootacamund in the Neilgherry Hills (ten thousand feet above the sea-level), obtained six almost equally good photographs. It was, as many readers perhaps will remember, the resemblance be- tween the six photographs of each set, and between the two sets, which finally overthrew the theory held by some that the corona is a phenomenon of our own atmosphere, or else is something ' at the moon,' as the theorists (somewhat ungrammatically) expressed their view. A view derived from these photographs is given in the latest edition of my treatise on the sun. During the same eclipse Janssen, the French astronomer, succeeded in discerning the solar dark lines in the spectrum of the corona. In other words, he succeeded in proving that the coronal light is in great part reflected sunlight. The corona till last July had been regarded as an appendage of varying extent and figure, usually some- what radiated, and occasionally throwing out streamers to great distances, but probably nowhere exceeding two millions of miles in range outwards from the sun. It had been maintained by some (as by myself) that the THE SUN IN HIS GLORY. 75 observed extension of the corona is simply limited by the conditions under which eclipses are necessarily studied, *the real extension being far greater. In the first edition of my treatise on the sun, written and in great part printed before the eclipse of December 1870, I remarked that ' the zodiacal light indicates the exist- ence of a lens-shaped region around the sun within which cosmical matter is profusely strewn, and that if the zodiacal light could be traced yet farther to- wards the sun's place, the increase of lustre [observed towards its core and near the sun's place below the horizon] would continue, and that therefore all round the sun there would be seen a luminosity correspond- ing precisely with the observed aspect of the corona.' In other words, I expressed the opinion, which many at the time thought rather extravagant, that the zodiacal light is but the outer part of the solar corona. Considering that the zodiacal (for so it may more con- veniently be called) extends certainly some eighty million miles from the sun, while the corona had only been traced at that time to a distance of a million miles, and was not even regarded by all astronomers as demonstrably a solar appendage, it will be understood that my opinion not only seemed to many an extrava- gant one, but was regarded by some as a theory which never could be submitted to any valid test. If, however, the corona could be traced to a dis- tance of eight million miles in the direction of the zodiacal, the proof of connection between the two would be sufficient ; for the zodiacal has been traced 76 THE POETRY OF ASTRONOMY. to within a distance of eighty million miles of the sun. I expressed my belief that this could be done, and showed how the difficulty should be attacked. If the bright inner corona were concealed from view by an interposed screen, protecting the eyes of the observer, the faint outer corona could, I said, be traced most probably to a much greater distance than heretofore, even if its extension into the zodiacal were not clearly recognised. During the eclipse of 1870, Prof. Newcomb, of Washington, tried this method, but failed in obtain- ing any satisfactory result. The weather was hazy, his station not well suited for an observation of so delicate a nature, and the circumstances in other respects were unfavourable. In July 1878, however, Prof. Newcomb renewed the attempt at a station high above the sea level and in beautifully clear weather. He succeeded in tracing the light of the outer corona to a distance of about six degrees or some ten million miles from the sun. Prof. Langley, of Pittsburg (a very skilful observer of the sun), achieved a similar success. And Prof. Cleveland Abbe succeeded in tracing luminous streaks and streamers to a distance of five millions of miles. Thus at length the gap between the corona and the zodiacal light has been completely bridged over. What was before but a probable surmise has become a demonstrated fact. The sun, we see, enormous as is his bulk, is yet, so far as volume is concerned, little more than a point within the mighty volume of which THE SUN IN HIS GLORY. 77 he is the nucleus. Assigning to the zodiacal the figure of a flattened spheroid (like the earth's, only the flat- tening is far greater) having for its greatest diameter one hundred and sixty million miles, and for its least or axial diameter about twenty million miles, the sun's volume is less than one eight-hundred-thousandth part of the volume of this mighty disc of cosmical matter. Albeit we must remember that the mass of the sun we see exceeds in yet greater degree that of all the material of the zodiacal disc, though the volume of this tremendous disc so enormously exceeds his. Of this mighty system is the sun we see the nucleus. What lies below that glowing surface we do not know. But of what lies around it we have obtained some degree of knowledge, slowly and pain- fully, though much yet remains to be learned, and far more will ever remain unknown. Here, as in so many other departments of science, the saying of Laplace is justified, ' What we know is little, the unknown is immense.' WHEN THE SEA WAS YOUNG. WE are best able to realise the fact that our earth is a globe-shaped orb, one among many such orbs peopling space, when we contemplate the wide expanse of ocean. Although- the teachings of astronomy place the real figure of our earth beyond all possibility of question, it is nevertheless not readily rendered sensible to observation. 78 THE POETRY OF ASTRONOMY. Whatever science may teach us, we usually see the earth as a generally level surface arched over by a dome of sky, which, whether clear or veiled by clouds, deceives us as to the earth's true extent and figure. Not only is this apparent shape of the sky deceptive, suggesting a somewhat flattened dome rather than the visible half of a space which, if regarded as bounded at all, should appear as bounded by a perfectly spherical surface, but the sky, seeming to spring from the visible terrestrial horizon, appears to have an arch of very limited extent. Under ordinary conditions we uncon- sciously regard the portion of the sky which lies next to the visible horizon as some five or six miles from us at the utmost, 1 while the part overhead seems not more than two or three miles from us. Where the air is exceptionally clear the extent of the sky-vault ap- pears somewhat greater ; but ordinarily some such con- ception as we have indicated is suggested respecting the size and shape of the dome which the heavens appear to form over our heads. And accordingly, when 1 That the mind does not, in its unconscious action, attribute a very great distance to the horizon is shown by the strange illusion produced during balloon ascents. As the balloon rises the horizon seems to rise up all around the aeronaut, so that the visible portion of the earth beneath him seems to assume the shape of a vast basin. If the mind assigned its true distance to the circle where land and sky seemed to meet, this illusion would not occur ; for there can of course be no doubt that the apparent rising of the horizon all round the rising balloon is due to the idea present in the aeronaut's mind that, while he rises perceptibly from the earth, the circle forming the visible land-horizon ought perceptibly to sink, which it would do if it were as near as it had been unconsciously assumed to be. WHEN THE SEA WAS YOUNG. 79 we try to realise the idea that the earth is a globe, we unconsciously picture it as a globe enclosed within the sky-vault, which we conceive as extended below the horizon so as entirely to surround the earth. Accord- ing to this conception the earth would have a diameter of no more than some thirteen or fourteen miles ; and reason at once rejects this conception as altogether in- adequate. But where there is a wide expanse of ocean, whether partially limited or not by land-scenery, the real extent of the terrestrial globe is suggested, though not actually indicated. The mind recognises, from the appearances presented to the eye, that the ocean has a curved surface of enormous extent; while the arch of the sky is recognised as manifestly not springing from the visible horizon, itself thrown much farther away (if the eye is well raised above the sea-level) than when an ordinary land-surface limits the range of view. When the air is very clear, so that objects many miles beyond the water- horizon can be distinctly seen, the sense of the real vastness of the terrestrial globe is still more strongly impressed on the mind, especially if the objects so seen are such that their actual distance and position can be recognised. For instance, a portion of elevated land-surface seen beyond the sea-horizon does not so strongly suggest real remoteness as a ship ' hull down,' unless there should happen to be land nearly at the distance of the sea-horizon, so that by the greater distinctness of such nearer land the remoteness of other land seen above the horizon-line is indicated. 1 1 For ihe same reason an ocean scene at night is seldom so sug- So THE POETRY OF ASTRONOMY But apart from the effect produced, as it were in- stinctively, by the actual appearance of the ocean, another effect is produced on the mind by the con- sideration of the ocean's real nature. Of all terrestrial features the ocean is the one which best deserves to be regarded as cosmical. Rather, perhaps, it should be said that the division of a planet's surface into land and water is the characteristic most readily to be recog- nised when the planet is viewed from some other celes- tial orb ; so that when we contemplate our ocean we are regarding a feature of the earth as a planet one, too, whereof others besides the inhabitants of the earth may be cognisant. The thought that we may thus be sharing our- impressions of the earth's condition with beings of some other world that, in however diverse a degree, inhabitants of Venus, or of Mercury, or per- haps even of Mars, may be able to note that very feature which we are considering brings forcibly before the mind the fact, otherwise so hard to realise, that this earth of ours is a globe travelling like the other planets round the sun, rotating on its axis as we see the other planets rotating; and that, in fine, of all those orbs which astronomy presents to us as distri- buted and moving so variously through space, the earth is that one which we are able to examine under the most favourable conditions. So that an astronomer gestive of the earth's real nature as a daylight view of the ocean ; for the curvature of the ocean-surface cannot be clearly recognised at night, nor usually can any objects far beyond the sea-horizon be perceived at all, still less their true distance appreciated. WHEN THE SEA WAS YOUNG. 8 1 at such times comes to recognise an astronomical and cosmical, rather than a merely terrestrial, interest in the contemplation of our earth. He finds his science brought into close connection with terrestrial researches, since these afford the only means available for examin- ing one among the orbs which form the subject of his study. And although his observations may serve to render him very doubtful whether among all the orbs in space there is a single one which very closely re- sembles the earth, yet he finds reason also to believe that in general respects the earth's past and future condition illustrate well the significance of phenomena presented by orbs now very unlike her. So that the astronomer finds a new interest in contemplating the earth as one among the bodies to which his science relates. It, is not merely with regard to space, but with regard to time also, that her aspect, thus viewed, becomes suggestive. This globe, to which we are bound by the chains of a universal force, is not only among the unnumbered and all-various globes scattered throughout infinite space, but we perceive in her the traces of processes carrying back our thoughts over un- numbered aeons in the past, the germs of effects belonging to periods as immense in the remote future. In this respect the study of the ocean is especially suggestive. For of all things terrestrial the ocean is at once the most ancient and the one which will endure longest. Mountains and hills have from time imme- morial been taken as emblems of the long-lasting. The Bible speaks of ' the utmost bounds of the ever- Q 82 THE POETRY OF ASTRONOMY. lasting hills;' compares 'the precious things brought forth by the sun and moon ' with the 4 chief things of the ancient mountains and the precious things of the lasting hills;' and, as a supreme type of the Almighty's power, Habakkuk says : * Grod stood and measured the earth ; and the everlasting mountains were scattered, the per- petual hills did bow.' But, in reality, the mountains are young compared with the ocean, 1 while for ages after our present mountains have disappeared the same ocean whose waves beat now upon our shores will lave the shores of continents as yet unformed. 1 It is related in the Life of John Herschel that when he was still a. boy he asked his father, the great astronomer, William Herschel, what he thought was the oldest of all things. ' The father replied, after the Socratic method, by putting another question : " And what do you yourself suppose is the oldest of all things ? " The boy was not successful in his answers ; whereon the old astronomer took up a small stone from the garden- walk : " There, my child, there is the oldest of all the things that I certainly know." ' The biographer from whom we have quoted says that we can trace in that grasp and grouping of many things in one, implied in the stone as the oldest of things, as forming one of the main features which characterised the habit of the younger Herschel's philosophy. But in truth the stone speaks to the thoughtful mind of something far older than itself not, indeed, older in respect of mere existence as matter (for all matter is eternal ; and in this sense the bud that flowered yesterday is no less ancient than the substance of the time-worn hill, or the waters of the everlasting ocean), but older in the sense wherein that which fashions is older than the thing fashioned. For the stone upon the garden-walk at Slough had either been rounded by the waves of ocean, or had been shaped by the running waters of brook or river formed by rains, the proceeds of evaporation from ocean's surface. Nay, even passing to still earlier periods of the stone's history leaving, that is, the consideration of its formation as a stone to consider the formation of its substance its substance was gathered at the bottom of the sea when the ocean was already more aged than the oldest mountains now existing. WHEN THE SEA WAS YOUNG. 83 But even those periods of the ocean's history which are thus brought before our thoughts the vast ages during which the land-surface of the globe has been constantly changing, rising and sinking alternately ac- cording to the varying pressures exerted by the earth's interior forces, and the ages yet to come, during which like changes will take place are as nothing compared with the duration of three stages of the ocean's history, one of which we now purpose to consider. The ocean's entire existence under its present aspect is one of these stages ; of the others, one preceded and the other will follow the present stage at intervals of probably many hundred millions of years ; while the waters comprising the ocean presented during the first stage, and will present during the coming, or third stage, an appear- ance utterly unlike that of the ocean in the present era of its existence. It is now admitted by almost all students of science that the earth, and the solar system of which she is a member, reached their present condition by processes of development. The exact nature of those processes may be matter of doubt and uncertainty, just as the exact nature of the process of development by which animal types have reached their present condition may be doubtful. But exactly as biologists hold by almost universal consent the general doctrine of development, though they differ as to the exact course along which such development proceeded, so every astronomer of repute believes in the evolution of the solar system by natural processes, though different ideas may be enter- G 2 84 THE POETRY OF ASTRONOMY. tained as to the exact history, either of the solar system as a whole, or of its various members, during long past geons of ages. Whatever theory of evolution we adopt, however, or in whatever way we combine the various theories which have been advanced, one fact in the past history of our earth stands out with unmistakable distinctness. The whole frame of the globe on which we live, and move, and have our being, was once glow- ing with intense heat. Whether we consider the earth's frame with the geologist, or study with the astronomer the nature of the planets' movements and the evidence so afforded respecting prior conditions of the solar system, we are alike forced to this conclusion. At a very remote period the whole substance of the earth must have been molten with intensity of heat ; at a still more remote period the whole of that sub- stance must have been gaseous with a heat still more intense ; and these stages of the earth's history, remote though they were, and continuing so long that, accord- ing to our modes of measuring time, they were practi- cally everlasting, were yet but two among a series of eras whose real number, no doubt, was to all intents and purposes infinite. Now when we go back to even the nearer of those two eras we find that we must conceive of our ocean during that era as utterly unlike the seas which now encompass the earth. Its substance was the same, or nearly so, but its condition must have been altogether different. No water could for a moment rest upon the intensely hot surface of a globe raging with heat ex- WHEN THE SEA WAS YOUNG. 85 ceeding that of a smelting furnace. There could not have been during that era oceans of liquid water, though all the water of our present oceans surrounded the earth then as now. The water must at that time have existed in the form of mixed vapour and cloud ; that is, it must have been spread through the air partly as pure aqueous vapour and partly in those aggregations of minute liquid globules and vesicles of water forming visible cloud-masses. There must also at that time, as now, have been various kinds of cloud- forms an outside layer consisting of the light feathery cirrus clouds, below that a layer of the cumulus or 6 woolpack ' clouds, and below that again a deep layer of the densest nimbus or rain-clouds from which perfect sheets of rain must at all times have been falling ; not however, to reach the glowing surface of the earth, but to be vapourised in their fall, and in the form of vapour to pass upwards again. We say that all this must have been ; because, in point of fact, however doubtful we may feel as to many details of the earth's condition in the remote era we are considering, there can be no doubt whatever as to the general facts in- dicated above. We have only to enquire what would happen at the present day if the earth's whole frame were to be gradually heated until at last the surface glowed with a heat equal to that of white-hot iron, to perceive that, whatever other changes might take place, the ocean certainly would be entirely evaporated boiled off, so to speak. But the water thus added to the earth's atmospheric envelope in the form of vapour 86 THE POETRY OF ASTRONOMY. could not possibly remain wholly in that form. At a great distance from the glowing earth the aqueous va- pour would find a cooler region, and higher still would be exposed to the actual cold of space. Hence there would follow inevitably the formation of clouds of the various orders, cirrus, cumulus, and nimbus, not pro- bably in absolutely distinct layers, but the cirrus commingled with the cumulus, the cumulus with the nimbus, and the whole series of cloud-layers affected by the most violent disturbances, partly from the continual rushing upwards of freshly-formed vapour, partly from the continual rarefactions and condensations of the air under the varying conditions to which it would be sub- jected through the continual changes of the watery en- velope. For at every change from the form of pure aqueous vapour to the cloud-form, an enormous amount of heat would be developed, while corresponding quan- tities of heat would be withdrawn in vapourising other masses of watery matter. The depth of the atmo- spheric region throughout which these stupendous pro- cesses were continually in progress must far have ex- ceeded the depth of the cloud-regions of our own atmosphere. For the same heat which prevented the water from resting on the earth's surface must have pre- vented the heavier rain-clouds from approaching within many miles of that surface without being turned into pure aqueous vapour. Again, not only would the layer of rain-clouds, thus raised many miles above the earth's surface, be also many miles in depth, but the heat pre- vailing throughout the layer would in turn prevent a WHEN THE SEA WAS YOUNG. Sf layer of cumulus clouds from being formed, except at a great height above the rain-cloud layer. In like manner the cirrus or snow-cloud layer would be raised high above the layer of the cumulus clouds. And each of these layers, besides being separated from the next below by a deep intermediate space of commingled cloud-forms, would also be of great thickness. Hence we may fairly assume that the extreme range of the lightest and highest clouds in that era of the earth's history must have been many miles from the earth's surface, even if the atmosphere then contained no greater amount of matter (other than its watery consti- tuents) than at present. But we have reason for believing that, besides the oxygen and nitrogen now present in the air, there must have been at that remote era enormous quantities of carbonic, chloric, and sul- phurous gases besides an excess of oxygen ; and all these, with the aqueous vapour (alone far exceeding the entire present atmosphere of the earth), expanded by a tremendous heat. This heavily -loaded atmosphere must therefore have extended much farther, we may even say many times farther, from the earth than her pre- sent aerial envelope. It is not at all unlikely that the outermost part of the cloud-envelope was then several hundred miles from the earth's surface, itself raised, through the expansive effects of heat, many miles above the level it was to assume when cooled. In attempting, indeed, to conceive the effects produced by that tremendous heat with which, most certainly, the whole frame of our earth was once instinct, we are far 88 THE POETRY OF ASTRONOMY. more likely to fall short of the reality than to exceed it, partly because the physical processes concerned are so far beyond our ordinary experience, but much more because they operated on so inconceivably vast a scale. While it cannot but be regarded as certain (that is, as not less assured than the theory of cosmical develop- ment itself) that during a very remote and long-lasting period the water now forming our seas surrounded the earth in the form of mixed vapour and cloud, yet this consequence of the development theory, however certain, is so remarkable that one would wish to see it confirmed, if possible, by some evidence derived from actually ex- istent worlds. Now as the various orbs peopling the universe occupy all regions of space, so they must pre- sent all the various phases through which each orb has to pass with the progress of time. It would be absurd to suppose, for instance, that every star (that is, every sun) peopling space is passing through exactly the same period of sun-life as our own sun, no less absurd to suppose that every planet is passing through the same period of planet-life, or each moon through the same period of moon-life. But it is in reality seen to be as absurd, when once we open our eyes to the real meaning of the astronomy of our day, to suppose that among the millions of millions of bodies which exist even in that mere corner of space which is measured by the range of our most powerful telescopes, there are not illustrations of every stage of the exist- ence of worlds in space, from the first known to us, the vaporous, to the sun-like, and thence through all the WHEN THE SEA WAS YOUNG 89 forms of world-life down to the stage of absolute refri- geration or planetary death. Some among these varie- ties must exist within the solar system, and therefore admit of being telescopically examined, unless we sup- pose that by some amazing accident all the members of the solar system are passing through the same exact stage of world-life. But this, though it is the theory commonly accepted (because of a species of mental in- dolence which makes the most uniform theory appear of easiest acceptance), is in reality the most glaringly improbable, or rather the most utterly impossible, theory it ever entered the heart of man to conceive. It is as though one who knew that a number of ships, unequal in size and power, had set out at different times from various ports on long sea-journeys, should assume, as the most probable opinion respecting their position at any time selected at random, that they were riding all abreast upon the long crest of some great ocean roller. But regarding the planets of the solar system as presumably in various stages of world-life, according to what law may we expect to find them ranged in point of age ? May we take the outermost as the oldest, and the innermost as the youngest ? According to the de- velopment theory conceived by Laplace, we might do so ; though even then the various ages assigned to the several planets would only be arranged in the order of their actual antiquity, not with reference to the youth, maturity, and decadence of planetary life. A planet younger than another in years might be older in deve- lopment ; just as an animal twenty years old might be 90 THE POETRY OF ASTRONOMY. aged, while another thirty years old might scarcely have reached maturity. Moreover, it begins to be recognised that Laplace's theory of the formation of our solar sys- tem from without inwards does not present the whole truth, even if it presents the most characteristic feature of the system's process of development. Other pro- cesses have been at work, and even still continue to be at work, which may have helped to complete the fashion- ing of interior planets while outer planets still remained unfinished. Indeed, it is more than suspected that Jupiter may still be growing, and that Saturn may not even have assumed his final planetary form. 1 But undoubtedly the most important consideration is the first mentioned. Among planets so unequal in size and mass as those of the solar system it cannot be but that the duration of planet-life and of its several periods must differ very largely. If all the planets, then, had been fashioned simultaneously, they would now have reached very different stages of progression. Not only so, but even enormous differences in the epochs of planet- ary formation would probably be more than cancelled by these varieties in the rates of growth and development. Shall we, then, take quantity of matter as the main guide for determining the relative duration of planetary life and of its various stages ? Experiment will readily show whether and to what degree such a guide might 1 Something of this sort is hinted at by Laplace himself, when he says of Saturn's rings that they seem to him to be 'des preuves toujours subsistanles de Textension primitive de 1'atmosphere de Saturne, et de ses retraites successives.' WHEN THE SEA WAS YOUNG. gi be trusted. It is manifest that the chief question to be determined is the relative rate of planetary cooling through the various stages, from the time when a planet is a mere mass of vapour, down to the time when its whole substance is entirely refrigerated. Suppose, then, we take two globes of iron, one two inches and the other one inch in diameter, and, heating them both to a red heat in the same fire, set them aside to cool. From the result we can form an opinion whether the larger or smaller of two similar and similarly heated orbs will cool the more quickly, or whether size has little or no influence on the rate of cooling. The result of the experiment leaves us no room for doubt on this point. Long after the smaller globe has ceased to glow the larger still shows its ruddy lustre, while a still longer interval separates the time when the smaller globe can be handled from the time when the larger has cooled down to the same extent. We infer, then, that size, or rather quantity of matter, most importantly affects a body's rate of cooling. Indeed, a little consi- deration shows that this might have been expected. For a body can only part with its heat from its surface. Now the surface of the larger globe in our experiment is four times as great as that of the smaller, and there- fore the larger gives out moment by moment four times as much heat as the smaller, when both are at the same temperature ; but the larger has eight times as much matter in it as the smaller, and therefore eight times as much heat to part with, both starting from the same temperature. Naturally, therefore, since the 92 THE POETRY OF ASTRONOMY. larger, with eight times as much heat to give out, ex- pends that supply only four times as fast, the heat sup- ply of the larger lasts longest. We should expect the supply to last about twice as long ; and, but for some minor considerations which affect the practical carrying out of the experiment, that would be the relative dura- tion of the heat-emission from the two globes. Only of course it does not follow that the test by touch would correspond with the law here indicated, for the surface of a metal globe may be cool enough for handling while the interior is still exceedingly hot. It is, indeed, the consideration last indicated which prevents the careful student of science from accepting as demonstrated certain conclusions which have been somewhat confidently advanced respecting the time re- quired by our own earth for cooling down to its present condition. The experiments of Bischof, for example, upon basalt have been quoted as showing that our globe would require 350 millions of years to cool down from 2,000 to 200 Centigrade, and the process has been re- ferred to as if it were long since completed, so that that period certainly might be reckoned as belonging to the earth's past ; yet an enormous portion of the earth's globe may still possess a degree of heat between those limits, and possibly nearer to the higher limit than to the lower. Yet while it is in our opinion an altogether hopeless task to attempt to deduce absolute time-measures, either experimentally for the determination of our earth's an- tiquity, or theoretically for the comparison of other WHEN THE SEA WAS YOUNG. 