# Littell's Living Age/Volume 134/Issue 1728/Is the Moon Dead?

From The Cornhill Magazine.
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. Formerly, 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 telescope, 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 Fesolé,
* Or in Val d'Arno, to descry new lands,
* Rivers, or mountains, on her spotty globe.
While again, in the fifth book, Raphael 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 Galileo, in his interviews 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 scrutinized, 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 phenomena, in an article which appeared in the Cornhill Magazine for August, 1873; and for this reason (especially 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 considerable density. The moon has, in fact, passed through the same changes as our own earth, though not necessarily 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 vapor 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 atmosphere 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 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 he 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 subserved and never come to subserve that one chief purpose for which the orbs peopling space can be supposed to have been formed, if purpose indeed reigns throughout 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 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.
Combining together several considerations, viz., first that the moon must have been fashioned as a planet many millions of years before the earth, 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), 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 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 confidently assert that fully a hundred millions of years have passed since that era of her history. And as the 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 moon's 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 very strange feature, though it is often dismissed as if there were nothing very remarkable about it. In whatever 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 evidence 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 original 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 beginning set rotating so slowly that its day a 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 recognizable advantages, and many most manifest inconveniences (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.
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 substance than the moon exerts on hers. On the other hand, the relative 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 disconnected) 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 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 nothing 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 attributed 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 favor than either of those hitherto considered. Before the moon became a separate planet her frame, then vaporous, must have been enwrapped in the vaporous 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 enwrapping vapor-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 continually 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 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 competent, 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 by the contracting earth, cannot have 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 earthwards had its origin during the birth itself of our satellite. We may, indeed, find in that peculiarity one of the strongest arguments in favor 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 interested — 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 vigor of life might be supposed 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 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 favor 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.
We had occasion in the article on the moon, referred to above, to notice the theory advanced by Frankland in this country respecting the way in which the lunar air and seas have been caused to disappear; but we did not then enter into any very careful discussion of that theory, our purpose leading us to consider other matters. But in this place the theory must occupy a larger share of our attention. In passing, we may remark that the originator of the theory was Seeman, the German geologist; but it was independently 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 unknown 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 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 existence; and, secondly, to be incapable, if it existed, of exercising the supposed effect.
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 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 considerable 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 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 wineglass (supposed to be cone-shaped) and the surface of the water. Suppose the wineglass 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 proportion of the water might be drawn off, and the ocean surface still remain almost as great as before. We may 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 seventy-two hundredths of the entire surface of the earth, we may assume that the lunar oceans covered between thirty-six and twenty-four hundredths of the entire surface of the moon. It will he 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 depending 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 vapor." And he proceeds thus to analyze 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 fourteen and a half 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 existing 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 considered. That diminishes as the centre of the planet is approached. What we have really to consider is the 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 increases. 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 pressures the most solid substances become plastic. Steel behaved like a liquid in Tresca'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 pressures, 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 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 utmost one hundred, miles. Now one hundred 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 one hundred miles) in about the proportion of four to one. So that even on the assumption that all the external parts of the moon, to the depth of one hundred 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 pressure 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 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, considered in the essay entitled "When the Seas were Young," the seas are floating in the form of cloud and vapor 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 ​contracting 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 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 probably 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 Rutherford in America, and by De la Rue in this country, were in reality only photographs of a peeled orange, the crater Tycho representing one end of the core, and the radiations 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, indeed, 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 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 disturbance 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 contracted 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 consequence 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 permanently 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 synchronizes with the period during which life exists on the planet's surface. During all 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 vapor. 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 vapor) 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 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 easily 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 refrigeration 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 withdrawn 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 required 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 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 nebulæ or masses of luminous star-vapor 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 condition 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 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 recognized 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 Linné, is far better explained in this way than as the result of volcanic 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 Mädler 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 which had been so clearly seen with Mädler's small telescope; but though they found a crater it was nothing like the crater described by Mädler. 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 otherwise. Sir John Herschel pronounced somewhat confidently in favor 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 steep crater-walls like those which seem formerly to have girt about this deep cavity."
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 death. 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, becoming