93 planets' development with hers, we can nevertheless very confidently infer that some planets must be far less advanced than the earth towards planetary maturity, and that others must have passed beyond such maturity to extreme old age, if not to decrepitude or even to planetary death. When we consider, for instance, that the quantity of matter in Jupiter exceeds three hun- dred-fold that in our earth r s globe, we cannot doubt that the stages of Jupiter's existence as a planet must exceed the corresponding stages of the earth's exist- ence many times in duration. We cannot argue, indeed, directly as follows, as some have done : Since Jupiter contains three hundred times as much matter as the earth, the globe experiment described above shows that Jupiter would take nearly seven times as long as the earth in completing any given stage of planetary cooling, for if one globe contains three hundred times as much matter as another it will exceed this other nearly seven times in diameter. Nor can we proceed to argue that, since Bischof's experiments indicate 350 millions of years for one stage of the earth's cooling, Jupiter would require more than 2,350 millions of years for that stage, and so must be at least 2,000 millions of years behind the earth in development, from the consideration of that stage alone, and probably some 10,000 millions of years behind the earth alto- gether, in such sort that some 10,000 millions of years hence Jupiter will be in the same stage of planetary existence that our earth is now passing through. The definiteness of such statements as these makes them 94 THE POETRY OF ASTRONOMY. more attractive to many than more general statements, but they cannot be relied upon. All that can be safely alleged and manifestly so much can be safely alleged is that planets like Jupiter and Saturn, exceeding the earth enormously in quantity of matter, must have required far longer periods of time for the various stages of planetary development, and must consequently be as yet far less advanced towards planetary maturity. It follows, equally of course, that bodies like Mars, Mercury, and the Moon, as well as the moons of Jupiter, Saturn, and Uranus, being so much less than the earth in mass, must require much less time for the various stages of their development, and may be regarded as having probably long since passed the era corresponding to that through which our earth is now passing. 1 It would be, therefore, to Saturn and Jupiter that the telescopist would turn for indications of the existence of ocean waters in the state wherein our own ocean must once have existed. Instead of holding the opinion, commonly expressed in our books of astronomy, 1 Only it is to be noted that the smaller the orbs considered the smaller the periods of their existence, and the less, therefore, the probability that differences so arising would cancel differences in the actual epoch of first formation. For instance, suppose that the above reasoning about Jupiter could be relied upon in points of detail as well as in its general sense. Then we see that a difference of no less than 2,000 millions of years comes in as affecting one stage only of the history of that planet and of our own earth ; but if instead of comparing our earth with Jupiter, containing three hundred times more matter, we compared her with an orb which she exceeded in the same degree, we should find that the smaller orb would require about 25 millions of years for the stage which lasted 350 millions of years in the earth's case a difference of only 275 instead of 2,000 millions of years. WHEN THE SEA WAS YOUNG. 95 that, unless very strong evidence is presented to the contrary, other planets ought to be regarded as pro- bably like our earth, we ought (at least if we accept, as every astronomer does, the doctrine of cosmical evo- lution) to expect to find Jupiter and Saturn in some far earlier stage of planetary existence, and only on the strength of absolutely overwhelming evidence to admit the possibility that they may resemble the earth. Seeing, however, that every particle of evidence yet obtained respecting those planets favours the belief that they are in that early stage of development in which we should expect to find them, while many parts of the telescopic evidence are such as cannot possibly be interpreted on any other theory, it would seem to be only by an amazing effort of scientific conservatism that the old view, originally incredible and opposed by all the telescopic evidence, is retained in our books of astronomy, as though it had been the subject of some such demonstration as Kepler gave of the laws which bear his name, or Newton of the laws of gravity. Without entering here at length into the evidence relating to the age of the planets Jupiter and Saturn, or rather to their present stage of development, we shall consider how their appearance corresponds with that which the earth must be supposed to have pre- sented when the waters now forming her oceans enve- loped her in the form of commingled vaporous and cloudy masses. We have seen that at that remote epoch the earth must not only have been completely cloud-enwrapped, 96 THE POETRY OF ASTRONOMY. but that the outermost of her cloud-layers must have been raised hundreds of miles from her real surface. Measured, then, by an observer on some other planet, her apparent dimensions would then have been far greater than at present, for her outermost cloud-layer would be measured, not her true body. Thus judged, then, to have a much greater volume than she really has, she would be regarded (supposing her total mass to have been determined, as it might readily have been, from the motions of her moon) as having a mean den- sity much less than that of her actual globe. How much less we do not know, because we cannot deter- mine the extent to which her own frame would be expanded, her atmosphere swollen, and the various cloud-layers floating in it thrust away, so to speak, from her intensely heated surface. But it may well be believed that her apparent diameter would be so increased that (her volume being increased necessarily in a much greater degree) her estimated density would be much less than her present density. Now this pre- cisely corresponds with what we find in the case of Saturn and Jupiter, each of these planets having a very small density compared with the earth's, though the tremen- dous attractive power residing in their enormous globes would, if unresisted, lead to a high degree of com- pression, and therefore to great density. The evidence afforded by the spectroscope renders it highly impro- bable that these planets are formed of other substances than those forming the earth, or of the same substances in very different proportions. We know that the WHEN THE SEA WAS YOUNG. 97 attractive energy of these planets' masses must act out yonder precisely as the energy of our earth's mass acts throughout her frame. Experiments assure us that no cavities can possibly exist in the interior of a planet, so that Brewster's ingenious attempt to account for the small density of Saturn and Jupiter, by supposing these planets to be but hollow shells, fails altogether to remove the difficulty. There remains, then, only the supposition that these planets' attractive energies are in some way resisted, and the natural effect of those energies, extreme compression, prevented. And we find just the required explanation in the theory (to which we had been already led on a priori grounds) that these planets are still young and therefore in- tensely hot, the waters one day to form them being thus raised into their atmospheres, enveloping the planets in enormously deep and complex layers of mingled cloud and vapour, the planets' real globes lying far within these cloud- envelopes, and being also themselves greatly expanded by the tremendous heat with which their substance is instinct. Not only is this the only available explanation of the small density of the planets Jupiter and Saturn, but it is a manifestly sufficient explanation. It is next to be noticed that certain very striking phenomena would result from the great depth of the earth's vapour-laden and cloud-laden atmosphere, dis- turbed not only by tremendous hurricanes moving horizontally, but also by vertical movements of great energy and velocity. Conceive the descent of vast 98 THE POETRY OF ASTRONOMY. sheets of water towards some intensely-heated portion of the earth's surface, and the effect of their rapid con- version into vapour. The mass of vapour thus formed, being much lighter than the surrounding atmosphere, would rise just as heated air from a chimney rises in the surrounding cooler and therefore heavier air ; only with much greater rapidity, because the vapour of water is far lighter than heated air, and the atmo- sphere of the remote period we are considering was far denser than our present air. The mass of vapour would rush upwards to an enormous height in a very short time, and, coming from a region relatively near the centre of the earth to a region farther away, it would be affected by the difference in the rate of rotational movement at these different levels. For instance, at the present surface of the equator the movement due to rotation has a velocity of rather more than a thousand miles an hour, while at a height of a hundred miles above the surface the air is carried round with a velocity twenty-five miles greater per hour. If, then, a body or a mass of vapour were shot upwards from the equator to a height of a hundred miles, it would, while at that height, lag behind the surrounding parts of the air, and, in fact, would travel backwards at the rate of twenty-five miles an hour. If the matter propelled upwards were vaporous, and when at the higher level became condensed into cloud, a trail of clouds would be formed along a lati- tude-parallel, and, as observed from some other planet, the earth would appear to be girt round by a whitish WHEN THE SEA WAS YOUNG. 99 band parallel to the equator. The deeper the envelope of mixed vapour and cloud, the more readily would such bands form; and remembering the tremendous energy of the causes at work, the whole frame of the earth glowing with intensest heat, and keeping the whole mass of water now forming our oceans in the form of mixed cloud and vapour, we cannot doubt that well-marked belts must almost at all times have existed in the earth's cloud-envelope. The earth, then, would have appeared as a belted planet, resembling the planet Jupiter (or Saturn without his rings), but on a minia- ture scale. It is, indeed, common enough to find the belted aspect of Jupiter and Saturn compared with the probable present aspect of the earth, because of the existence of a zone of calms near the equator, bounded on the north and south by the trade-wind zones, and these in their turn by the zones of the counter-trades. But there is not the slightest reason for supposing that these so-called zones could be recognised by an observer viewing the earth from without. Still less reason is there for supposing that they would, even if recognis- able, resemble in the remotest degree the well-defined bands surrounding the globes of Saturn and Jupiter. Such as they are, too, they would be found obeying the influence of the sun as the ruler of the day and also of the seasons ; they would be also limited to sea-covered regions ; and, in fine, they would correspond much more nearly with the appearances presented by the planet Mars (where occasionally for a few hours por- tions of bands, not complete zones, are seen across the H 2 100 THE POETRY OF ASTRONOMY. Martian seas) than with anything shown on the discs of Jupiter and Saturn. What we see on these giant planets corresponds closely, however, with what we should expect to find in the case of planets whose vapour-laden and cloud-laden atmospheres are so deep as to form a considerable portion of the disc seen and measured by astronomers. For the belts of these giant planets show no dependence whatever upon the progress of day and night, or of the long years of Saturn and Jupiter, but behave in all respects as if generated by forces residing in the planets them- selves: their well-defined shapes also corresponding exactly with what we should expect from the mode of formation indicated above. But, returning to the earth, it is manifest that cloud-belts formed in the way we have described would not be permanent. Sometimes they might continue for several weeks, sometimes perhaps even for months ; but frequently they would be formed in a few hours, and last but for a few days, or not even, perhaps, for an entire day. So that the belts of the planet earth, viewed in those times from some remote world, would present changes of appearances, sometimes occurring slowly, sometimes rapidly. Now this precisely corre- sponds with what is observed in the case of the belted planets Jupiter and Saturn. Sometimes the belts remain, though undergoing constant changes of form, for weeks or months together, while sometimes they vanish very soon after their formation. Again, it is clear that other changes than the form- WHEN THE SEA WAS YOUNG. IOI ation or dissipation of cloud-belts would affect the deep cloud-laden atmosphere of the planet. Hurri- canes and tornadoes would rage from time to time, and sometimes for long periods together, in an atmosphere where processes of evaporation and condensation, with all the rapid variations of temperature occasioned by them, were continually taking place on a scale compared with which that of the most tremendous tropical storm on the earth in our time is utterly insignificant. The effects of such hurricanes and whirling storms would be visible from without through the displacement of the great cloud-masses forming the belts. Sometimes cyclonic storms would produce great circular openings in the cloud-belts, through which the darker depths below would be brought into view. These openings would be visible from without as dark spots on the lighter background of the belts. At other times the uprush of columns of heated vapour, condensing as soon as it reached the higher regions of the planet's atmosphere, would cause the appearance (to an observer outside the earth) of rounded masses of cloud, which, because of their strong reflective power, would seem like spots of white upon the background even of a light belt, and show still more markedly if they appeared above one of the dusky bands corresponding to lower cloud-levels. And besides changes due to great dis- turbances and rapid movements in the cloud-envelopes, the changes resulting from evaporation and condensa- tion proceeding quietly over extensive portions of these cloud-regions, would be discernible from without. The 102 THE POETRY OF ASTRONOMY. observer would see dark spaces rapidly forming, where some higher cloud-mass which had been reflecting the sun's light brightly, evaporated, and so allowed part of a lower cloud-layer to be seen. Where the reverse pro- cess took place, large masses of transparent aqueous vapour rapidly condensing into cloud, the formation of bright spots would be observed. How closely all this corresponds with what now takes place in the deep vapour-laden atmosphere of Jupiter, will appear from the following account by South of the appearance and rapid disappearance of an enormous dark spot on one of the belts of Jupiter : ' On June 3, 1839, 1 saw with my large achromatic, immediately below the lowest [edge] of the principal belt of Jupiter, a spot larger than I had seen before ; it was of a dark colour, but certainly not absolutely black. I estimated it at a fourth of the planet's " longer" diameter. I showed it to some gentle- men who were present ; its enormous extent was such that, on my wishing to have a portrait of it, one of the gentlemen, who was a good draughtsman, kindly undertook to draw me one ; whilst I, on the other hand, extremely desirous that its actual magnitude should not rest on estimation, proposed, on account of the scandalous unsteadiness of the large instrument, to measure it with' a telescope five feet in length. ' Having obtained for my companion the necessary drawing instruments, I went to work, he preparing him- self to commence his. On my looking, however, into the telescope of five feet, I was astonished to find that the large dark spot, except at its eastern and western WHEN THE SEA WAS YOUNG. 103 extremities, had become much whiter than any of the other parts of the planet, and ' in thirty-four minutes from the first observation, ' these miserable scraps ' (that is, the two extremities of the original spot) ' were the only remains of a spot which, but a few minutes before, had extended over at least 22,000 miles.' Again, Webb, in his singularly useful little treatise, ' Celestial Objects for Common Telescopes,' thus describes certain small whitish spots seen for a time on the planet's dusky belts : Recently, c minute white roundish specks about the size of satellites ' have been seen < on the dark southern belts. Dawes first saw them in 1849 ; Lassell in 1850, with his Newtonian reflector, two feet in aperture. Dawes has since given several striking drawings of them,' and they have been seen with a nine-inch telescope by Sir W. K. Murray, in Scotland. 6 They are evidently not permanent. Common tele- scopes have no chance with them, or with similar traces which Lassell has detected (1858) on the bright belts.' But, indeed, many pages might be occupied with the account of appearances on Jupiter's belts, indicating the progress of changes such as could not be looked for except in the case of a planet enveloped by an exceed- ingly deep atmosphere laden with enormous masses of cloud and vapour. In the case of Saturn such appear- ances are less often and less clearly recognised, doubt- less because the planet lies so much farther away. For it should be remembered, in comparing the accounts which observers give of the two planets, Jupiter and Saturn, that these orbs are studied under very different. 104 THE POETRY OF ASTRONOMY. conditions, a telescope nearly twenty times as powerful being required to show Saturn as to show Jupiter with equal distinctness. One circumstance seems to us to merit attention here, of which, so far as we know, no explanation has ever yet been attempted. There is sometimes to be observed along the belts of Jupiter, and in particular along the great equatorial belt, a certain regularity of marking, giving to the belt affected by it somewhat of the appearance of a ring marked with a series of regular elliptical mouldings ; or, to use Webb's description, the belts throw out dusky loops or festoons, < whose elliptical interiors, arranged lengthwise, and sometimes with great regularity, have the aspect of a girdle of luminous egg-shaped clouds surrounding the globe.' ' These oval forms,' he proceeds, ' which were very conspicuous in the equatorial zone (as the interval between the belts may be called) in 1869-70, and of which the vestiges still remain (in 1872-73), have been seen in other regions of the planet, and are probably of frequent re- currence. It is by no means easy to assign a reason for this prevalent configuration, which sometimes shows itself in a solitary ellipse, seen by Gledhill and Mayer in 1869-70.' Several considerations suggest themselves when we study these peculiarities thoughtfully. First, the enormous size of these oval cloud-masses indicates that they are formed in a very deep atmosphere they have a length and breadth often of nine or ten thousand miles, and sometimes (as in the case of the great solitary oval seen by Mayer and Gledhill) the extreme WHEN THE SEA WAS YOUNG, 1 05 length of an oval cannot, after every allowance for possible exaggeration in the drawing, be computed at less than 30,000 miles. The regularity of their shape indicates that they are due to the operation of some cause at work below, and whose action, extending all around some central region, leads to a regular form, having, like the oval, a centre of symmetry. But the enormous size of the ovals indicates that the centre of disturbance must lie very deep down. One cannot, indeed, fairly estimate its probable depth at less than thousands of miles. Now, if we ascribe each of the oval clouds, seen when a belt looks like a girdle of egg- shaped mouldings, to a region below the cloud-stratum, we should have to suppose a girdle of such regions ; in other words, that the real surface of the planet was not only zoned by such regions of disturbance, but the zone divided regularly up into equidistant regions of disturb- ance alternating with regions of calm. This theory is not only improbable in itself, but, since we have seen that the existence of belts of cloud arises from the lagging of cloud-masses thrown up from lower depths, we perceive that there is no reason for supposing the real surface of Jupiter to be divided zone-wise, still less for supposing the zones to be at any time divided regularly along their length. The cloud-masses lying along different parts of a zone come thus to be regarded as owing their position, not to the position of the region of Jupiter's real surface immediately underlying them, but to the time when the vapours forming them were carried upwards from the neighbourhood of the true IO6 THE POETRY OF ASTRONOMY. surface. A regular series of oval cloud-masses, then, would be explained simply as a series which had been formed over one and the same part of Jupiter's true surface, but at successive equal intervals of time, the causes leading to the upthrowing of the vapour being alternately active and quiescent. Now, we know that such uniform, or nearly uniform, alternation of activity and rest is a phenomenon frequently to be observed in terrestrial phenomena, and very readily to be explained. For the energetic action of any particular process in nature will bring about, by its very energy, the action of the reverse process, which, again, will bring the former into work, the two alternating with gradual diminution of intensity, just as a pendulum swung in one direction is by that very motion caused to swing in the opposite direction, then back again, until gradu- ally the alternate motion is brought to an end. 1 So that this explanation of the occasional regular disposi- 1 We see an interesting astronomical illustration of such alter- nate action in the formation of successive envelopes around the head of a comet. These are generally seen to be arranged with great uniformity, envelope within envelope, separated by well- marked interspaces of transparent matter ; and they rise gradually from the nucleus, the outer envelopes disappearing, and new en- velopes forming within. Now, the formation of the visible envelope implies a process of one kind (possibly condensation), while the 1 ransparent space between indicates a process of the reverse kind (possibly evaporation) ; so that the regular arrangement of en- velopes and spaces shows that there must be an alternation of these processes at nearly uniform intervals. And though the forces causing either process are, so far as we can perceive, at work all the time, we can quite readily understand how first one, then the other, prevails, each by its very prevalence for a while bringing about conditions favourable to the prevalence of the other. WHEN THE SEA WAS YOUNG. IO/ tion of enormous oval cloud-masses in a zone girdling the whole frame of Jupiter, while corresponding well with conclusions to which we had been already led, is far simpler and better in accordance with observed phenomena than the idea of a series of equidistant centres of disturbance around a zone of Jupiter's real surface. It should be added, as in our opinion placing the real nature and method of formation of Jupiter's belts beyond a peradventure, that the cloud-surface in different latitudes of the planet's globe turns round at different rates, the equatorial portion moving fastest. This, of course, could not be the case if we saw any- where the real surface of the planet, or even if the depth of its atmosphere were small in proportion to the planet's apparent diameter. Next we may note yet another remarkable feature which the earth must have presented to observers on other worlds during the first stage of our ocean's history. With an atmosphere so deep as she then had, in which many layers of cloud were floating at various depths, it could not but happen that from time to time such changes would take place, either by the rapid appearance or by the rapid disappearance of extensive cloud-masses at high levels, that her shape would seem to be distorted. Indeed, this is only supposing that from time to time high cloud-layers formed or vanished in a part of the earth's atmosphere chancing at the moment to form a portion of the outline of her visible disc, instead of forming part of a belt in the mid por- tions of the disc. Accordingly, to an observer viewing 108 THE POETRY OF ASTRONOMY. the earth from without, her shape would not always appear perfectly circular, or rather of that figure almost circular, but very slightly elliptical, which in those re- mote times, as now, must have corresponded to the proportions of her real globe. Cloud-layers floating very high in the earth's extensive atmosphere would cause her disc to bulge out slightly but perceptibly, if they chanced to be so placed as to form the outline of that disc, while regions where for a while the higher layers were wanting would (under the same circum- stances) appear slightly depressed below the mean out- line of the disc. It might very well happen that these irregularities would usually be too minute to be de- tected ; that effect called irradiation, which slightly expands the apparent outline of every bright object seen on a dark background, would go far to hide such peculiarities. Yet sometimes they would be too marked, probably, to escape notice, supposing only the observer's station were well placed for the observation of the earth ; as, for instance, if at that remote time there were creatures living on the moon, and able to examine the earth from that convenient distance. Especially when it chanced that raised portions of the earth's outline lay between two depres&d portions, or a depressed portion between two raised portions, the observer would have a good opportunity of recognising the irregularity so resulting. He would perceive in one case that the outline had two somewhat flattened parts with a sort of corner between them, while in the second case there would be flattening between two WHEN THE SEA WAS YOUNG. 109 corners. Of course, in neither case would the corners or the flattened parts be well marked ; they would, in fact, only be just discernible by the most scrutinising observation. It might, however, have happened at times that whole zones of cloud-layers would lie higher than usual, while adjacent to them were zones where only the lower cloud-layers were formed for the time being. During such periods the whole disc would appear out of shape, at least to very keen vision. Now, precisely such peculiarities have been recog- nised in the case of Jupiter and Saturn, the two planets which, as already seen, we should expect from a priori considerations to be in the cloud-enveloped condition, and whose exceedingly small mean densities compel us either to believe that they are so, or else to adopt the conclusion that they are framed of materials quite different from those constituting our own earth. For that careful observer Schroter, the contemporary, and in some orders of observation the rival of Sir W. Herschel, notes that at times he could not but suspect that the outline of Jupiter was imperfectly rounded, being in places slightly flattened. 1 In the case of 1 It may, indeed, be noticed as remarkable that such a peculi- arity, if it exists, h&s not been more commonly observed ; but in reality it would be very readily overlooked and might even be altogether imperceptible with many telescopes superior to Schroter's. It was but a few years ago that certain irregularities of the moon's surface, so extensive as to modify her outline when they chance to be so placed as to form part of it, were detected by Mr. Cooper Key, though the moon must quite often have been observed at times when the peculiarity should have been noticed; and he detected the peculiarity by a process corresponding in fact to the spoiling of his telescope, at least temporarily. It was a silvered- 1 10 THE POETRY OF ASTRONOMY. Saturn, not only have occasional local irregularities been noticed, but the planet has sometimes been observed to be for a time quite markedly out of shape, bulging out in the regions corresponding to the earth's temperate zones, and compressed (relatively) in the equatorial and polar regions. It would be easy to dismiss such observations as due to optical illusion if they had been made by mere amateurs. But Schroter was no amateur telescopist : few ever surpassed him in skill, and none in zeal and patience. The peculiarity in Saturn's figure, again, was first observed by Sir W. Herschel when at the height of his fame as a telescopist ; and it has since been observed by such astronomers as Sir J. Herschel, Airy, the Bonds of Harvard (than whom no better observers ever lived), Coolidge, and many others, while the practised and certainly not imaginative workers at Greenwich Observatory have recorded, in the account of their year's work, that ' this year Saturn has from time to time assumed the square-shouldered aspect.' It is impossible to reject such testimony, though beyond all question the normal condition of Saturn is not the ' square-shouldered,' as some have supposed. It is certain, from multiplied observations and measurements, that Saturn usually presents the glass reflector ; and he removed the silvering, so that the glass itself reflected the rays, but much less perfectly, of course, than the polished silver. He thus had a much fainter image of the moon, and, the effects of irradiation being removed, the flattening at the edge of the disc could be recognised. It is so great, when the moon is in one particular position, as to give two flat edges which would form sides of a twelve-sided polygon if the rest of the disc's outline were similarly shaped. WHEN THE SEA WAS YOUNG. Ill figure of a perfect ellipse, flattened like the earth at the poles, but in far greater degree. It is equally certain, therefore, that the square-shouldered aspect is but an occasional peculiarity. It is explained quite simply and naturally when we regard Saturn's real globe as deep embosomed within his cloud-laden atmosphere a view of the planet (we again and again repeat) which a priori considerations, as well as his exceedingly small apparent density, absolutely force upon us. On the other hand, those who reject as utterly incredible, or at least sensational, the belief that the giant planets are passing through a stage of planetary existence through which our earth has certainly passed, insisting on regarding all the planets as in the same stage of their existence notwithstanding the enormous a priori probabilities against such a supposition, are not only compelled at the very outset to adopt the opinion that Saturn and Jupiter must be formed of materials altogether unlike those constituting our earth a view much more opposed to their theory of general resem- blance than the one we have here indicated but when observations such as those we have been describing are brought under their notice they are compelled either to reject them as optical illusions (an explanation which will account for anything), or else to adopt the conclu- sion that disturbances have taken place in the solid framework of a planet compared with which the most tremendous earthquakes would be the merest child's play. Thus their very preference of observation to theory, and of the ordinary to the sensational, forces 112 THE POETRY OF ASTRONOMY. them in this case either to reject multiplied observa- tions as mere illusions, or to adopt a theory of planet disturbance which is not sensational merely, but utterly extravagant and incredible. When our earth's deep atmosphere bore the waters of her present seas floating aloft in the form of vast cloud-masses above her fiery surface, a remarkable peculiarity of appearance must occasionally, though perhaps only as a rare phenomenon, have been observ- able. Suppose that while a telescopist on Venus or Mercury was contemplating the earth, one of those rapid changes described in the preceding part affected cloud-layers forming the earth's visible outline at the moment of observation. The earth's apparent figure would then not only be distorted by the change, but the actual progress of the change would take place under the observer's eye. Most probably no change of the kind could have been detected by direct observation, many circumstances with which telescopists are familiar rendering an observation of the kind peculiarly difficult. But supposing the observer to have watched the earth when the moon was about to pass in transit across her face, and that the moon appeared at the moment close to that part of the earth's outline where such changes were taking place ; then it would be possible, on account of this favourable conjuncture, to recognise the change of outline. For instance, if the apparent outline chanced to be raised above its usual position when the moon was very close, the two outlines that of the moon and that of the earth would seem to be in WHEN THE SEA WAS YOUNG. 113 contact before they really were ; but if, just at that time, the high cloud-layer which formed the raised part of the earth's outline were rapidly to disappear, then her outline would shrink in that place, and no longer appear to touch the moon's. Or again, it might happen that an observer of the moon, watching the great globe of the earth as it moved over the star- strewn heavens, would see its outline pass over and conceal some conspicuous star, but in a few minutes perceive the star reappearing outside the same part of the earth's outline. The observer would then know that the outline must have shrunk. In these and like ways observers outside the earth might in those remote times have seen the evidence of very active processes of change taking place in her deep cloud-laden at- mosphere. Now appearances such as these cannot be expected to occur frequently in the case of Jupiter or Saturn. The changes themselves which could alone produce them are infrequent, and the conditions under which the changes could alone be detected occur but seldom ; so that the chance of a change occurring just where and when it could be detected are very small indeed. Yet in one case certainly astronomers have detected j ust such a change in the outline of Jupiter. It would be difficult nay, we venture very confidently to say that it is impossible otherwise to explain what is described by the late Admiral Smyth, one of the most careful and skilful of modern astronomers : ' On Thurs- day, June 26, 1828,' he says, ' the moon being nearly i 1 14 THE POETRY OF ASTRONOMY. full and the evening extremely fine, I was watching the second satellite of Jupiter as it gradually approached to transit its [the planet's] disc. My instrument was an excellent refractor, of 3 inches aperture, and 5 feet focal length, with a power of 100. It appeared in con- tact at about half-past ten, by inference, and for some minutes remained on the edge of the limb ' (that is, on the outline of the disc), l presenting an appearance not unlike that of the lunar mountains coming into view during the first quarter of the moon, until it finally disappeared on the body of the planet. At least twelve or thirteen minutes must have elapsed, when, acci- dentally turning to Jupiter again, to my astonishment I perceived the same satellite outside the disc. It was in the same position,' as to level, ' where it remained distinctly visible for at least four minutes, and then suddenly vanished.' This narrative is so surprising, even when explained in the simple manner which our theory of Jupiter's condition suggests, and still more so on the usual theory of Jupiter's condition, that it may be well to pause for a moment to inquire whether there may not have been some mistake. Admiral Smyth was a skilful observer, as we have already stated. His state- ment alone would have great weight. Still one may admit the bare possibility of an optical illusion, similar to what is described in Brewster's ' Natural Magic,' the satellite seen after the immersion being a mere trick of the mind, a ' blot on the brain which would show it- self without.' Smyth himself supposed so, for he says : WHEN THE SEA WAS YOUNG. 115 * As I had observed the phenomena of Jupiter and his satellites for many years, without any remarkable irre- gularities, I could not but imagine that some optical or other error prevailed, especially as the satellite was on this ' (i.e. the hither) < side of the planet.' And pro- bably the phenomenon thus dismissed by Smyth him- self would not have been heard of, but for the fact that two other observers chanced to witness it. ' A few days afterwards,' proceeds Admiral Smyth, * I received a letter from Mr. Maclear, Biggleswade, informing me that he had also observed the same, but that he had considered it a " Kitchener's wonder " ' (old Kitchener, the telescopist, having been apt to recount every optical illusion by which he was perplexed as a real pheno- menon). ' And about the same time,' adds Smyth, * Dr. Pearson, having favoured me with a visit, asked me whether I had noticed anything remarkable on the 26th ; for that he had, in accidentally looking at Jupiter, seen the second satellite reappear ! Here, then, were three observers, at distant stations, with telescopes of different apertures, all positive as to the extraordinary deviation from rule. It may be borne in view that Biggleswade is twelve miles from Bedford ' (the place of Smyth's observatory ; and South Kilworth, Dr. Pearson's residence, is thirty-five). Mr. Maclear's telescope was rather smaller than Admiral Smyth's ; while Dr. Pearson's was a much more powerful instru- ment, twelve feet long, and nearly seven inches in aperture. ' Explanation,' calmly remarks Mr. Webb, in speaking of this phenomenon, c is here set at defiance ; i 2 Il6 THE POETRY OF ASTRONOMY. demonstrably neither in the atmosphere of the earth nor Jupiter; where and what could have been the cause ? At present we can get no answer.' But it is not the part of the true student of science thus to resign the attempt to explain a phenomenon merely because it is unusually perplexing. In this case we can reason directly from the observed fact to its inter- pretation, apart from those a priori considerations which in the present essay have led us to regard such a phenomenon as one to be looked for in Jupiter's case. First, the observation was certainly not an optical illusion, for three persons made it independently; secondly, it was demonstrably not due to terrestrial atmospheric causes, for it was seen from three stations far apart ; thirdly, it was demonstrably not caused by any action of Jupiter's atmosphere on light proceeding from the satellite, for the satellite was between Jupiter and the observer ; fourthly, the satellite cannot really have stopped, gone back on its path, and then resumed its onward course, unless the laws of nature were sus- pended a theory we may dismiss in a scientific inquiry ; for a similar reason, fifthly, we may dismiss the idea that the whole mass of Jupiter moved in abnormal fashion. There remains only one possible interpreta- tion viz. that the outline of Jupiter's disc had changed in position ; in fact, in whatever way we explain hoiu this happened, the observations may be regarded as proving unmistakably that it did happen. Now the supposition that Jupiter's outline altered leaves us still much to wonder at. For let us consider WHEN THE SEA WAS YOUNG. 1 1/ the extent of change necessary to account for what was observed. Smyth may have been mistaken as to the time intervals he mentions in his account, since he does not seem to have taken them from the clock. The interval, which he supposed to have lasted twelve or thirteen minutes, may in reality not have lasted more than five or six ; and the time during which, after reappearing, the satellite continued visible, may not have lasted more than two minutes instead of four, as roughly estimated. But, taking only eight minutes as the total interval between the first and second disap- pearance, we have to account for marvellous changes in the apparent position of the planet's outline. For in eight minutes the second satellite would travel about 4,000 miles, and the outline of Jupiter must have changed by that amount, seeing that at the first disappearance the visual line to the satellite just touched the planet's apparent edge, while at the second disappearance the visual line to the second posi- tion of the satellite, 4,000 miles from the first, touched the planet's edge in its now changed position. Pro- bably the difference was even greater; Smyth's own estimate of the time would make it at least 8,000 miles: but 4,000 miles will be enough to deal with. It is not necessary to suppose that the planet's appa- rent outline, as ordinarily seen, shrank inwards by the whole of this amount. More probably the outline bulged beyond its normal position at the time of the first disappearance, and presently shrank below its normal position, bringing the satellite again into view, Il8 THE POETRY OF ASTRONOMY. and remaining thus depressed until the second dis- appearance had taken place. We may suppose, then, that at the beginning the surface forming the apparent outline was (at the place where the satellite's transit began) about 2,000 miles above the usual mean level, while afterwards it was much below that level. Two thousand miles being less than the fortieth part of the diameter of Jupiter, we can readily understand why even so enormous an apparent expansion or contraction should not have noticeably affected the symmetry of the planet's apparent figure. Indeed, with ordinary telescopic power the outline of Jupiter is so expanded by irradiation, that much greater changes of level would be so far masked as to escape attention. But we are not greatly concerned to reason at this stage as though the theory that the planet's outline changed required to be defended against objections. For it is absolutely certain that the outline must have changed. The visual line to the satellite certainly passed several thousand miles nearer the planet's centre at the time of the first disappearance than at that of the second, yet in both cases touched the apparent outline, which must therefore have shifted by as many thousands of miles, unless the satellite itself had stopped and re- treated, or the whole bulk of the planet had shifted ; neither of which events could occur except by a miracle. Now the changing of the outline, though marvellous, is not miraculous, and, being demonstrably the only non-miraculous interpretation of the observed event, must be accepted as the true interpretation the event WHEN THE SEA WAS YOUNG. 119 itself, observed as it was by three skilled astronomers, having certainly occurred. This being so, the outline of Jupiter having cer- tainly changed for awhile on that particular occasion, which theory, we would ask, should be rejected as fanciful and sensational the ordinary theory, accord- ing to which the solid crust of Jupiter must, after rising 2,000 miles at least, have sunk through 4,000 miles? or the theory that a cloud-layer, floating at least 2,000 miles above the usual level of the highest visible cloud-layer of Jupiter, melted quickly into the form of invisible vapour, and thus a layer lower than usual by as many thousand miles came into view, forming for the time the planet's apparent outline in that place ? According to the first theory, a surface much larger than the whole surface of our earth sank through a depth greater than the whole distance from the earth's surface to her centre. The intense heat which is regarded with such disfavour by followers of the old-fashioned ideas (really based on the Ptolemaic astronomy), if it had had no existence before, would have been generated by so tremendous a downfall, which indeed could not have taken place without vul- canian heat, exceeding in intensity what the other theory presents as the natural consequence of Jupiter's mode of formation. According to this second theory, the rising of the cloud-layer even to so great an eleva- tion as 2,000 miles above the usual level of the highest Jovian clouds, was an exceptional phenomenon indeed, but by no means incredible ; while the rapid dissipa- 120 THE POETRY OF ASTRONOMY. tion of the cloud was not only quite easily to be ex- plained, but corresponded with changes which have been observed to take place among cloud-layers seen on the disc itself. If a vast cloud-layer can disappear in a few minutes from view, above one part of the planet's surface, so also it can above another. One part may chance to lie on the visible disc of the planet ; another may chance to lie on the edge of the disc ; for these parts of the disc only bear relation to our point of view, not to the planet itself ; and while a change occurring in one part would make a belt or spot seem to form or disappear, one occurring in the other position would make the apparent outline of the planet seem to bulge or shrink, as the case might be. Nay, we may add one consideration which would render the dissipation of a high cloud-layer in the position where Jupiter's outline appeared swollen, even more naturally to be accounted for than the often observed dissipation of a cloud-layer on the disc itself. For the cloud-layer which vanished on that occasion had just been carried into sunlight by the planet's rotation ; and we can readily understand how the solar heat, slight though its effects may be compared with those of Jupiter's own internal heat, might bring about the dissolution of a cloud-layer which chanced to be in that critical stage where a slight cause will bring about either rapid formation or rapid dissipa- tion of visible cloud. The chief difficulty, of course, in the theory, or rather the most surprising result of the demonstrated fact that Jupiter's visible cloud-layer thus changed, re- WHEN THE SEA WAS YOUNG. 121 sides in the enormous depth we have to assign to the cloud-supporting atmosphere. We have already shown in these pages l that, cceteris paribus, the atmosphere of Jupiter would be much shallower layer for layer than our earth's, simply because the planet's mighty attractive power would more strongly compress it. That it is manifestly not thus compressed indicates, as we then showed, the intensity of the heat pervading its whole extent. But that it should range to a height of thousands of miles above the true surface of the planet, does certainly seem at first amazing. Yet be it remem- bered that not only is such an inference demonstrably correct, as we have just shown, but it also follows neces- sarily from the comparison already instituted between Jupiter and the earth in respect of mass and density. If we assign to the solid globe of Jupiter the same mean density as the earth has or, rather, if we imagine the totality of material, whence millions of years hence his solid globe is to be formed, gathered into a globe having the same mean density as the earth we find for this globe a diameter of 53,000 miles, less than his present apparent diameter by nearly 32,000 miles ; so that the level of his surface in that condition would lie 16,000 miles below his present surface, the space between the two surfaces, or the total shrinkage of Jupiter's volume, amounting to about 930 times the volume of this earth on which we live. As we have every reason to believe that (in a general sense) all the planets are constructed of the same materials not very 1 Cornhill Magazine for May, 1872. 122 THE POETRY OF ASTRONOMY. differently proportioned, we are compelled to admit this vast expansion of Jupiter's present dimensions, and can therefore very well understand even such mighty changes of apparent surface-level as the observation of Admiral Smyth, Sir T. Maclear, and Dr. Peacock cer- tainly shows to have taken place. But now, reverting to our earth's history during the period corresponding to that through w^hich Jupiter is now passing, let us consider whether the ocean, con- verted by heat into great cloud-masses floating through hundreds, if not thousands, of miles above the glowing surface-crust, would not produce yet other appearances such as distant observers might have been able to note. When the shadow of the moon falls now upon the earth during a solar eclipse, it may either wholly or in part reach the actual surface of the earth, or be inter- cepted partly or wholly by cloud-layers. If an observer on Venus or on Mercury were to watch the earth when undergoing eclipse in this way, the apparent shape of the shadow would not be in any appreciable degree modified by such variations in the manner of the sha- dow's fall, unless very powerful telescopes were em- ployed. For the cloud-layers of our air lie but a few miles above the surface of the earth, 1 and the apparent 1 Much less is known than might be respecting the height of the loftier cloud-layers. Coxwell and Glaisher, in their highest aerial flights, saw the cirrus clouds apparently as high above them as when seen from the ground. The height of such clouds could be quite easily determined by taking photographs, with suitably ad- justed instruments, from either end of a measured base-line a mile or two in length. WHEN THE SEA WAS YOUNG. 12$ displacement of a part of the moon's shadow, inter- cepted by a cloud-layer, would be correspondingly small, and in fact undiscernible from Venus or Mercury. But if the atmosphere were very deep, and the cloud-layers separated from each other and from the earth by hun- dreds of miles, the case would be different. To illus- trate the nature of the appearances which might be expected, let us consider the case of a balloon suspended in full sunlight above a layer of fleecy clouds, the layer intercepting a portion of the sun's light, but not all of it. If the layer intercepted all the sun's light, then, of course, a shadow of the balloon would be thrown upon the cloud-layer, this shadow appearing as one, whether seen from the balloon itself, or from the higher parts (let us say) of a lofty mountain reaching far above the layer of clouds. But, the layer not intercepting all the light, a portion of the rays pass on to illuminate the ground everywhere except where the balloon has inter- cepted the sun's rays. That is to say, there is another shadow on the ground upon the prolongation of lines drawn from the balloon to the shadow on the clouds. These two shadows seen from the balloon itself would appear as one, both lying in the same direction ; but they would be separately discernible from a station on the mountain height. Neither would appear quite black ; for the higher would lie on clouds through which the observer would receive light from the illuminated ground below, which he would partially see, while the lower shadow would be seen through the illuminated 124 THE POETRY OF ASTRONOMY. cloud-layer whose light would partially conceal the blackness of the shadow. If the cloud-layer were very thin, the upper shadow would be the least distinct ; if the clouds without being dense yet suffered but a small quantity of direct sunlight to pass between and through their fleecy texture, the upper shadow would be very dark, the lower scarcely visible. Now replace the bal- loon by the moon, and the observer upon the moun- tain height by a distant astronomer on Venus or Mercury, and we perceive that at times, when (in the distant period we are considering) the shadow of the moon fell on a very lofty layer of fleecy clouds, while the shadow so falling would be plainly visible, another fainter one would be discernible on a lower cloud-layer, whose existence and relative position would in this way be indicated to the thoughtful observer. Or, if many layers of thin and fleecy clouds, or a single deep layer of such clouds, existed, then either a set of shadows get- ting fainter and fainter at each successive layer 1 would be seen, or else a long cone of shadow passing through the range of the deep cloud-layer. Now let us see whether Jupiter, the most con- veniently placed of all the younger planets for purposes of observation, shows such appearances as these. Let 1 The shadows themselves would not grow fainter and fainter, but would be black right through the range along which they would lie ; for no part of the sun's rays would reach any one of the spaces in shadow. But seen as they would be through partially trans- parent cloud-layers, and seen also as the partially illuminated cloud-layers would be through the shadows, these necessarily would grow less and less distinct the deeper they lay. WHEN THE SEA WAS YOUNG. 12$ it be premised that ordinarily we could not expect to see them, except on very rare occasions, when some exceptionally thin and fleecy cloud-layer, lying very high, received the first shadow, allowing another to be formed on a cloud-layer lying many hundreds of miles below. It would probably be as rare to detect such appearances, supposing them specially searched for (which has never yet happened), as it would be to observe such a phenomenon as the reappearance of a satellite. And manifestly the lower shadow must be hundreds if not thousands of miles below the upper to be separately seen, since the shadow of a satellite would be about 2,000 miles in diameter, and the earth is so close to the sun compared with Jupiter that the line of sight to the planet is never more than slightly inclined to a line from the sun to the planet. Manifestly, if we looked exactly in the same direction as the sun's rays fall, we should not see the shadow at all ; looking in a direction slightly inclined, we see the shadow thrown somewhat on one side of the satellite (never very far) ; a lower shadow would be thrown somewhat farther in the same direction, but only (in proportion) very slightly. To be thrown as much as 2,000 miles on one side so as to seem clear of the first shadow, the distance of the lower layer from the upper must be several thousand miles. As for seeing such a cone of shadow as is referred to in the last sentence of the preceding paragraph, that could scarcely ever happen. In fact, if the requisite conditions existed, the chances would be that the lengthened shadow would be too faint to 126 THE POETRY OF ASTSOXOMY. be Mjm at H- In like manner it might chance that where in reality there mas a second shadow it would not be discernible, and the only i^mytJM* effect be that the first shadow weald not appear so dark as nsoaL Probably, on the whole, these being the aetoal condi- ader ma eonaider that it shooM be aU but to look for any such phi'maiM'ga as we have the recorded observations of the Let IBS see how tins may be, however. Turning to Webb's little work, 'Celestial Objects for Common Tele- scopes,* in which we may always expect to find the record of -'-- the shadow of the nearest satellite when it the disc. Gorton saw it grey of the second satellite by Buffnam, Birt. and Graver. South many jean ago pabfched in one of the pabtie joornals a most interesting observation. I Ml * A* A T m..mt-nA- mi, n rtm 1m4- T giq*aiJy irgiM. ta^j. *flltif1*T in f < " nMn H r * DOt 1 am to its tenonr, which was, that in his great of two shadows of satellites to be flttymffi 1 by a feint *"pK^fy by its of which could be just detected with a telescope of (I believe) five feet,' in focal length. in Chamber* 5 ! 4 Descriptive Astronomy,' it is that on April 5, 1861, Mr. T. Barneby saw the of the third satellite first in the shape of a broad dark streak soch as the cone of the shadow would WHEN THE SEA WAS TOr - " : i - -_- ::- - - - -- '. Z.-T". -1- i. May 1874: 'I wtt to eri 128 THE POETRY OF ASTRONOMY. points to the conclusion that it is not seen at a.11 in either of them, but that all we see of Jupiter consists of semi-transparent materials. The particular fact from which this inference would be drawn, is, that the dark sides of the suspended or projected masses are not suffi- ciently hard or sharply denned for shadows falling upon an opaque surface, neither are they sharper upon the light background than upon the dark.' This point Mr. Brett proceeds to deal with by reasoning which has a special value because relating to a subject in which he is an expert. ' The laws of light and shade upon opaque bodies,' he remarks, ' are very simple and very absolute ; and one of the most rudimentary of them is that every body has its light, its shade, and its shadow, the relations between which are constant ; and that the most conspicuous and persistent edge or limit in this association of elements is the boundary of the shadow ; the shadow being radically different from the shade in that its intensity is uniform throughout in any given instance, and is not affected by the form of the surface on which it is cast, whereas the shade is distinguished by attributes of an opposite character. Now if the dark spaces adjoining the light patches on Jupiter, which I have called shadows, are not shadows at all, but shades, it is obvious that the opaque surface of the planet on which the shadows should fall is concealed ; whereas, if they are shadows, their boundaries are so soft and undefined as to lead to the conclusion that they are cast upon a semi-transparent body, which allows the shadow to be seen, indeed, but with diminishing dis- WHEN THE SEA WAS YOUNG. 129 tinctness towards its edge, according to the acuteness of its angle of incidence. Either explanation of the phenomenon may be the true one ; but they both lead to the same conclusion namely, that neither the dark belts nor the bright ones are opaque, and that if Jupiter has any nucleus at all, it is not visible to us. ... By the kind invitation of Mr. Lassell I had an opportunity, on the 20th of April, of examining the disc with his twenty-feet reflector of two-feet aperture, and I found this large instrument confirm my impressions concern- ing the shadows in the most satisfactory manner.' There remains one peculiarity in the appearances resulting from the earth's condition during the remote period we are dealing with, which might possibly, though perhaps barely, have been detected by observers on Venus or Mercury. The shadow cast by the earth upon the moon that is, the true shadow, not the mere penumbra has a round shape, corresponding to the fact that the body casting it is a globe. But of old, when irregular cloud-masses and cloud-layers, various in shape and extent were suspended in the deep atmosphere of our planet, it must necessarily have happened that at times the outline of the sha- dow was irregular, and that in a marked degree. The irregularity, in fact, would correspond closely in degree with the occasional irregularity of the earth's apparent figure arising from the same cause (though it is pos- sible that it might have been at times more clearly discernible, as not affected to quite the same degree by irradiation). Now here is a peculiarity which we could K 130 THE POETRY OF ASTRONOMY. not expect to recognise in the case of our heretofore chief test-planet, Jupiter. No telescope yet made by man, probably no telescope man ever will make, would show peculiarities in the shape of Jupiter's shadow on one of his satellites. No one has ever yet claimed to have seen the outline of that shadow at all, far less to have been able to discern its true shape ; and it is not likely that anyone ever will. But in this case the planet Saturn may help us; for his shadow is not merely cast at times upon the small discs of his distant moons, but rests constantly upon the broad expanse of his mighty rings While Saturn whirls, his steadfast shade Sleeps on his luminous ring and that shadow we can study, despite the vast dis- tance of the planet, with a fair chance of detecting peculiarities in its shape, should such at any time exist. Let it be noticed at the outset that it is perfectly easy to calculate what the shape of the shadow should be, if Saturn were a solid globe and the rings' surface perfectly flat. The astronomer knows that at one time, on these assumptions, the shadow would be hidden, at another visible above or below the planet's globe ; at one time to the east of the globe, at another to the west, and always with an elliptical (but very nearly circular) outline, not quite sharply denned, but with a slight fringe of shading only discernible in powerful telescopes. In like manner we may note, in passing, that the shape of the rings' shadow on the WHEN THE SEA WAS YOUNG. 131 globe would always be calculable ; and we know that, when visible at all, it should appear as a black curved streak, either above or below the ring, and perfectly smooth in outline. Again, whatever irregularities there may be in the level of the rings can very little affect the apparent shape of either shadow, because we know from the edge view of the rings that such irregularities are slight compared with the thickness of the rings, which itself is not great. So that any irregularities of a marked character in either shadow must be referred to that cause alone which is competent to produce them ; viz. irregularity in the cloud-layers and cloud- masses floating in the deep atmosphere of the planet. So much premised, let us see what the records gathered by astronomers have to tell us on this point. We turn to a series of papers on the planet Saturn in the 'Intellectual Observer' for 1866, by Mr. Webb, and we find the portion relating to the shadows open- ing thus : ' From an early period, irregularities have been remarked in the form of the shadows which the globe and ring mutually cast upon each other.' Mr. Webb deals first with the shadow of the ring, with which at present we are not directly concerned ; though, of course, any irregularities in that shadow, like the irregularities in the shadows of Jupiter's moons, already described, indicate the depth and the occasionally irre- gular arrangement of the cloud-envelopes. Mr. Webb, in fact, after describing such irregularities, rejects, first, the theory that they are caused by irregularities in the ring ; secondly, the theory that the globe's sur- K 2 132 THE POETRY OF ASTRONOMY. face is irregular ; and, thirdly, the theory that the ring has an atmosphere through which the sun's rays are irregularly refracted in fine, * passing over this diffi- culty as insoluble,' which is not a very satisfactory result. Going on to consider the shape of the shadow of the planet on the rings, he mentions, first, how such first-rate observers as Sir W. Herschel, Lassell, Dawes, and Secchi saw the outline of the shadow concave, in- stead of convex. Next, Dawes on one occasion saw the shadow irregular in outline where it crossed the bright ring. In October, 1852, Lassell saw the sha- dow on both sides of the globe. The younger Bond, of Harvard, Mass., saw the same; on November 2, saw the shadow winged. November 3, Tuttle saw the shadow on both sides, on which he naivqly asks : ' What can this mean ? ' On November 29, De la Eue saw the shadow on both sides, and wrote : ' This is very re- markable, but there can be no question as to the fact ; ' both shadows looked ' like objects seen by mirage ' a remarkable expression. Then we find these observers, and others of equal repute, describing the shadow as having horns, ears, a ' roof ' (pictured with two project- ing eaves), an inlet, a single ear, a reversed edge. Secchi writes : ' L'ombre assez curieuse, elle est ren- versee et ondulee.' On one occasion Bond saw two shadows one black, the other ' a narrow, ghost- like shade.' Of this faint shadow he says: 'I was much impressed by the fact that the outline was pre- served perfectly, while the intensity of the shadow was very feeble.' Was not this certainly either the faint WHEN THE SEA WAS YOUNG. 133 shadow of a deep partially transparent cloud-layer, or a dark shadow seen through such a layer ? After enumerating a number of such cases, Mr. Webb proceeds : ' Thus far extend our facts. What shall we say in explanation of them ? Can we charge them upon personal or instrumental peculiarities ? l It seems not possible, since, in the main, they are agreed upon in England and Italy, and Malta, and India, and the United States. Some of the most sin- gular statements, it is true, come from America alone. But, as they have often the concurrence of more than one observer, so the optical capacity of a telescope, which in favourable air would bear distinctly a power stated to be 1,560, leaves small chance of appeal.' (He might have added that the American astronomers were second to none in observing skill, and that the American skies are particularly favourable for observations of the class in question.) 4 In fact, it is,' Mr. Webb proceeds, 4 a remarkable circumstance that the mystery of the subject has increased under closer, more powerful, and more extended scrutiny. Some of the phenomena may admit of a more or less probable solution. For instance, the apparent concavity of outline might be explained as a deception similar to those optical perver- sities illustrated by Mr. Proctor,' in an article on Saturn's square-shouldered aspect. ' But the " ears " projecting, even when the true shadow was invisible 1 We have altered a word here, and perhaps marred the sen- tence ; but the original word * equation ' would have no meaning for many readers of these pages. 134 THE POETRY OF ASTRONOMY. the two shadows, when one only should have been seen the " roof " and " inlet," and the varying depths of shade in different parts, are alike too clearly attested for doubt, and too incomprehensible for explanation.' (Gela depend.) ' We might take refuge to a certain extent in the idea of varied curvatures in the shadowed surfaces ; and, in order to meet the objection arising from the evanescent thinness of the rings,' we might ' speculate on some force emanating from the sun dis- turbing the level of the rings. But even after we have ventured this daring ' (and, in fact, impossible) ' effort, we find other features as intractable as ever. Some things look like effects of an atmosphere very irregu- larly distributed round the ball, and possessed of pro- perties greatly dissimilar to those of ordinary gases ; but this is undiscoverable, just where it ought to be most apparent,' where the remoter parts of the ring meet the outline of the disc obliquely. But there is not one of these phenomena which cannot be explained by the theory of a very deep atmosphere, not ' irregularly distributed,' or ' possessing properties greatly dissimilar to those of ordinary gases,' but irregularly laden with cloud-masses. In fact, these occasional peculiarities in the shadow are thus brought into exact correlation with the peculiarities observed occasionally in the planet's shape, as noted in the first part of this paper. We might note here other circumstances in the earth's youthful condition. For instance, from time to time the ruddy glow of her intensely heated surface WHEN THE SEA WAS YOUNG. 135 must have been visible through breaks in her cloud- layers ; and just such occasional views of Jupiter's heated surface seem to have been obtained on those occasions when the usually cream-white equatorial belt has shone with a ruddy colour. But this consideration, and others connected with the quantity of light re- ceived from Jupiter and Saturn, have already been dealt with at considerable length in these pages. It appears to us, in fine, that all the evidence, both a priori and a posteriori, corresponds with the theory which we have brought before the reader, that a planet during its extreme youth has its oceans floating in the form of cloud-masses and cloud-layers in a very deep atmosphere. We have seen reason, first, for believing that the intense heat of a planet, for many ages after its first formation, would keep the oceans in this cloud- like condition. Then, looking around for planets such as we might suppose to be much younger than the earth, we have seen that Jupiter and Saturn, the giant planets of the solar system, are probably the youngest (in this sense), always excepting the sun, which is in an earlier stage than any member of his family. And, considering what appearances a planet with a very deep cloud-laden atmosphere might be expected to present, we have found that just such appearances are presented by the planets Jupiter and Saturn, the phenomena described not being seen at all times, but occasionally, and in varying degree, precisely as we should expect from the variable causes producing them. We have also seen that the small density of the giant planets 136 THE POETRY OF ASTRONOMY. cannot readily be otherwise explained than by the theory that we do not see their real surface, but the outer surface of cloud-layers enveloping them. More- over, while not a single fact known about the great planets is opposed to this theory, there are some facts, as we have seen, which cannot possibly be explained on any other theory. But when so much as this can be said of any theory, the theory may be regarded as established. When the earth and sea were young, then, the earth's whole frame was intensely heated. Her real surface was doubtless partly solid and partly liquid then, as now ; but the solid portion glowed with ruddy and in places with white heat, while the liquid portions, instead of being water, as now, were formed of molten rock. Above this surface, with its ' tracts of fluent heat,' was the fiery atmosphere of that primeval time, enormously deep, complex in constitution, bearing enormous masses of aqueous vapour, and every form of cloud and cloud-layer, swept by mighty hurricanes whose breath was flame, drenched with showers so heavy that they might rather be called floods, and tor- tured by the uprush of the vaporous masses formed as these floods fell hissing on the earth's fiery surface. After myriads of centuries came the time when the surface so far cooled as no longer to glow with ruddy light, and no longer to reject by vaporising the waters which fell upon it. Then a fearful darkness pre- vailed beneath the still mighty canopy of cloud ; for only little by little, by very slow degrees, would the WHEN THE SEA WAS YOUNG. 137 water descend upon the earth's surface. Some, indeed, have thought that it was this stage of the earth's past which was described in the Bible words : ' The earth was without form and void, and darkness was upon the face of the deep ; ' noting, in particular, that the coming of light (because of the descent of the waters upon the earth, according to this view) was followed by the separation of waters under the firmament from waters above the firmament, (that Expanse of liquid, pure, Transparent, elemental air,) the waters under the heaven being next gathered to- gether into one place, and so forth. But we must confess that this interpretation of the narrative, some- times called the vision interpretation, seems to us very far-fetched and unnatural ; though we are in no way concerned here to oppose it, deeming it only necessary to mention that, for our own part, we cannot doubt that the writer of the narrative wished to be under- stood as describing what really occurred, not appear- ances shown to him in a vision. A question which has long been regarded as among the great mysteries of nature the question, How did the seas become salt ? seems to us to find a ready solu- tion when we consider that the ocean once formed the earth's cloud-envelope. We may, in fact, regard the oceans as holding in solution what was washed from the earth or otherwise extracted from its substance during the ages when the waters of ocean were passing from their former to their present condition. For 138 THE POETRY OF ASTRONOMY. then all the conditions assisted the action of the waters themselves the intense heat of the earth's crust and of the atmosphere, the tremendous atmo- spheric pressure, and consequently the high boiling point (so that the waters first formed on the earth's heated crust must have been far hotter than is boiling water at the present time), and the presence also in the atmosphere of many vapours which would greatly help the decomposing action of the water itself. Con- sider, for instance, the following description, abridged from a paper by Dr. Sterry Hunt, the eminent Cana- dian chemist and geologist. After showing that car- bonic acid, chlorine, and sulphurous acids would be present in enormous quantities in the primeval atmo- sphere, besides, of course, still vaster quantities of the vapour of water, he proceeds : ' These gases, with nitrogen and an excess of oxygen, would form an atmosphere of great density. In such an atmosphere, condensation would only take place at a temperature far above the present boiling point ; and the lower levels of the earth's slowly cooling crust would be drenched with a heated solution of hydrochloric acid, whose decomposing action, aided by its high temperature, would be exceedingly rapid. The primitive igneous rock on which these showers fell probably resem- bled in composition certain furnace slags or volcanic glasses.' The process of decomposition would continue ' under the action of the heavy showers until the affi- nities of the hydrochloric acid were satisfied. Later larger quantities of sulphuric acid would be formed, WHEN THE SEA WAS YOUNG. 139 and drenching showers of heated solutions of this energetic dissolvent would fall upon the earth's heated crust. After the compounds of sulphur and chlorine had been separated from the air, carbonic acid would still continue to be an important constituent of the atmosphere. It would be gradually diminished in gravity,' through chemical processes resulting in the formation of various clays, ' while the separated lime, magnesia, and alkalies,' changed into bicarbonates, would be carried down to the sea in a state of solu- tion.' Here we seem to see a fair account rendered of the enormous quantity of matter forming collectively what is called the brine of the ocean, and containing, besides common salt (chloride of sodium), sulphuric acid, magnesia, soda, sulphate of lime, and other substances. The theory that these substances have been washed from the earth's surface by causes such as are now in progress, would not, we think, be seriously enter- tained if the vast amount of matter thus present in the waters of the sea were remembered and consi- dered. Brine forms, on the average, about 3^ per cent, of sea-water. Hence, if we take the average depth of the ocean at two miles, 1 or, roundly, 10,000 1 In Maury's Physical Geography of the Sea there is a passage which we take to be one of the most amusing ever written in a work of the kind. The irlea would seem to have occurred to him of estimating how much surface the salts of the sea would cover to the depth of a mile ; and while in the midst of the calculation, he would seem to have grown weary of it. At least we cannot other- wise understand how he came to pen the following singular remarks: 'Did anyone who maintains that the salts of the sea 140 THE POETRY OF ASTRONOMY. feet, it follows that, if all the water of the sea were evaporated, there would be left a deposit of salt ave- raging 350 feet in depth all over the present floor of were originally washed down into it by the rivers and the rains ever take the trouble to compute the quantity of solid matter that the sea holds in solution as salts ? Taking the average depth of the ocean at three miles, and its average saltness at 3^ per cent., it appears that there is salt enough in the sea to cover to the thickness of one mile an area of several millions of square miles.'' (The italics are ours.) This passage reminds us of one in an early volume of Household Words, where a very amusing account was given of the stores of wine in the London Docks, over which the writer is sup- posed to be shown, collecting materials, but also tasting wine, as he proceeds. The gradually increasing effect of the wine-tasting is indicated very humorously. In one of the later stages of his progress, the writer enters into a computation of the amount of wine wasted in the process of cleansing the glass with wine. (We write from memory, and possibly, as many years have passed since we read the passage, we may not be correct in details.) Assuming so much wasted at each cleansing, so many visitors, each tasting so many times, and so forth, ' then,' says the writer, ' it may be shown that in each year 800 bottles, or it may be 8,000 bottles, of wine are wasted. And should any one object that there is a con- siderable difference between 800 and 8,000, all we have to say is that the principle is the same,' &c. Captain Maury passes on, however, without any allusion to the somewhat unexpected vague- ness of his conclusion. ' These millions of cubic miles of costal salt have not made the sea any fuller,' he proceeds. < All that solid matter has been received into the interstices of sea-water without swelling the mass ; for chemists tell us that water is not increased in volume by the salt it dissolves. Here we have, therefore, an economy of space calculated to surprise even the learned author himself of the Plwality of Worlds.' All which, so far as appears, is a propos de bottes. Within the same page, which, we submit, is inferior to Maury's usual style, we find him, in dealing with the question, What was the Creator's main object in making the sea salt ? advancing the startling proposition that all the objects of the salts of the sea are main objects.' (The nature of the context, which is serious, even solemn, will not allow us to suppose that any pun was here intended.) WHEN THE SEA WAS YOUNG. 141 the sea. This would correspond in quantity to salt covering all the present land surface of the earth to a depth of a thousand feet, or to a deposit two hundred feet deep over the entire surface of the globe ; so that the idea of its having been washed from the land is altogether inadmissible. It may, indeed, be urged that, as the process of washing down from the land is continually going on, only a sufficiency of time would be needed to account for any quantity whatever of sea- salt. But apart from the fact that only a certain thickness of the solid crust, and that thickness by no means very great, could be drawn upon for the supply, and that the very continuance of the process shows us that even that portion of the earth's crust has not been drained of its salts, there is every reason to believe that the extraction of salt from the sea is going on and has been going on for many ages past at fully as great a rate as the addition of fregh salts. Although the process of evaporation cannot remove the salts, these, as Maury justly notes, can be ex- tracted by other processes. ' We know,' he says, ' that the insects of the sea do take out a portion of them, and that the salt-ponds and arms which from time to time in the geological calendar have been separated from the sea, afford an escape by which the quantity of chloride of sodium in its waters the most abun dant of its solid ingredients is regulated. The in- sects of the sea cannot build their structures of this salt, for it would dissolve again as fast as they could separate it. But here the ever-ready atmosphere 142 THE POETRY OF ASTRONOMY. comes into play, and assists the insects in regulating the salts. It cannot take them [the salts] up from the sea, it is true, but it can take the sea away from them ; for it pumps up the water from these pools that have been barred oif, transfers it to the clouds, and they deliver it back to the sea as fresh water, leaving behind the salts it contained in a solid state. These are operations which have been going on for ages ; proof that they are still going on is continually before our eyes ; for the ' hard water ' of our fountains, the marl-banks of the valleys, the salt-beds of the plains, Albion's chalky cliffs, and the coral islands of the sea are monuments in attestation.' We must, then, regard the salts of the sea as in the main dissolved from the solid crust during that remote period when the seas were young. The seas thus indicate to us the nature of those vast chemical processes through which the earth had to pass in the earlier stages of its history. If the present crust of the earth did not afford, as it does, the clearest evidence of a time when the earth's whole frame glowed with intense heat ; if we could not, as we can, derive from the movements of the celestial bodies, as well as from the telescopic appearance of some among them, the most certain assurance that all the planets, nay, the whole of the solar system itself, were once in the state of glowing vapour ; the ocean brine the mighty residuum, left after the earth had passed through its baptism of liquid fire, would leave us in little doubt respecting the main features at least of the earth's WHEN THE SEA WAS YOUNG. 143 past history. The seas could never have attained their present condition had not the earth which they encompassed when they were young been then an orb of fire. Every wave that pours in upon the shore speaks to us of so remote a past that all ordinary time-measures fail us in the attempt to indicate the length of the vast intervals separating us from it. The saltness of the ocean is no minor feature or mere detail of our globe's economy, but has a significance truly cosrnical in its importance. Tremendous indeed must have been the activity of those primeval pro- cesses, fierce the heat of those primeval fires, under whose action sixty thousand millions of millions of tons of salts were extracted from the earth's substance and added to its liquid envelope. [Since this essay was in type, a paper has been read before the Astronomical Society by Mr. Brett, describing observations alto- gether inexplicable, except by the theory we have advocated above. They relate to the movements of two large white spots on Jupiter's chief belt. Both these spots were so shadowed as to indicate that they were in reality bodies of globular shape no doubt rounded masses of cloud, floating in the relatively transparent atmosphere of the planet. The fact that they are wholly immersed in the semi-transparent material of the planet is indisputable,' says Mr. Brett, 'since they gradually disappear as they approach the' edge of the disc, 'and in no case have been seen to project beyond it.' The distinguishing peculiarity of these bodies was, however, their rapid motion, as though gaining on the planet's rotation. The average motion was estimated by Mr. Brett at about 165 miles per hour, but this estimate would have been somewhat reduced had he taken into account, as he should have done, the changing position of the earth, relatively to Jupiter. Still, even after adding to this reduction all that can possibly be attributed to errors of observation, there remains a considerable motion of these cloud- masses, each of which was about half as large as the whole 144 THE POETRY OF ASTRONOMY. IS THE MOON DEAD ? THE idea generally prevailing, among astronomers, respecting the moon's condition is that she is a dead planet, an orb which circles around the sun like her companion planet the earth, but is not, like the earth, the abode of living creatures of any sort. For- merly, indeed, other views were entertained. It was thought that the dark regions were seas, the bright regions continents a view embodied by Kepler in the saying, ' Do maculas esse maria, do lucidas esse terras/ But the telescope soon satisfied astronomers that there are no seas upon the moon. It has been noted that in two well-known passages of the ' Paradise Lost,' in which Milton touches on the work of Galileo with the tele- scope, he speaks of lands, mountains, rivers, and regions, but not of oceans or seas, upon the moon. Thus, in describing the shield of Satan, he compares it to The moon, whose orb Through optic glass the Tuscan artist views At evening from the top of FesolS, Or in Val d'Arno, to descry new lands, Rivers, or mountains, on her spotty globe. globe of the earth ! It may, perhaps, be thought that we have here attached too much weight to the telescopic observations of one who is skilled rather in art than in science ; and in fairness it must be admitted that about half Mr. Brett's observations have been re- garded more than doubtfully by astronomers. But this observation, like the one described in the body of the above essay, depends only on accuracy in estimating the apparent position of two spots on the planet's face; and so skilful a draughtsman as Mr. Brett cannot have made any large error in an observation of the kind.] IS THE MOON DEAD? 145 While again, in the fifth book, Eaphael views the earth As when by night the glass Of Galileo, less assured, observes Imagined lands and regions in the moon. We may well believe that had Gralileo, in his inter- views l with Milton, described appearances which (with his telescopic power) resembled seas or oceans, the poet would not have used so vague a word as ' regions ' in the third line of the last quoted passage, where the word ' oceans ' would so obviously have suggested itself. From the very beginning of the telescopic observation of our satellite, it became clear that no seas or oceans exist upon her surface. And as telescopic power has increased, and the minute details of the moon's surface have been more searchingly scrutinised, it has been seen that there are no smaller water regions, no lakes, or rivers, not even any ponds, or rivulets, or brooks. But indeed, while the close telescopic scrutiny of the moon was thus showing that there are no water surfaces there, it was becoming also clear that no water could remain there under the sun's rays ; that is, on the parts of the moon which are illuminated. For it was found that the moon has an atmosphere so rare that water would boil away at a very low temperature indeed. How rare the lunar atmosphere is we do not certainly know; but a number of phenomena show that it must be very rare indeed. Some of these have been already considered, along with other lunar phem> 1 See Milton's Areopagitica. L 146 THE POETRY OF ASTRONOMY. mena, in an article which appeared in the ' Cornhill Magazine ' for August 1873 ; and for this reason (espe- cially as that article has since been republished) we do not here enter into this portion of the evidence, our object being to discuss here certain relations which were not dealt with in that earlier paper. But now that astronomers have almost by unanimous consent accepted the doctrine of the development of our system, which involves the belief that the whole mass of each member of the system was formerly gaseous with intensity of heat, they can no longer doubt that the moon once had seas and an atmosphere of consider- able density. The moon has, in fact, passed through the same changes as our own earth, though not neces- sarily in the same exact way. She was once vaporous, as was our earth, though not at the same time nor for so long a time. She was once glowing with intensity of heat, though this stage also must have continued for a much shorter time than the corresponding stage of our earth's history. Must we not conclude that after passing through that stage the moon was for a time a habitable world as our earth is now ? The great masses of vapour and of cloud which had girt our moon's whole globe, even as in the youth of our earth her seas enwrapped her in cloud form, must at length have taken their place as seas upon her surface. The atmo- sphere which had supported those waters must at first have been dense by comparison with the present lunar atmosphere, perhaps even by comparison with the present atmosphere of our earth. Then the glowing IS THE MOON DEAD? 147 surface of the moon gradually cooled, until at length the moon must have been a fit abode for life. But whether, when thus swept and garnished into fitness for habitation, the moon actually became an inhabited world, is a question which will be variously answered according to our views respecting the economy of nature in this respect. Those who hold that nature makes nothing in vain, will need only to ask whether the support of life is the one sole purpose which a planet can subserve; if that should appear probable, they would at once decide that the moon must during its habitable stage have been inhabited. Others who, looking around at the workings of nature as known to us, perceive, or think they perceive, that there is much which resembles waste in nature, will be less confident on this point. They may reason that as of many seeds which fall upon the ground, scarce one subserves the one purpose for which seeds can be supposed to have been primarily intended, as many younglings among animals perish untimely, as even many races and types fail of their apparent primary purpose, so our moon, and possibly many such worlds, may never have sub- served and never come to subserve that one chief pur- pose for which the orbs peopling space can be supposed to have been formed, if purpose indeed reigns through- out the universe. But we are not here concerned to inquire carefully whether the moon ever was inhabited ; we care only to show the probability, the all but certainty, that the moon during one stage of her existence was a habitable L 2 148 THE POETRY OF ASTRONOMY. body, leaving the questions whether she ever actually had inhabitants, and what (if she had) their nature may have been, to the imagination of the reader. Most certainly there is little reason for believing that on this point men will ever have any real information for their guidance. But it will be well, in thus considering the past of our moon for the purpose of forming clearer conceptions as to her present condition than telescopic evidence can supply, to examine not only in what respects she re- sembled our earth, but also those circumstances in which there is reason to believe that she differed. In the first place, whatever opinion we form as to the exact nature of the processes by which the solar system attained its present stage of development, we cannot doubt that our moon was formed earlier than the earth. In considering the solar system we may be doubtful whether an exterior planet was fashioned before or after an interior planet, a larger planet before or after a smaller planet, and so on ; but we could not for a moment doubt, even if the sun's present aspect did not assure us of the fact, that the fashioning process was completed earlier for each one of the planets than for the central body. In thus speaking, it will be understood that we are referring to the stage of cosmical existence when the great bulk of a planet's material has been gathered in, and the planet has assumed the condition of a separate globe of matter. So long as any considerable portion of the materials out of which a planet was to be formed, continued so placed and so IS THE MOON DEAD? 149 moving as to be still associated with the sun's exterior portions, neither the sun nor the planet could be re- garded as having yet assumed their final form ; and it is manifest that, when planet after planet had taken its substance from the general stock of matter, the sun must still have had long processes of change to undergo before he could gather in his substance in such sort as to become a definite orb, separated by a well-marked interval from the nearest member of his system. In the same sense it is clear that long after our moon had been formed as a separate body circling round the centre of gravity of the mass from which she herself had been framed and the earth was hereafter to be framed, the earth must have been but an inchoate planet, a chaotic mass of vaporous matter, without even as yet, in all probability, a solid or liquid nucleus. Thus started in planetary life long before the earth, the moon must also have possessed far less heat when thus first formed than the earth when formed long after. The inherent heat of a planet in the first stage of its existence, must depend on the quantity of matter it contains, whichever theory of development we adopt. If a planet were solely formed, as Laplace's theory supposes, from nebulous matter left behind after the contraction of the great nebulous mass of the system, the heat of the planet, when its vaporous mass had so contracted as to give the planet separate exist- ence, would depend on the quantity of vapour ; seeing that in every part of the process of contraction heat would be evolved in proportion to the quantity of con- ISO THE POETRY OF ASTRONOMY. tracting vapour and the energy of the contractile force, this energy being itself proportioned to the mass or quantity of vapour. If, on the other hand, a planet were formed solely by gathering in cometic and meteoric matter, then the planet's original heat would depend on the energy with which such matter was drawn in ; and this energy would greatly depend on the quantity of matter gained by the planet. Assuming, as far more probable than either theory, that planets were formed by a combination of both processes, of course what is true of each process considered separately is true of the two combined. Applying this reasoning to the moon, whose mass is but about one eighty-first part of the earth's, we see that when she was first formed, her heat, though doubtless intense, was far less than the heat of the earth at the corresponding stage of planetary life. Here, then, was a second cause tending to shorten the duration of the moon's life as compared with that of our earth, or rather tending to throw back, and that not by a little but by millions of years, the habitable period of the moon's existence. But, thirdly, the moon must have passed much more rapidly through all the stages of planetary life than our earth. We have already shown (' Cornhill Magazine' for August, pp. 157, 158) that the rate of a planet's cooling depends on the quantity of matter contained by the planet. If we applied the same method to the moon which we there applied to Jupiter with the same caution, however, as to any exact reliance to be placed upon the result we should infer IS THE MOON DEAD? 151 that each stage of the moon's existence would fall short by many millions of years of the corresponding stage of the earth's. Thus, the experiments of Bischoflf, there referred to, indicate some 350 millions of years as the period required for the cooling of the earth from 2,000 centigrade to the heat of boiling water. Now the experiment on heated iron globes, also described there, would show that, as the moon's mass is but about the eighty-first part of the earth's, the moon would require but about three thirteenth parts l of this period, or about eighty millions of years, to pass through the same stage of cooling. Here, then, there is a difference of no less than 270 millions of years for one stage only of the moon's cooling. And although, as we pointed out in the essay to which we have just referred, exact reliance cannot be placed upon the method of calculation here employed, yet the real difference is as likely to exceed as to fall short of the result just indicated. It is certain, or as nearly certain as anything not actually demonstrated can be, that the difference must be measured by tens of millions of years. There is yet one other circumstance which must have tended to shorten the moon's existence as a habit- able world. We know that the earth's vulcanian forces, by which her frame is modelled and remodelled so as to be continually refitted for the purposes of life, depend on the energy of terrestrial gravitation. This we shall presently have to consider more at length ; at present 1 It will be found that 4 multiplied twice into itself gives a number not much greater than eighty-one. 152 THE POETRY OF ASTRONOMY. we need merely note that it is terrestrial gravitation, drawing continually inwards the rock strata of our earth's frame, which results (on the conversion of the mechanical energies thus arising, into heat) in the generation of the pressures on which earthquakes, volcanoes, and other such phenomena depend. The relative smallness of lunar gravity must have caused these processes to continue for a much shorter time in her case than in our earth's. Combining together these considerations viz. first that the moon must have been fashioned as a planet many millions of years before the earth ; secondly, that her original heat must have been greatly less than that of the earth (corresponding to a reduction of many millions of years in the time required for cooling down to the habitable condition) ; thirdly, that each stage of the moon's cooling must have lasted less by many millions of years than the corresponding stage for the earth's cooling ; and lastly, that lunar gravity being so much less than terrestrial gravity the moon's vulcanian vitality must have lasted for a much shorter time than the earth's we perceive that the moon must have passed that stage of her history which corresponded to that through which our earth is now passing, many many millions of years ago. It would probably be no exaggeration whatever of the truth to say that more than a thousand millions of years have passed since the moon was a habitable world. But we may quite con- fidently assert that fully a hundred millions of years have passed since that era of her history. And as the IS THE MOON DEAD? 153 changes which she has undergone since then have occurred at a much more rapid rate than* those by which the earth is now passing on and will continue to pass on, for ages yet to come, towards planetary decrepitude, we may assert with equal confidence that the moon is passing through a stage of planetary existence which the earth will not reach for many hundreds of millions of years yet to come. The moon, thus regarded, presents to us a most interesting subject of study, because she illustrates, in general respects if not perhaps in details, the condition which our earth will attain in the remote future. Let us then examine the principal features of the moon those which may be regarded as characteristic, which at any rate distinguish her from the earth and consider how far it is probable that our earth will one day present similar features. We can also inquire how far the moon's present condition may be regarded as that of a dead world, in this sense that she can neither now be, nor (under any conceivable circumstances) hereafter become, once again a habitable world, as formerly she presumably was. There is one very remarkable feature of the moon's motions which is commonly not explained as we are about to explain it, but in a way which would correspond better with the general views indicated in this article, than the interpretation which seems to us preferable. We refer to the circumstance that the moon's rotation on her axis takes place in precisely the same time as her revolution around the earth. This is, in reality, a 154 THE POETRY OF ASTRONOMY. very strange feature, though it is often dismissed as if there were notm'ng very remarkable about it. In what- ever way the arrangement was brought about, it is absolutely certain that the earth had her share in the work ; and again, no matter what explanation or set of explanations we accept, we find most interesting evi- dence suggested as to the moon's past condition. According to one account, the moon was originally set spinning at a rate closely corresponding to her present rotation rate, and the earth, having by her attractive power somewhat elongated the moon towards herself, acted on this not perfectly round body in such sort as gradually to coerce its motion of rotation into exact agreement with its motion of revolution. It is known that this would necessarily happen if the ori- ginal approach to agreement between these motions had been sufficiently close. If we adopted this view, we should find ourselves in presence of the somewhat remarkable fact that the small moon was in the begin- ning set rotating so slowly that its day lasted as long as a lunar month. Such a rotation, as the result of some process of systematic evolution, could be readily accepted ; but that this motion, which presents no recognisable advantages, and many most manifest in- conveniences (for creatures living in the moon), should have been specially communicated to the moon by the creative hand, would not be an acceptable theory, even if we were not forced by overwhelming evidence to throw special creative acts very much farther back (to say the least) than the formation of our moon, or of any part of the solar system. IS THE MOON DEAD? 155 Another explanation which has been offered runs as follows. When the moon had oceans, the earth must have acted on those oceans in the same way as the moon now acts on the oceans of our earth. In one respect the earth must have acted more energetically, in another less. Being very much (eighty-one times) more massive than the moon, the earth necessarily exerts much more force on the moon's substance than the moon exerts on hers. 1 On the other hand, the re- lative difference between the pull on the nearest and remotest parts of the globe is less in the case of the earth drawing the waters of the moon (in old times) than in the case of the moon drawing the waters of the earth ; for the moon is a much smaller globe than the earth ; and this difference is the really effective force in the production of tides. Also it is probable that the moon never had -a relatively large ocean-surface, as will presently be shown, and small seas (probably dis- connected) could not be swept by a great tide-wave. Still we may suppose that there was once a tidal wave, greater or less, sweeping athwart the lunar seas much in the manner of our own tidal wave. Now, our tidal wave is beyond doubt slowly checking the earth's motion of rotation, for the wave travels so as to meet 1 In one sense the moon pulls the earth just as strongly as the earth pulls the moon, for gravity is not a force which one body exerts on another solely, but a mutual force. But what mathema- ticians call the moving force exerted by the earth on the moon is eighty-one times greater than the corresponding force exerted by the moon on 1 he earth ; for the mutual attraction between these bodies has in the former case to move the moon, whereas in the latter it has to move the much larger mass of the earth. 156 THE POETRY OF ASTRONOMY. the motion of rotation, which therefore to some slight degree it opposes. This will go on, until at length the rotation has been so reduced that the tidal wave no longer affects it ; or, in other words, until the earth's period of rotation corresponds with the period of the tidal wave, viz. with the lunar month. Hundreds of millions of years will pass before that happens ; but then we have seen that the moon may fairly be regarded as illustrating the earth's condition hundreds of millions of years hence. Accordingly, there is no- thing absolutely incredible in the theory that during the remote ages when the moon had seas the tidal wave which traversed them, continually retarding the moon's motion of rotation, gradually coerced it into absolute agreement with her motion of revolution around the earth. Still it must be admitted that the theory is not very easily to be accepted as it stands. The seas of the moon were probably less in relative extent, even when at their largest, than those of Mars now are, and such seas could have no tidal waves which even in thousands of millions of years could reduce the moon's rate of rotation in any considerable degree ; and, as we shall presently see, the duration of the era when the moon had seas can hardly have been measured by periods so vast. On the whole, while we may admit the probability that at some very distant time in the past the earth may have exerted influences on lunar seas resembling those which the moon now exerts on our seas, it does not appear to us probable that the peculiar feature we are now considering can be attri- IS THE MOON DEAD? buted either wholly or in very large degree to the retarding influence of tidal waves upon the moon. One other theory remains which seems to have more in its favour than either of those hitherto consi- dered. Before the moon became a separate planet her frame, then vaporous, must have been enwrapped in the ^aporous frame of the earth. While this continued the moon was necessarily compelled to move as a portion of the earth's outer envelope, and therefore, of course, turned upon her axis in the same time that that exterior portion of the earth revolved. So soon as the contraction of the earth's vaporous frame left the moon outside, she was free if she could to change her rate of rotation ; that is to say, the earth's enwrap- ping vapour-masses no longer prevented the moon from changing her rotation rate. And there were two causes at work, either of which, if in action alone, would have markedly changed the moon's rate of turning on her axis. One was the gradual contraction of the moon's frame in cooling. This would have made her turn more quickly on her axis. The other was the con- tinually gathering in of meteoric matter from without, which was a process taking place probably far more rapidly then than now, seeing that the meteoric systems now remaining are the merest residue of a residue compared with those existing hundreds of millions of years ago. This process would tend to make the moon turn more slowly upon her axis. However, the former process would probably operate far more effectively, and thus the moon would on the whole have acquired 158 THE POETRY OF ASTRONOMY. a more rapid rate of rotation, and the coincidence between rotation and revolution existing when she first had separate existence would have disappeared. But there was all the time a force at work to check the tendency to change in this respect. The earth was there, exerting that very force which we have already described in considering another theory a force f om- petent, we may infer, to check the tendency to a slow increase in the moon's rate of rotation, and to preserve that relation which existed when the moon was first formed. We say that the competence of this force may be inferred meaning that the observed coincidence between the moon's rate of turning round upon her axis, and her rate of revolution around the earth, shows that the force was sufficient for that purpose. A similar force exerted by the sun upon the earth since she was first separately formed has not proved competent, as we know, to make the earth turn on her axis in the same time exactly that she travels round the sun that is, in a year. Nor have any of the planets been forced to behave in this way. But we can readily understand that a great difference should exist between the formation of a planet which, having an enormously high temperature when first formed, would have an enormous amount of contraction to undergo ; and the formation of a subordinate orb like the moon, which, though no doubt intensely hot when first thrown off l by the contracting earth, cannot have 1 We here use the words ' thrown off ' as equivalent to ' left behind.' The theory that the moon was thrown off by the earth, or IS THE MOON DEAD? 159 been nearly so hot as a planet at the corresponding stage of its existence. On the whole, there are (so it seems to us) good reasons for believing that that peculiar law of the moon's motion which causes the same lunar hemisphere to be constantly turned earth- wards had its origin during the birth itself of our satel- lite. We may, indeed, find in that peculiarity one of the strongest arguments in favour of the theory that our solar system reached its present condition by a process of development, since on no other theory can a satisfactory solution be obtained of the most striking peculiarity of the moon's motions. But the inhabitants of earth are more directly in- terested not for their own sake, but for the sake of their remote descendants in the subject of the moon's present airless and waterless condition, regarded as the result of systematic processes of change. If we can ascertain what those processes may have been, and if we should find that similar processes are taking place, however slowly, on the earth, then the moon's present condition has in a sense the same sort of interest for us that a man in the full vigour of life might be sup- posed to find in the study of the condition of aged persons, if through some strange chance he had never had an opportunity of observing earlier the effects of old age upon the human frame. The inhabitant of earth who contemplates the moon's present wretched condition, may be disposed like Lydia Van den the earth by the sun, is altogether inconsistent with mechanical possibilities. 160 THE POETRY OF ASTRONOMY. Bosch when she saw Madame Bernstein's shaky hands and hobbling gait to hope we ' sha'n't be like her when we're old, anyhow ; ' but the probabilities are in favour of a young world following in the same path which those now old have followed, and so reaching the same condition. If the moon is really a much older world than the earth and we have seen that in all probability she is then she presents to us a picture of the condition which our earth will hereafter attain. It may be well to mention here, but to mention only, the facts which are held by astronomers to show that the moon has a very rare atmosphere and no water- surface. In the first place, then, we know that, if the moon's atmosphere were not exceedingly rare, we should never see the sun totally eclipsed, some portion of the sun's light being brought into view round the moon's edge, even at the time of central eclipse, by the refracting power of the atmosphere. Nothing of this kind happens, however. It might once have been thought that the ruddy light of the prominences was the refracted sunlight, but the spectroscope has shown beyond all possibility of question that this is not the case. Secondly, the moon, as she moves over the heavens, blots out stars from view, and, advancing, brings them again into view, quite sharply and suddenly. If she had an atmosphere which was not extremely rare, they would be gradually obliterated, and as gradually reappear. Next, the shadows of the lunar mountains are appreciably black, whereas we know that an atmo- sphere (unless exceedingly rare) would bend and reflect IS THE MOON DEAD* l6l the solar rays in such sort as to take off the blackness of the shadows. There is, fourthly, no appreciable twilight in the moon, the boundary line between light and darkness being always sharply denned : now we receive light after sunset from portions of our at- mosphere fully forty-five miles above the sea-level, where the atmospheric pressure is probably scarcely the eight-thousandth part of that at the sea-level. And though it would not be safe to infer that the lunar air is so rare as that, seeing that one or two observers have suspected the existence of a twilight circle on the moon, yet it may quite safely be concluded, from this and the other considerations named above, that the lunar atmosphere must have a very small density Probably the density may be about one five-hundredth part of the density of the air we breathe at the sea- level ; but at the highest possible estimate the density of the lunar atmosphere cannot exceed the two-hundredth part of the density of the air we breathe. With an atmo- sphere such as this, the boiling point would be very close to freezing point, and the gentlest warmth from the rays of the rising sun would suffice to boil water away that is, to turn it entirely into vapour. It will be under- stood, then, how utterly impossible it is that even the smallest water-surface should continue under the rays of the lunar mid-day sun day lasting on the moon about a fortnight of our time. 1 1 Mr. Boyle, a telescopist of New York, announced some few years since that lie had detected pools of water upon the moon, having doubtless been deceived by some of those bright spots M 1 62 THE POETRY OF ASTRONOMY. The theory advanced by Frankland in this country respecting the way in which the lunar air and seas have been caused to disappear must now be considered. In passing, I may remark that the originator of the theory was Seeman, the German geologist ; but it was inde- pendently advanced by Frankland in England, Stanislas Meunier in France, and Sterry Hunt in America. In the first place, it is to be noted that no other theory seems available. Of three others which have been advanced, only one, Hansen's, according to which the seas and atmosphere of the moon have been drawn by lunar gravity to the farther or unseen hemisphere of the moon, needs serious refutation. (The other two are Whiston's theory, that a comet carried off the lunar seas and air ; and the theory whose author is un- known to us that the lunar seas, and later the lunar atmosphere, have been frozen through the intensity of cold to which, in the long lunar nights, the moon is exposed.) But this theory is no longer entertained by which seem to possess a mirror surface. It will be seen from what is stated above that no such pools could possibly remain as such under the sun's rays, even assuming that such pools might exist during the lunar night, when, however, the temperature would necessarily be far below the freezing point. Some, by the way, suppose that the question whether any processes resembling vege- tation can take place on the moon is on the same footing as the question whether small portions of the moon's surface can be covered with water. But this is a mistake. It is altogether im- probable, no doubt, that there is any vegetation on the moon, but it is not absolutely impossible, whereas it is utterly impossible that any water-surface could be seen there. Whatever water may have been imagined by telescopists during their surveys of lunar scenery has been as unreal as the water sometimes seen by travellers on African deserts. IS THE MOON DEAD? 163 astronomers, simply because it has been shown that the peculiarity of the moon's shape which had suggested the theory has been found, first, to have no real exist- ence ; and, secondly, to be incapable, if it existed, of exercising the supposed effect. 1 The theory independently advanced by the four students of science named above is simply this, that seas formerly existing on the surface of the moon have 1 The idea was that the moon, though nearly spherical, is some- what egg-shaped, the smaller end of the egg-shaped figure being directed towards our earth. Now, while it is perfectly clear that on this supposition the greater part of the moon's visible half would be of the nature of a gigantic elevation above the mean level, and would therefore be denuded (or might be denuded) of its seas and the denser parts of the air formerly covering it, yet it is equally clear that all round the base of this monstrous lunar eleva- tion the seas would be gathered together, and the air would be at its densest. But it is precisely round the base of this part of the moon, or, in other words, round the border of the visible lunar hemisphere, that we should have the best chance of perceiving the effects of air and seas, if any really existed; and it is because o the absolute absence of all evidence of the kind that astronomers regard the moon as having no seas and very little air. It is worthy of notice that Hansen's theory was anticipated by the author of that clever little pamphlet called TJie Lunar Hoax, who places the human inhabitants (the Bat-men) in the regions near the edge of the lunar disc, on the strength of some such views as Hansen advanced a quaiter of a century later. Recently the Chicago Times published several columns of lunar-hoax matter, purporting to be an account of observations made in France with a new and exceed- ingly powerful reflecting telescope. The observations made with this instrument showed a number of lunar folks, whose movements rendered it manifest that they were prisoners undergoing some kind of penal servitude, the visible lunar hemisphere being a sort of Botany Bay or Cayenne for lunar offenders, while the other hemisphere is a comfortable place of abode for good moon people. But what an unhappy state of things is here suggested 1 Conceive a world one-half of whose surface is required as an abode for its malefactors 1 1 64 THE POETRY OF ASTRONOMY. been gradually withdrawn into the moon's interior, and that a similar process, but chemical rather than mechanical, has led to the withdrawal of the greater portion of the air which formerly enveloped the moon's frame. It may be well, first, to inquire whether the moon is likely to have had originally an atmosphere of con- siderable density and oceans of considerable extent. Supposing, for the sake of argument, that the materials of the moon's mass (including air and water) were originally proportioned as to quantity very much like those of our earth's mass, it is easily seen that the quantity of air above each square mile of the moon's surface, at the time when the moon had reached the stage of planetary development through which our earth is now passing, must have been very much less than the quantity of air now existing above each square mile of the earth's surface. For, the moon's mass being about an eighty-first part of the earth's, the mass of the lunar air must have been about an eighty-first part of the mass of our present atmosphere. But the moon's surface bears a much greater proportion to the earth's, being about a thirteenth. Whence it follows that, on the assumptions we have made, the quantity of air above each square mile of the moon's surface would be only about one sixth part of the quantity above each square mile of the earth's surface. And this air being drawn downwards only by lunar gravity, which has but about a sixth part of the energy of our terrestrial gravity, would be less compressed in the same degree on this IS THE MOON DEAD? 165 account. One-sixth of the quantity of air being thus compressed with one-sixth the amount of force, it is clear that the density of the lunar air in that stage of the moon's existence would only be about one thirty- sixth of the density of our air. Similar reasoning applies to the water, except as to the compression under lunar gravity. The average quantity of water to each square mile of the moon's surface would be but about one sixth part of the quantity there is for each square mile of the earth's surface. The relative extent of the lunar oceans would not be less in precisely the same degree, however. For, speaking generally, the bed of the ocean slopes downwards from the shore-line in such a way that more than half, or a third, or a fourth, or so on, would have to be removed to diminish the surface by a half, a third, or a fourth, or so on, respectively. We may illustrate our meaning here by considering the relation between the quantity of water in a wine-glass (supposed to be cone-shaped) and the surface of the water. Suppose the wine-glass full at first, and the circular surface of the water to be three square inches, then if five-sixths of the water are thrown out, so that only one-sixth remains, the surface will not be reduced to one-sixth its former extent that is, to one-half of a square inch but will be about nine-tenths of a square inch. It is clear that in the case of an ocean having a bottom very steeply sloping near the shore-line, and nearly level elsewhere, a large propor- tion of the water might be drawn off, and the ocean- surface still remain almost as great as before. We may 1 66 THE POETRY OF ASTRONOMY. assume as a mean and sufficiently probable hypothesis that the lunar oceans had a relative surface equal to between one-half and one-third of the present relative surface of the terrestrial oceans. That is to say, our oceans covering about 72 hundredths of the entire surface of the earth, we may assume that the lunar oceans covered between 36 and 24 hundredths of the entire surface of the moon. It will be seen presently that some importance attaches to this question of the probable surface of the seas on the moon, a portion of the evidence for the theory we are examining depend- ing on this relation. Let us next consider in what way the withdrawal of the lunar oceans into the moon's interior probably took place. On this point, Frankland's presentation of the theory is undoubtedly defective. In fact, it has been the weakness of the theory in this respect, as presented in England, which has in all probability prevented it from receiving the attention here which it fairly deserves. 'The cooling of the moon's mass must,' said Frankland, ' in accordance with all analogy, have been attended with contraction, which can scarcely be conceived as occurring without the development of a cavernous structure in the interior. Much of the cavernous structure would doubtless communicate, by means of fissures, with the surface, and thus there would be provided an internal receptacle for the ocean, from the depths of which even the burning sun of the long lunar day would be totally unable to dislodge more than traces of its vapour.' And he proceeds thus to analyse IS THE MOON DEAD? 1 67 the amount of space which would be rendered available for the retreat of the lunar oceans. 'Assuming the solid mass of the moon to contract on cooling at the same rate as granite, its refrigeration through only 180 of the Fahrenheit thermometer (the difference between the boiling and the freezing points) would create cellular space equal to nearly 14 J millions of cubic miles, which would be more than sufficient to engulf the whole of the lunar oceans, supposing them to bear the same proportion to the mass of the moon as our own oceans bear to that of the earth.' But in reality no such cavernous structure could possibly be developed in the interior of a planet like the moon. Frankland's mistake, here, is similar to that made by Brewster and others, who have suggested that possibly the small mean density of the outer planets might be due to the existence of great void spaces in the interior of those bodies. So soon, however, as we make the roughest calculation of the pressures exist- ing in the interior of even a small planet like the moon, we perceive that there could be no cavities. The most solid materials steel, adamant, platinum become plastic under pressures far less than those brought into action by the attractive energy of a planet's mass upon all parts of its interior, except those not far from the surface. Be it noticed that it is not, as some seem to suppose who have written on this subject, the force of gravity at different depths which has to be con- sidered. That diminishes as the centre of the planet is approached. What we have really to consider is the 1 68 THE POETRY OF ASTRONOMY. pressure produced by the weight of the superincumbent mass above any given level, and this of course becomes greater and greater as the depth below the surface in- creases. If the rigidity of the solid substances forming the solid crust of a planet were such that any amount of pressure could be borne without impairing it, then of course the various layers of the crust would form a series of arches, stronger and stronger with approach to the centre, because of the increased compression, and therefore the increased density of their substance. There is no a priori reason, perhaps, why this should not be so. Compression, for example, might increase the rigidity or force-resisting power of the materials of the earth's substance in such sort that mines might be dug to any depth, and horizontal tunnelling carried out from the lowest parts of any mine. But experiment shows that the fact is otherwise. Under great pres- sures the most solid substances become plastic. Steel behaved like a liquid in Tre sea's experiments, affording the most conclusive evidence that at a depth of ten or twelve miles no steel walls, however massive, could defend a cavernous space from the surrounding pres- sures, which would simply crush in the steel until it formed one solid mass without interstices at least with no interstices which could be seen if the steel were afterwards brought up from that depth to be cut open and examined. It will be readily understood that at the depth of ten or twelve miles there can be no caverns into which the water of the oceans could be bodily withdrawn. Extending similar considerations to IS THE MOON DEAD? 169 the moon, we perceive that there can be no caverns in the moon's interior at a greater depth than sixty or seventy, or at the utmost 100 miles. Now 100 miles is less than the twentieth part of the moon's diameter, and the entire mass of the moon exceeds the mass of the outermost layer (to a depth of 100 miles) in about the proportion of four to one. So that even on the as- sumption that all the external parts of the moon, to the depth of 100 miles, contracted in such a way as to leave cavernous spaces in the manner conceived by Frankland, there would not be nearly enough space for the lunar oceans, supposing them to bear the same proportion to the moon's mass which our ocean bears to the mass of the earth. But, though cavernous spaces would not form throughout the interior of a planet, room would yet be found, even to the degree conceived by Frankland, for the waters of the planet. The greatest possible pres- sure to which the most solid rock can be exposed would not fill the capillary spaces which exist throughout the material of the rock, while the pressure on the water at great depths would force it into even minuter than capillary spaces. This has been conclusively shown during experiments entered upon for another purpose viz. to determine the compressibility of water. For when in 1661 Florentine academicians tried to compress water which had been enclosed within a globular shell of gold, they found that the water under great pressure forced its way through the pores of the gold, and stood on the outside of the globe like dew ; and since that I/O THE POETRY OF ASTRONOMY. time the experiment has been repeated with globes of other metals, a similar result being obtained. It follows from these considerations that, as a planet cools, more and more space is formed for the retreat of the planet's seas ; and that in all probability in the extreme old age of a planet, when its whole frame to the very centre has been sufficiently cooled, space enough is thus formed to hold all the water which had once adorned the planet's surface. If we consider the whole history of the moon's cooling, partly as indicated by her actual aspect, partly by the evidence given by the aspect of other planets, and partly as justly inferrible from the laws of physics, we shall find abundant reason for believing that her seas at any rate might thus have been withdrawn. During the earlier stages of a planet's history, con- sidered in the essay ' When the Sea was Young,' l the seas are floating in the form of cloud and vapour above the planet's surface. In the next stage, when the crust is still hot, but not too hot for the waters to rest upon it, the process of cooling must take place more rapidly in the crust of the planet than in the planet's interior. All this time, then, the crust would be con- tracting upon the nucleus a process which would leave no cavernous spaces between the crust and the nucleus for the waters to retreat to. From time to time the contracting crust would give way, exactly as a non- contracting crust would give way under the pressure of an expanding nucleus. The scene of such a catastrophe 1 See p. 77. IS THE MOON DEAD? I/ 1 would be marked thereafter by a great crater at the place where the crust first gave way, and a series of radiating streaks marking the places where the crust was split open all around that spot. The signs of events such as these in the moon's earlier history are very manifest. There is the great lunar crater Tycho, which is clearly visible to the naked eye, near the lower part of the disc of the moon ; and from this as a centre radiations extend in all directions, some of which run right across the visible lunar hemisphere, and pro- bably extend right round the moon. These also can be seen with the naked eye ; and they are so well marked in photographs of the moon that some supposed the earlier photographs by Draper and Kutherfurd in America, and by De la Eue in this country, were in reality only photographs of a peeled orange, the crater Tycho representing one end of the core, and the radia- tions corresponding to divisions between the sections of the orange. Besides this most remarkable case, there are six others, centres of radiating streaks on the moon's visible hemisphere, and doubtless others upon the unseen hemisphere. We have here clear evidence of the tremendous nature 'of the forces which were at work throughout the moon's frame in the earlier stages of her history, the disturbance in particular by which the radiations from Tycho were made having apparently wracked the whole frame of the moon. Directly, in- deed, these considerations do not affect the theory we are considering, because no large portion of the lunar seas can by any possibility have retreated beneath the THE POETRY OF ASTRONOMY. surface during this stage of her existence. But as showing the enormous store of heat which existed at that time (by far the larger part of which must have remained unexhausted when the next stage began), the consideration of these amazing evidences of disturb- ance has an important though indirect bearing on our subject. After the crust had parted with the greater portion of the heat which it had possessed when first formed, it would cool, and therefore would contract but slowly. The nucleus, on the other hand, which had before con- tracted more slowly than the crust, would now contract more rapidly, leaving spaces between itself and the crust. And then two things would happen. One would be the manifestation of vulcanian energy in con- sequence of the heat generated by the crust as it crushed its way downwards upon the retreating nucleus. The other would be the influx of water wherever it found access to the cavernous spaces between the crust and the nucleus. It is probable that before this vulcanian era of the moon's history was completed a considerable portion of the lunar waters had taken its place perma- nently beneath the crust. It should be noticed that this era corresponds with a part of the earth's existence which is as yet far from being completed, even if it can be regarded as much more than begun. It is far from unlikely that the era during which a planet's crust is thus kept in constant activity by the retreating motion of the nucleus synchronises with the period during which life exists on the planet's surface. During all IS THE MOON DEAD? 1/3 this period, which may have lasted tens of millions of years, not only were portions of the waters of the moon gradually taking up their place in cavernous spaces between the crust and the retreating nucleus, but another process must have been at work to exhaust the lunar seas. When water falls upon a large land-surface in the form of rain, so that the surface is thoroughly drenched, a portion probably disappears permanently from the water-circulation of the globe. Of course, the greater portion is conveyed into the sea in the form of running water. Then, again, the drying of the surface means that the water which had moistened it is taken into the air again in the form of aqueous vapour. And this eventually assumes the form of visible cloud, and after sundry changes (during which it may many times in turn appear as cloud or disappear as vapour) it falls again in rain, and may be either restored in this way directly to the sea from which it came, or so fall on land-surface as to run into some stream communicating by brook, rivulet, river, and estuary with the ocean. And some portion of the water which falls on land-surfaces, passing below the surface, feeds internal streams, and eventually appears again in the form of spring-water. But it cannot be doubted that a portion of the water which falls on dry land soaks its way downwards, very slowly, perhaps, but steadily and continuously, thus removing itself from sight, and pro tanto diminishing the planet's surface- . waters. How much of the water would be removed by these 1/4 THE POETRY OF ASTRONOMY. causes, before the last stage of all began (at least the last change of a planet's existence as a body undergoing change) is not easity determined. Probably a quarter or a third of the water forming the original oceans of a planet might be withdrawn in one or other of these ways, leaving the rest to be removed during the refrige- ration of the nucleus itself a process requiring many millions, possibly hundreds of millions, of years for its completion. In whatever way the withdrawal of the lunar seas was accomplished, it is certain that every particle of water has disappeared from the surface of the moon ; and as there are clear signs of the former existence of extensive lunar seas, apart from the strong a priori considerations showing that the moon must once have had water on her surface, we have little choice but to admit that the waters of the moon have been with- drawn by such gradual processes as have been described above, and consequently that the era of the moon's existence as a habitable world is really removed from the present epoch by the enormous time-intervals re- quired for the completion of those processes. In fact, we can see clearly pictured on the moon's face the evidence which shows that she has passed through all the stages of planetary life, from the time when her whole frame was glowing with intensity of heat, down to the period when she had reached the condition which our earth in the remote future must attain that of a cold dead orb, neither living itself (regarding physical changes as corresponding with vitality) nor capable of IS THE MOON DEAD? 175 being the abode of living creatures. Extending the range of our survey, we find in the giant planets, Jupiter and Saturn, the evidence of an earlier stage than any of which the moon's present aspect affords direct evidence. The sun presents a yet earlier stage, while the gaseous nebulae or masses of luminous star-vapour scattered through the immensity of space illustrate the earliest of all stages of cosmical existence of which we have any direct evidence. On the other hand, we see in Mars, with his small ocean-surface and rare atmosphere, the picture of a stage intermediate between that through which the earth is now passing, and the decrepit or death-like condition of the moon. Mercury, if we could examine his condition more satisfactorily than is the case, would probably illustrate a stage somewhat nearer to the moon's present condition. Venus, on the other hand, so far as can be judged, though a somewhat smaller planet than the earth, is in a somewhat earlier stage of planetary existence. Although the moon may be regarded as to all intents and purposes dead, it must not be supposed that no changes whatever take place upon her surface. On the contrary, some of the peculiarities of the moon's condi- tion must tend to cause even more rapid changes of certain orders than take place in the case of our own earth. Thus the great length of the lunar day, and the moon's waterless condition and rare atmosphere, must help to cause a comparatively rapid crumbling of the moon's surface. During the long and intensely hot lunar day the rock substance of the moon's surface 176 THE POETRY OF ASTRONOMY. must expand considerably, for it is raised to a degree of heat exceeding that of boiling water. During the long lunar night the surface is exposed to a degree of refrigeration far exceeding that of the bitterest winter in the Arctic regions, and must contract correspondingly. This alternate expansion and contraction must gradually crumble away all the loftiest and steepest portions of the moon's surface, and will doubtless, in the long run that is, some few hundreds of millions of years hence destroy all the most marked irregularities of the moon's surface. The cases of change which have been recognised by telescopists who have carefully studied the moon's surface may all, without exception, be referred to this process of gradual but steady disintegration. The most remarkable case hitherto known, for example, the disappearance of the lunar crater Linne, is far better explained in this way than as the result of vol- canic outburst. This case has recently been described as follows, by the present writer : In the lunar Sea of Serenity there was once a deep crater, nearly seven miles across, a very distinct and obvious feature, even with the small telescope (less than four inches in aperture) used by Beer and Madler in forming their celebrated chart. But, ten years ago, the astronomer Schmidt, a selenographer of selenographers (who has in fact given the best energies of his life to moon-gazing), found this crater missing. When he announced the fact to the scientific world, other astronomers, armed with very powerful instruments, looked for the crater IS THE MOON DEAD* 1/7 which had been so clearly seen with Madler's small telescope ; but though they found a crater, it was nothing like the crater described by Madler. The present crater is scarcely two miles in diameter, and only just visible with powerful telescopes ; all around it there is a shallow depression, occupying a region about as large as the whole crater had been before. It seems impossible to doubt that a great change has taken place here, and the question arises whether the change has been produced by volcanic activity or other- wise. Sir John Herschel pronounced somewhat con- fidently in favour of the former hypothesis. 'The most plausible conjecture,' said he, ' as to the cause of this disappearance, seems to be the filling up of the crater from beneath, by an effusion of viscous lava, which, overflowing the rim on all sides, may have so flowed down the outer slope as to efface its ruggedness, and convert it into a gradual declivity casting no stray shadows.' ' But how tremendous the volcanic energy,' we note in the passage referred to, 'required to fill with lava a crater nearly seven miles in diameter, and more than half a mile deep ! The volcanic hypothesis seems on this account utterly incredible, for if such energy resided in the moon's interior we should find her whole surface continually changing. Far more probable seems the idea that the wall of this crater has simply fallen in, scattering its fragments over what had once been the floor of the crater. The forces at work on the moon are quite competent to throw down N 178 THE POETRY OF ASTRONOMY. steep crater-walls like those which seem formerly to have girt about this deep cavity.' l That the kind of vitality evidenced by such changes as these still exists in the moon's frame, is not merely probable but certain. Other changes, however, which were once supposed to have been observed, must be dismissed as having had no real existence. The effects of various kinds of illusion have to be taken into account in considering such phenomena. Thus the theory that a process of monthly change, due perhaps to vegetation, affects the floor of the large lunar crater Plato (called by Hevelius the greater Black Lake), is now rejected, because the supposed change has been shown to be a mere effect of contrast. The apparent change is of this nature : As the sun first begins to rise above the floor of the crater or, in other words, as the light of the filling moon gradually flows over the crater the floor appears bright, getting brighter and brighter as the sun rises higher and higher, up to a certain point. But afterwards the floor darkens, be- coming darkest towards lunar midday. Lastly, as the lunar afternoon progresses, the floor of Plato gets gra- dually lighter again. The mid-day darkening was attributed to some process of vegetation or else to che- mical changes. It has no real existence, however, but is due simply to the effect of contrast with the great brightness of the crater-wall all around, which is formed of some very white substance, and looks peculiarly bright and lustrous at the time of lunar mid-day, 1 The present writer, in the Spectator for June 24, 1876. IS THE MOON DEAD* 179 so that contrasted with it the floor looks peculiarly dark. On the other hand, during the morning and evening hours, the black shadow of the crater-wall is thrown across the floor, which by contrast looks brighter than it really is. This explanation has indeed been denied very confidently by some who formerly advo- cated the theory that lunar vegetation causes the darkening of the floor ; but there can be no doubt of its justice, for no one (not prejudiced in favour of a theory) who has tested the matter experimentally, eliminating the effects of contrast, has failed to find that there is no real darkening of the floor of Plato. It seems as certain as any matter not admit- ting of actual demonstration can ba that the moon is, to all intents and purposes, dead. Her frame is indeed still undergoing processes of material change, but these afford no more evidence of real planetary life than the changes affecting a dead body are signs of still linger- ing vitality. Again, it seems certain that the processes through which the moon has passed in her progress towards planetary death, must be passed through in turn by all the members of the solar system, and finally by the sun himself. Every one of these orbs is constantly radiating its heat into space, not indeed to be actually lost, but still in such sort as to reduce all to the same dead level of temperature, whereas vitality depends on differences of temperature. Every orb in space, then, is tending steadily onwards towards cos- mical death. And, so far as our power of understanding or even of conceiving the universe is concerned, it M 2 l8o THE POETRY OF ASTRONOMY. seems as though this tendency of every individual body in the universe towards death involved the ten- dency towards death of the universe itself. It may indeed be said that since the universe is of necessity infinite, whereas we are finite, we cannot reason in this way from what we can understand, or conceive, to con- clusions respecting the universe, which we cannot even conceive, far less understand. Still it must be admitted that, so far as our reasoning powers can be relied upon at all, the inference, from what we know, appears a just one, that the life of the universe will have practically departed when the largest and therefore longest-lived of all the orbs peopling space has passed on to the stage of cosmical death. So far as we know, there is but one way of escape from this seemingly demonstrated, but in reality incredible, con- clusion. May it not be that as men have erred in former times in regarding the earth as the centre of the universe, as they have erred in regarding this period of time through which the earth is now passing as though it were central in all time, so possibly they may have erred in regarding the universe we live in, and can alone comprehend, as though it were the only universe ? May there not be a higher order of universe than ours, to which ours bears some such relation as the ether of space bears to the matter of our universe ? and may there not, above that higher order, be higher and higher orders of universe, absolutely without limit ? And, in like manner, may not the ether of space, of which we know only indirectly though very certainly, IS THE MOON DEAD? l8l be the material substance of a universe next below ours, 1 while below that are lower and lower orders of universe absolutely without limit ? And, as the seem- ingly wasted energies of our universe are poured into the universe next below ours, may it not well be that our universe receives the supplies of energy wasted (in seeming) from the universe next in order above it ? So that, instead of the absolute beginning and the absolute end which we had seemed to recognise, there may be in reality but a continual interchange between the various orders of universe constituting the true universe, these orders being infinite in number even as each one of them is infinite in extent. We find our- selves lost, no doubt, in the contemplation of these multiplied infinities ; but we are equally lost in the contemplation of the unquestioned infinities of space and time amidst which our little lives are cast, while the mystery of infinite waste, which seems so inscru- table when we consider the universe as we know it, finds a possible interpretation when we admit the existence of other orders of universe than the order to which our lives belong. Thus should we find a new argument for the teaching of the poet who has said Let knowledge grow from more to more, But more of reverence in us dwell, That, mind and soul according well, May make our music as before, But vaster ; 1 The work called the Unseen Universe presents a portion of the evidence to this effect, but unfortunately the style of that work is not sufficiently lucid to bring its reasoning within the range of the general non-scientific reader. 1 82 THE POETRY OF ASTRONOMY. a new significance in the vision of him who said See all things with each other blending, Each to all its being lending, All on each in turn depending ; Heavenly ministers descending, And again to heaven uptending, Floating, mingling, interweaving, Kising, sinking, and receiving Each from each, while each is giving On to each, and each relieving Each the pails of gold ; the living Current through the air is heaving; Breathing blessings see them bending, Balanced worlds from change defending, While everywhere diffus'd is harmony unending. THE MOON'S MYRIAD SMALL CRATERS. SINCE Galileo first turned a telescope upon the moon, the lunar craters have been among the wonders and mysteries of astronomy. It is not merely or even chiefly the vast size of some of these objects which excites astonishment. Indeed, it might almost be inferred from what we know of the moon's size and general structure, that her volcanic energies would be more effective, though not greater, than those of our own earth. The really surprising characteristic of the lunar surface is the amazing number of the lunar craters. Even Galileo, though with his weak telescope he could see but a few of the craters which really exist in the moon, compared those in the south-western part of the moon's disc to the eyes in a peacock's tail. With each THE MOON'S MYRIAD SMALL CRATERS. 183 increase of telescopic power, more and more craters have been seen. Kegions supposed to be comparatively smooth have been found, on closer scrutiny with higher powers or under more favourable conditions, to be covered with minute craters. The slopes of the larger craters, even in some cases their floors, have been found to be strewn with small crater-shaped depressions. In line, almost the whole surface of the moon may be said to be pitted with depressions of all sizes, from mighty gulfs three or four hundred miles across, down to minute saucer-shaped shallows, such as only the most powerful telescopes will reveal. I propose to enter here into a brief consideration of the probable cause of the smaller lunar craters. Un- questionably the feature may be regarded as marking a characteristic distinction between the moon and our own earth. It may well be that the moon is an old world, while our earth is comparatively young ; but, for my own part, I cannot consider that the earth can come during the progress even of millions of years to resemble the moon in details, however closely she may hereafter resemble the moon in general respects in the ab&enca of water, for instance, in the tenuity of her atmosphere, and so forth. The course I propose to follow is one which, I think, may with advantage be pursued in a great number of cases in which as yet it has been little followed. Starting with the views now generally entertained respecting the origin and structure of the solar system, I propose to inquire what might in all probability be 1 84 THE POETRY OF ASTRONOMY. expected to happen in the special case of our own moon ; comparing the results to which we seem led, in this way of viewing the matter, with the results of actual obser- vation. In other words, I am going to follow an a priori method of reasoning, testing the conclusions to which it may lead by a posteriori considerations. It is now generally admitted that the various members of the solar system reached their present con- dition by processes of development. Few, however, among those who have studied the theory of cosmical evolutions for themselves, are disposed to accept un- questioningly Laplace's idea that the whole solar system was once a great mass of gaseous matter. It is only, indeed, by carefully closing the mental eye to the results of modern physical researches, that a theory of the kind can for a moment be entertained. I will not here consider the multitudinous objections against the so-called nebular hypothesis, regarded as the sole hypo- thesis of the origin of the solar system. Nor, on the other hand, will I consider here in detail the arguments in favour of the theory that the various members of the solar system acquired no small portion of their present bulk by a process of aggregation. Let it suffice to mention that the theory of planetary and solar growth, by the gathering in, during past ages, of immense quantities of meteoric and cometic matter, is one which has this immense advantage over the nebular theory, that it assumes the former action of a process which is going on at this present time ; while also, as regards the materials forming the masses of the sun and planets, THE MOON'S MYRIAD SMALL CRATERS. 185 this theory leads to inferences according well with known facts. I must, however, premise that neither the aggrega- tion theory alone nor the condensation theory alone can fully explain the observed present condition of the solar system. We must admit on the one hand that the several members of this system, including the sun, gathered in their substance in large amount from with- out* But we must also admit the former vaporous condition of the sun and planets, not indeed exactly in the way indicated by Laplace, for these bodies never could have had the enormous extension his theory re- quired and yet have retained coherence ; but that they were formerly far more expanded than at present, and were thus of very small density, may be regarded as to all intents and purposes certain. Indeed, the aggrega- tion theory would be insufficient to account for the formation of even a small portion of each planet's mass, unless we remembered that in the earlier stages of their existence the several planets were vaporous, and therefore much larger than in their later solid condition. For it would only be when thus expanded that they would gather, in their orbital motion around the sun, a sufficient quantity of meteoric or cometic matter. At present, for instance, our own earth, though she gathers in some 400 millions of meteors in the course of each year, yet gathers a quantity of matter so small compared with her own substance that in the course of 400 millions of years the earth's diameter would be increased only by a single inch. When the earth had a much 1 86 THE POETRY OF ASTRONOMY. smaller mass than she has now, however, but that mass vaporous and of small density, she would gather in many thousand times as much matter in each circuit round the sun, apart always from the fact that in those remote times the quantity of meteoric matter as yet not gathered in was many thousand times greater than it is at present. Now, we have in considerations such as these the means of explaining in some degree the peculiarities of the moon's state. In the first place, we must set the period during which the moon's globe was being fashioned by cosmic forces in a far more remote antiquity even than the corresponding period of the earth's history. How far back the last-named period should be set is not very easily guessed even in the roughest manner. Accord- ing to geologists, the interval during which the earth's crust has in general respects been in the same state as at present, must lie between 400 million years and 20 million years. The preceding period, during which the crust was cooling from the heat it possessed when first formed to a temperature such that living creatures could exist upon the crust, must have lasted at least 300 millions of years. The period preceding that again, when the earth had no crust, but was almost entirely vaporous, lasted probably many hundred millions of years. It must have been during this remotest of all the periods of the earth's own history that the moon was formed. But she must have been detached from the earth's mass, or rather left behind by the retreating THE MOON'S MYRIAD SMALL CRATERS. 187 vaporous mass of the earth, very early in this first stage of the earth's existence. Whether at this time the moon (which in any case contained far less matter then than she does now) existed as a single mass or as a number of small masses scattered round a ring-shaped region, is a point on which different views may be entertained. For my own part, though I cannot doubt that the substance of the moon once formed a ring around the earth, I think there is good reason for believing that when the earth's vaporous mass, receding, left the moon's mass behind, this mass must already have been gathered up into a single vaporous globe. My chief reason for thinking thus is, that I cannot on any other supposition find a sufficient explanation of one of the most singular characteristics of our satellite her rotation on her axis in the same mean time, exactly, as she circuits around the earth. This peculiarity in the moon's rotation is generally treated as though it were a natural and, so to speak, an antecedently likely arrangement, instead of being one of a very remarkable and unlikely nature. It is stated, very justly, that if the moon's original rate of turning had nearly coincided with her rate of travelling round the earth, in such sort that she would very nearly keep one side directed towards the earth during a single revolution, the earth's attraction on the elongated body of the moon would so operate as to compel the moon always to keep that side earthwards. The longer axis of the moon would sway backwards and forwards 1 88 THE POETRY OF ASTRONOMY. on either side of a line directed towards the earth, but would not be carried altogether round so as to bring the farther side of the moon eventually into full view. And as we know that such swayings, if they really take place, are very slight (for what is called the moon's libration or balancing has nothing to do with the sway- ing I refer to), it follows that originally the moon's rotation must have agreed very closely indeed with her rotation. All this is correct enough ; but what is commonly left unnoticed is the exceedingly improbable nature of the imagined coincidence, if the moon's rate of rotation and her rate of revolution had been indepen- dently communicated to her. Professor Grant, in his fine work the ' History of Physical Astronomy,' speaks of this coincidence as a relation which, though difficult to explain by the doctrine of chances, becomes very interesting and sug- gestive when it is considered as the result of Supreme Intelligence. But that method of dealing with the difficulty is not likely to be acceptable in these times, when men regard all the facts ascertained by observation as belonging to the domain of science. There is not a single department of scientific research in which men might not be checked at the outset by an explanation of that sort. Newton asked, Why does the moon travel round the earth and the earth round the sun ? and he proceeded in the scientific manner to find out. If he had been contented to answer, It pleased the Supreme Intelligence that these bodies should move precisely as they do, he would have manifested the fullness of his THE MOOWS MYRIAD SMALL CRATERS. 189 faith, but he would have lost the opportunity of effect- ing a very noble discovery, one too which affords grander conceptions of the mechanism of the universe than the mere motions which it explains. So here, in the case of the moon's rotation, it sounds well, perhaps, to say that we accept the observed fact as evidence of the wisdom of the Supreme Intelligence, and do not seek to know how it was brought about ; but this submission of the intellect to faith implies not only a certain in- tellectual languor, but also a doubtful, hesitating faith, I confess that for my own part I prefer the honest bluntness with which my valued friend Professor Newcomb presents this matter. ' That the adjustment,' he says, ' should be a mere matter of chance, without any physical cause to produce it, is almost infinitely improbable, while to suppose it to result from the mere arbitrary will of the Creator, is contrary to all scientific philosophy.' Now, there is a circumstance in the condition and movements of our own earth indicating a way by which the moon might have attained that peculiar rate of rotation. The tidal wave, which, roughly speaking, may be said to sweep twice a day round the earth in a direction contrary to her rotation, exerts a certain exceedingly small effect in slowing her rotation-rate, and thus in lengthening her day. This effect is so small that many millions of years must elapse before the day would be doubled in length, and many mil- lions of millions of years before the earth would turn at such a rate as to present always the same face THE POETRY OF ASTRONOMY. towards the moon, even if the present lengthening of the day continued constantly, instead of gradually diminishing from its present exceedingly minute amount. Now, if we suppose the moon to have existed for millions of millions of years, and to have had during the greater part of that time a deep ocean in which tides would be raised by the earth's attrac- tion, we can understand the possibility that an original rotation of the moon at something like the earth's present rate of turning might have been gradually reduced until at length the present slow rate of turn- ing once in 27 days had been attained to. But we require most tremendous time-intervals on such a theory, and moreover we require that the moon's con- dition should at one time and for a long time have been exceedingly unlike her present condition. The former difficulty is more serious than the latter ; for it is almost impossible to set back the formation of the moon farther than a few thousands, or at the most tens of thousands, of millions of years, whereas this theory would require that she should have been the scene of tidal disturbance during millions of millions of years. If we suppose that her own mass was wholly or partially fluid for millions of years, we to some degree escape this difficulty, for the tides which would in that case have been raised by the earth would have been far larger than mere tides in the lunar seas. Formerly this was the explanation which seemed to me the most probable. I find that Professor Newcomb regards it THE MOON'S MYRIAD SMALL CRATERS. IQI with some degree of favour. ' If the moon were once,' he says, ' in a partially fluid state, and rotated on her axis in a period different from her present one, then the enormous tides produced by the attraction of the earth, combined with the centrifugal force, would be accompanied by a friction which would gradually retard the rate of rotation, until it was reduced to the point of exact coincidence with the rate of revolution round the earth as we now find it. We therefore see in the present state of things a certain amount of probable evidence that the moon was once in a state of partial fluidity.' But while I still regard this theory as the true one, I recognise in a yet earlier stage of the moon's de- velopment the most effective part of the earth's action in modifying the rate of the moon's rotation. When the moon was in great part gaseous, at which time the earth was almost entirely gaseous, and probably ex- tended beyond the mass whence one day the moon was to be formed, this mass would be compelled to rotate very nearly in the same time as it revolved around the earth's centre. It may be compared to a mass of matter carried round by a whirlpool. Such a mass might have a slow independent rotation in the fluid ; but, speaking generally, we may describe its motion as corresponding to that which it would have if the fluid were so thick and viscid as only to allow the mass to move with it as it whirled round. If this were so in the moon's case, then when the contracting mass of THE POETRY OF ASTRONOMY. the earth left the moon outside, the moon would have just such a rate of rotation as she has at present that is, she would turn once on her axis as she circled once round the earth. And though, as the moon contracted, her rate of rotation would tend to alter, the action of the earth would be competent to overcome this ten- dency, compelling the moon to move always with the same face directed earthwards. Though there are difficulties in the theory thus presented, and though indeed it is altogether unlikely that the exact correspondence described in the pre- ceding paragraph ever really existed, I apprehend that there is no real objection to the theory that the ob- served peculiarity of the moon's rotation was chiefly brought about in this way that is, while the moon's mass was in great part vaporous. In a later stage, when the moon's mass was chiefly fluid, another large share of the work would be done. Only a very small part would thus be left for the time when the moon's surface had become solid but was still swept by ocean tides. In this way we not only attain an explanation which accords with accepted views respecting the past condition of the moon, as one of the members of the solar system, but we escape the necessity of imagining periods of time so long that even the tremendous periods which science recognises as appertaining to the past of our solar system seem small by comparison. For it is certain that a globe like the moon, having oceans like those of our own earth, and rotating once in twenty-four hours, would not be compelled by the THE MOON'S MYRIAD SMALL CRATERS. 193 earth's attraction to rotate once a month in less than a trillion (a million million millions) of years. It is well to notice, however, that no matter what physical interpretation of the observed peculiarity is accepted, we find in every case enormous time-intervals, during which the moon must have existed and have been subject to the earth's attraction. We are com- pelled to reject the idea that mere chance made the moon rotate as she does, keeping perfect time with her motion round the earth. We cannot accept the belief that, whereas the Supreme Intelligence allowed almost all the motions in the solar system to be completed in times no way related to each other, so that, for example, no exact number of days or months measure the year or any number of years, and that no exact number of hours or days measure the common lunar month or any other kind of month, or any number of any of these months, the Creator nevertheless saw fit in the Beginning to set the moon's turning motion in exact accordance with her motion round the earth a relation not only utterly useless (at least, no one has ever yet been able to con- ceive any possible use it could have), but positively dis- advantageous in more ways than one. It remains only that we should regard the relation as the result of physical processes : and so regarding it, we find that, in whatever way it was brought about, many millions or many hundreds of millions of years must have elapsed before the moon's movements received their final adjust- ment. Now let us revert to the theory which I advanced o 194 THE POETRY OF ASTRONOMY. originally in my book on the moon (p. 343, first edition), and which, as we have seen, Professor New- comb considers the most probable viz. that the moon's rotation-rate was determined when the greater part of her mass was fluid. Eemembering the exceeding remoteness which must be assigned to that era of her career, let us consider the conditions under which she has existed since. It will be observed that I do not insist on her prior existence as a vaporous mass, at least as an essential point in my present reasoning. It is not that I entertain any doubt that she was for a long time a vaporous mass ; but because it would be difficult to indicate any way by which any traces of what happened to her during that part of her existence could be detected. When she had become fluid, even, she would retain no trace of any of the accidents to which she would be exposed : luminous masses might fall upon her, but they would be absorbed into her fluid globe, leaving no sign of the encounter. It would not be till she began to lose her fluidity, as the fiery heat of her globe passed away, that any visible effects would result from the shocks and collisions to which she would be exposed. I pass on at once then to this era of the moon's existence. It is certain, in the first place, that at that time millions of millions of tons of matter, now forming part of the masses of the various members of the solar system, were travelling about as meteors. It would be utterly unreasonable to imagine that the process of meteoric indraught at present taking place on the earth THE MOON'S MYRIAD SMALL CRATERS. 195 is not also taking place on every member of the solar system, or that this process of growth, which all the members of the solar system are undergoing now, has not taken place during past ages, and will not take place during ages yet to come. But this is far from being all. Since we know that every meteor that falls upon this earth, or on any other planet, or on the moon, is there and then brought to the end of its existence as an independent body, we perceive that the process of meteoric indraught is one of diminishing activity. The supply of meteors is becoming slowly but steadily exhausted. Doubtless plenty yet remain, and will remain for millions of years yet to come. They never can be all consumed, in fact, any more than the air in the receiver of an air-pump can ever be exhausted by the process of pumping. Each stroke of the pump removes a certain volume of the rarefied air left in the receiver ; but as the air grows rarer and rarer the actual amount of air removed is diminished, and of course the air removed never can be the whole of the air left, since, by the very nature of the process of exhaustion, a small portion only of the contents of the receiver is removed at each stroke. So with the process of meteoric exhaustion. Every year the earth sweeps up or gathers in all the meteors encountered in its track, and each planet, in each of its circuits round the sun, does likewise ; but as the meteors become rarer and rarer the number swept up in any given time becomes less and less. Nor can all ever be swept up, since each planet, in each of its circuits, clears of meteors only a o 2 196 THE POETRY OF ASTRONOMY. very minute portion of the solar domain. The infer- ence as to the past is obvious. Many millions of years ago the number of meteors gathered in by any planet or satellite must have been enormously greater than it is at present. Now, the present rate of meteoric indraught is not altogether insignificant. It has been calculated that the earth gathers in, in the course of a year, as many as 400 million meteoric bodies, large and small, from the great masses which break their way through the air our shield against the meteoric artillery down to bodies so minute that a telescope would be required to make them visible in their rush through the air. This, be it remembered, is a result deduced from observation, and so deduced as certainly to fall short of the trut/h, not exceed it. In one sense the supply of meteoric matter seems enormous, while in another sense it is exceedingly small. If we assign to the meteors an average weight of only a single grain, we yet find that the earth grows a thousand tons in weight in three years, so that since the time of Abraham the earth's weight must have increased much more than a million tons. Probably one grain is too low an estimate of the average weight of these bodies. Professor Harkness, of Washington, has recently deduced from the known facts respecting meteors a result which accords closely with one which I myself enunciated in 1871 (as is natural, seeing that I used the same general evidence, and dealt with it in much the same manner). At the present rate of meteoric downfall, 400 million years or THE MOON'S MYRIAD SMALL CRATERS. 197 thereabouts would be required to increase the earth's diameter by a single inch. It may seem at a first view as though this result were altogether inconsistent with the theory that any considerable portion of the earth's mass has been de- rived from meteoric aggregation. But in reality, when due account is taken, first of the former expansion of the earth's globe when it was in the vaporous state, secondly of the enormous length of time during which the process of indraught has probably taken place, and thirdly of the fact that the present density of meteoric distribution must be exceedingly small compared with that existing hundreds of millions of years ago, it appears that ninety-nine hundredths of the earth's whole mass might readily have been gathered in by meteoric aggregation. I do not here dwell upon the evidence showing this, because it does not belong to my subject; but it seemed necessary to mention that, so far is any difficulty from arising in the way sug- gested that is, from the poverty of meteoric material that in reality the real difficulty is to understand how the earth remained so small when we consider how enormous must have been the quantity of meteoric matter in remote eras to account for so many millions of millions of meteors remaining still uncaptured. Now, the moon, travelling along with the earth in the remote ages to which our present inquiry relates, must have gathered in her own share of meteoric matter. At this present time, for instance, about thirty millions of meteorites, large and small, fall each 198 THE POETRY OF ASTRONOMY. year upon the moon. She passes through the same meteoric systems as the earth, and she can no more escape meteoric downfall as she thus rushes through these systems than the earth can. We may compare her companionship with the earth to that of a child with a grown person in a shower of rain. As many drops do not fall on the child as on the adult because the child is smaller ; but the child gets as thoroughly drenched as his grown companion, assuming neither to be protected by an umbrella. So the moon receives as many meteors on each square mile of her surface (on the average of many millions of years) as the earth does. Since her surface is about one-thirteenth of the earth's (more exactly two-twenty-sevenths), she re- ceives about one-thirteenth of the number of meteors which the earth encounters, or, taking the number above-mentioned for the earth, the moon's annual in- draught of meteors is at present about 30 millions. In passing, it is worthy of special notice that the downfall on each square mile of the moon is equal to the downfall on each square mile of the earth, on the average of long periods. It follows from this that the moon's present rate of growth from meteoric aggrega- tion is equal to the earth's. Not that the moon grows equally either in volume or in mass, for her annual growth in both respects is but about one-thirteenth of the earth's annual growth ; but as her surface is only a thirteenth of the earth's, a meteoric deposit of equal thickness is received each year by the moon and by the earth. And this has been true during millions of years THE MOON'S MYRIAD SMALL CRATERS. 199 past. Now if two bodies, unequal in size, were to grow equally in diameter year after year, they would become in the long run, to all intents and purposes, equal in size. Imagine a million miles added to the diameters of both the earth and moon ; then the earth would have a diameter of 1,008,000 miles, and the moon a diameter of 1,002,200 miles, and these numbers are practically equal the difference between them being very small compared with either. This is not a point of any im- portance as regards the future history of the earth and moon, for it is quite certain that neither will ever add half a mile to their present diameters, even though they should continue to travel as they now do for a million millions of years. But it is a point of extreme importance as respects the past of our earth and moon a circumstance which, so far as I know, no one has hitherto noticed. Suppose, for instance, we imagine the earth at some exceedingly remote epoch to have had only a thousandth of her present mass, so that at the same density her diameter would be only one-tenth that which she now has, and her surface one-hundredth of her present surface. Then if the moon existed at the same time, in the same state vaporous, fluid, or solid she would add as many miles to her diameter year by year from meteoric indraught as the earth would. And if this had continued to the present time, it would actually follow that the moon should have added to her diameter then (whatever it may have been) nine- tenths of the present diameter of the earth, or, roughly, 200 THE POETRY OF ASTRONOMY. about 7,000 miles. But the moon only has a diameter of about 2,160 miles altogether. It follows, therefore, that either the moon only had existence as a separate orb from the earth long after the earth had received the greater part of her present mass, or else the various stages of the moon's existence as a vaporous and as a fluid globe were very much shorter than the correspond- ing stages of the earth's existence. The latter is altogether the more probable explanation, and accords with what we should expect to happen during the cooling of the unequal masses of the earth and moon. But it is well to notice that our theoretical anticipa- tions in this respect are thus confirmed by reasoning of another kind. It has been calculated by Bischoff that the earth required 350 millions of years to cool from 2,000 de- grees to 200 degrees centigrade, or in other words the earth must have existed as a ball of fused rocks for about that time. It may readily be shown that the moon would have remained fluid during only about a fourth of the time, say about 80 millions of years. Now, during the greatest part of this long period the surface of the moon would be viscid rather than fluid ; and during the last ten or twelve millions of years of that period the moon's surface would be simply plastic. It would receive and retain any impressions which it might receive from without, much as the surface of a nearly dried pool of mud receives and retains the im- pressions of raindrops. Or rather, as such a surface, if stones be thrown upon it, allows the stones to pass THE MOON'S MYRIAD SMALL CRATERS. 2OI through, and shows thereafter a shallow depression where the stone had fallen, so if any large mass fell upon the moon's surface while in the plastic state, the mass would pass below the surface, and a circular saucer-shaped depression only would show where the mass had fallen. Let us suppose that the moon's surface was in this plastic state for only about three million years, remem- bering that, according to all that can be inferred from the experiments made by physicists and from the theoretical researches of mathematicians, this probably falls very far short of the truth. And next let us suppose that at the remote era to which we must refer that special stage of the moon's development, the density of meteoric distribution in the solar domain was only ten times as great as it is at present, remembering that this also is probably very far short of the truth. Now, among the meteors which fall each year upon the earth, few are large enough to break their way through the earth's atmospheric shield, without being either vaporised in their rush through it, or else caused to burst into a number of small fragments. Possibly over the whole earth some ten or twelve may thus fall in a year, one or two only being seen, because the chances are largely in favour of a meteorite escaping detection as it falls. If we suppose that at present only four such meteorites fall on the average each year upon the earth, and that therefore one only falls at present in the course of about three years upon the 202 THE POETRY OF ASTRONOMY. moon, we are certainly not taking an exaggerated esti- mate of the present rate of downfall of large meteoric masses upon our satellite. Of course a much larger number of meteoric bodies of all sizes reach the moon, for she travels on her course without the protection of an atmosphere, at least she has no atmosphere dense enough to ward off even the smallest meteors. So that, in reality, some 30 million bodies large and small must actually impinge on the moon's surface each year ; and probably some ten or twenty thousand are of the kind we call fire-balls. It is, however, to be noted that almost every mass which thus strikes the moon must be vaporised by the intense heat excited as it impinges on the moon's surface ; and even if this did not happen, 1 only one or two of the very largest which might so fall in the course of a century or so would be visible on the moon's surface observed under the most favourable conditions, with the largest telescopes made by man. Moreover, we may restrict our attention to the largest meteorites, in considering the moon's plastic era, for most probably at that time she had an atmosphere not far inferior to the earth's present atmo- sphere, as a shield against meteors. Putting one very large meteorite in three years as the present rate of downfall on the moon, it would 1 A certain proportion of meteoric masses reach the earth, and so, also, a certain proportion must reach the moon, with relatively small velocities. For instance, those which travel the same way, and either overtake or are overtaken with only the difference of their velocity and the velocity of the earth (or moon, as the case may be). THE MOON'S MYRIAD SMALL CRATERS- 203 follow that, at the remote period to which our re- searches relate, ten such meteorites would fall in three years. Thus, in the three millions of years during which the era may be safely assumed to have lasted, ten million very large meteorites fell, according to the moderate assumptions we have made, upon the plastic surface of our satellite. These would not correspond to the very largest meteorites or aerolites known to men, either as having fallen on the earth or as seen and measured while moving athwart the sky. From time to time bodies are seen whose diameter is esti- mated at several hundred yards ; and though no masses of this size have been known to reach the earth within the historic period, it must be remembered that the chances are usually in favour of the explosion of such meteorites into fragments as they pass through our air. I imagine, however, that the estimate of most of these bodies has been considerably exaggerated. 1 The point to be noticed here, however, is this, that a mass far too small to be discernible at the moon's distance, would produce a discernible mark if it fell on the moon's surface in the plastic era. A circular depression far larger in diameter than the falling mass would be formed at the place where it had pierced the 1 Though not quite to the extent imagined by Mr. Phipson in his treatise on Meteors, Aerolites, and Falling Stars. He has fallen into two mistakes, rather seriously affecting his conclusions : first, in taking the average height of great meteors above the earth as their average distance from the observer; and next, in supposing that a globe 206,000 times as far away as its diameter, subtends an angle of one minute, instead of an angle of one second only (a sixtieth part of a minute, that is). 204 THE POETRY OF ASTRONOMY. viscous crust. So that we might fairly take into account the downfall of all the very large meteorites that is, according to our estimate above, of some ten million masses as competent to leave marks such as could be recognised with powerful telescopes from our earth, supposing nothing happened in later stages of the moon's history to obliterate such marks. Among these ten million meteorites ten only in a thousand perhaps might be very large, so as to leave where they fell circular depressions from a quarter of a mile to a mile in diameter. For the diameter of the aerolites themselves, of course, would not be nearly so large as that of the circular depression left where they had fallen. In this case about a hundred million small shallow craters would be formed on the moon's surface during the plastic era. But again, among these very large aerolites, pro- bably some it might be only one in a thousand would be excessively large, from a quarter to half a mile perhaps in diameter. It is true, we know of no such mass having struck our earth within historic times, nor have any such masses been recognised in the earth's crust ; but so many instances are on record of the passage of masses apparently as large as 100 yards in diameter through our air, which but for the air would certainly have fallen with their full mass on the earth's solid surface, that we cannot but believe in the existence even to this day of many enormous meteorites, and in the probability that at long intervals they fall upon our earth's atmospheric shield. Thus during THE MOON'S MYRIAD SMALL CRATERS. 2O$ these three million years some hundred very large masses would fall upon the moon's plastic surface, leaving where they had pierced the moon's crust vast circular depressions, each far exceeding in diameter the mass whose downfall had produced it. Before proceeding to consider the result of such meteoric downfall on the moon's surface, I must remind the reader yet once more that, strange though these considerations which I am presenting to him may seem, they are based entirely upon known facts, and probably fall even far short of the truth. The nebular hypo- thesis, or some modification of that hypothesis, of the formation of the solar system is received by all astro- nomers of repute in the present day. The enormous duration of the various periods of planetary and lunar development has been demonstrated not only by experi- ments on the cooling of various substances, but by the study of our earth's crust. We know that meteors of all kinds still encounter the earth, and have no choice but to believe that, since so many now remain, the number existing millions of years ago must have been enormously greater. We know certainly that the moon in her journey round the sun must have en- countered her share of these meteoric bodies. And we cannot possibly doubt that any considerable meteoric mass falling on the moon's surface at any time during the long period when that surface was wholly or partially plastic, would leave a larger circular depression where it has pierced the crust. All these points may be regarded as certain ; at 206 THE POETRY OF ASTRONOMY. least, any doubts respecting them must be doubts affecting the general theory of the evolution of the solar system, and such doubts need not here be com- bated. But now the question arises whether the marks thus left upon the moon's surface would remain during the later stages of her existence down to the present time. It is certain that the surface of our own earth must once have been in a similar way pitted with the marks of meteoric downfalls, for she, like the moon, was in her growth Pelted with star-dust, stoned with meteor-balls, and the era when her surface was plastic to receive and to retain the marks of the meteoric hail-storm (before Man and his works and all that stirred itself Of its own motion could live upon it) lasted many millions of those cos- mical instants which men call years. Yet we know that of those impressions which the earth then received no traces now remain. Again and again has the sur- face of our earth been changed since then. By the denudation of continents, by the deposit of strata under seas, and by the repeated interchange of seas and continents, every trace of the primeval surface of our globe has long since been either removed or concealed. Would this have happened with the moon ? or if we are to judge by the evidence of what is, rather than THE MOON'S MYRIAD SMALL CRATERS. 2O? by the consideration of what would have befallen, has this happened with the moon ? As regards the probable sequel of the state of things which, as we have seen, must have existed when first the moon's surface solidified, it is not easy to form an opinion. On the one hand, there are reasons for sup- posing that for many long ages the moon would re- semble our earth in having an atmosphere and oceans, though probably the atmosphere would be far rarer than ours is now, and the oceans far more limited in extent. On the other hand, it is impossible to over- look the actual facts of the case, viz. that at present the moon has no atmosphere of appreciable density, and no ocean surface at all, while the theories which have been advanced to explain the removal of an atmo- sphere and oceans formerly existing are, to say the least, not altogether satisfactory. They might account perhaps for the disappearance of a very tenuous atmo- sphere, and the drying up (or rather the soaking in) of oceans of limited extent ; but scarcely for the dis- appearance of all signs of an atmosphere and oceans at all resembling those of our own earth. On the whole, I am disposed to think that those features of our moon which have been regarded as indicating the former existence of oceans as, for instance, the darkness of the low-level regions called seas, the existence of regions looking like alluvial deposits, and so forth may be regarded as indicating only the existence of regions which remained liquid long after the rest of the moon's surface had solidified. 208 THE POETRY OF ASTRONOMY. I would not deny the possibility, or even the proba- bility, that in these regions there may formerly have been considerable seas. Nay, they may possibly have been entirely sea-covered. But it certainly has not yet been proved that they ever were so. Of course when the moon's surface was partially solid or even merely plastic and partially liquid, all the liquid matter would seek the lower levels. The plastic surface only would retain the marks of meteoric down- falls: that is, the traces of the fall of those many thousands of large masses which we have seen must have struck the moon during her plastic era. Where the liquid surfaces existed, no such traces could be retained, any more than the marks of rainfall can be retained by the surface of the sea. On the one hand, then, if we suppose the atmo- sphere of the moon in remote times exceedingly tenuous and the seas very limited in extent, the effects of aerial denudation would be utterly insignificant com- pared with those which we recognise on the earth ; so that we might expect the signs of meteoric pitting to be very little disturbed during the comparatively short era of the moon's existence as a habitable world. On the other hand, we could not expect any traces of meteoric downfall to remain in the low-lying regions to which the liquid portions of the moon's surface formerly flowed. Only when this liquid matter had either solidified or been gradually withdrawn into the moon's interior, could irregularities be formed, retained, or recognised in these regions. THE MOON'S MYRIAD SMALL CRATERS. 209 If these a priori considerations are just, it would be found first, that the high-level regions of the moon would be marked by multitudinous small craters of all dimensions, from the minutest which the most powerful telescope could recognise to craters a mile or two in diameter ; secondly, that the low-level regions would present a different colour, and, as it were, texture, being formed of different matter which, retaining its liquidity longer, had necessarily come to form the lower lunar levels ; thirdly, that comparatively few craters, and those mostly small ones, would be found over these low-lying regions. To these probable features may be added, but with less antecedent likelihood, this that in the arrangement of the smaller lunar craters, pecu- liarities might sometimes be recognised indicating the occasional fall of a flight or string of meteors such as we sometimes see travelling athwart our skies even in these times when the supply of meteoric matter is all but exhausted by comparison with the wealth of meteors formerly existing. Now let us see how these anticipations accord with the facts. To avoid all possibility of prejudice I will take the account of lunar details from a work written by an official astronomer, one therefore not likely to consider even, far less to be prejudiced in favour of, speculations respecting the past history of the heavenly bodies (any more than a land surveyor or a civil engineer would be likely to dwell upon geological speculations respecting the soils or surfaces with which he has officially to deal). I must admit that Professor New- p 210 THE POETRY OF ASTRONOMY. comb, to whom I refer, differs entirely from most European official astronomers in this respect, as do others of his countrymen. In writing his treatise on astronomy he does not seem by any means to have thought it essential to eschew all consideration of the physical significance of observed facts. I would therefore have taken a description of the moon by some one else, some official astronomer of the purely surveying order ; but unfortunately the descriptions of the moon in their writings are too incomplete to be of interest or value ; and any thoughts as to the moon's probable conditions, either now l or in the remote past or future, would be sought in vain. Let us hear, however, how Professor Newcomb describes the features of the moon which specially concern us here. ' As the moon is now seen and mapped,' he says, 4 the difference between the light and dark portions is due merely to a difference in the colour of the material, much of which seems to be darker than the average of terrestrial objects. . . . Galileo saw that the brighter portions of the disc were [are] broken up with inequal- ities of the nature of mountains and craters, while 1 Not long ago, a picture which some ingenious artist had painted to represent a lunar landscape, was sent to the Astrono- mical Society, for exhibition at one of the evening meetings. Many remarks were made on the probable accuracy or inaccuracy of various features of this fanciful but attractive painting. (In some respects it was decidedly inaccurate.) At last the chief official astronomer rose, and many expected that remarks of con- siderable interest would be addressed to the meeting respecting the lunar landscape. His actual speech was simply as follows : ' Mr Chairman, I move that this picture be demitted to the floor.' THE MOON'S MYRIAD SMALL CRATERS. 211 the darker parts were [are] for the most part smooth and uniform. ... It is very curious that the figures of these inequalities in the lunar surface can be closely imitated by throwing pebbles upon the surface of some smooth plastic mass, as mud or mortar. . . . There is no more real smoothness in the regions of the supposed seas than elsewhere. The inequalities are smaller and harder to see on account of the darkness of colour, but that is all.' As to peculiarities of arrangement, Webb remarks on the tendency to parallel direction among craters, and local repetitions : ' Two similar craters often lie north and south of each other, and near them is fre- quently a corresponding duplicate. Two large craters occasionally lie north and south, of greatly resembling character, the southern usually three-fourths of the northern in size, from eighteen to thirty-six miles apart, and connected by ridges pointing in a south- west direction. Several of these arrangements are the more remarkable, as we know of nothing similar on the earth.' If the views above considered are just and it seems to me very difficult to controvert them the multi- tudinous small craters would be due to external action, and they would be earlier formations in the main than the larger craters due to the reaction of the moon's interior upon 'the contracting crust. Thus we might expect to find regions covered with small craters affected by the results of contractive processes and internal re- sistance to such contraction, in such sort that all the p 2 212 THE POETRY OF ASTRONOMY. small craters would be distorted and all similarly. Beer and Madler describe a lunar feature corresponding with what we should thus expect, speaking of * small craters entangled in general pressures, and squeezed into an oval form,' the effect being * like that of an oblique strain upon the pattern of a loosely-woven fabric.* It will be understood that I do not consider the larger features of the moon as necessarily or probably due to external action. I cannot see how the crust of the moon while plastic can have escaped being marked by multitudes of small craters ; and I do not think it likely that the pitting thus caused would be obliterated by subsequent processes of denudation. Thus I regard the crowded small craters which exist on the higher regions of the moon's surface as most probably due to meteoric downfall. But the crust thus pitted exter- nally would, during later stages (or possibly contem- porary stages) of the moon's progress, undergo changes resembling those which have affected our earth's crust. First, the crust contracting more rapidly than the nucleus, because parting more rapidly with its heat, would be exposed to tremendous strains, corresponding precisely with those which would result from the ex- pansion of a nucleus within an unchanging shell. It would probably be to this stage of the moon's develop- ment that we must refer the systems of radiating streaks which form so marked a feature of the lunar globe. THE MOON'S MYRIAD SMALL CRATERS. 21$ Secondly, the crust having cooled with comparative rapidity (though millions of years were probably re- quired for this process), the nucleus would in its turn begin to cool more quickly than the crust, having more heat to part with. Accordingly, spaces would form between the nucleus and the crust, were it not that the action of gravity would compel the crust to follow up the contracting nucleus. From this process two things would follow : first, massive corrugations would form on the surface of the moon ; in other words, mountain ranges and all orders of ridge-shaped ir- regularities ; secondly, the heat resulting from this mechanical process would, as in the case of our own earth even to this day, cause volcanic explosions, and result in the formation of mighty craters. But with these stages of the moon's development I am not at present concerned. It is with the multitudi- nous small craters which cover all the higher regions of the moon that I have sought to deal. It appears to me that whether we consider what must have happened as the moon passed through the plastic and semi-plastic stages of her existence, or whether we consider the evidence derived from the actual condition of the moon's surface, we are alike led to the conclusion that the innumerable small craters which cover the higher lunar levels have been caused chiefly by meteoric down- fall. When I first advanced this theory (in 1873) I had not yet fully recognised the evidence both a priori and a posteriori in its favour. I said then that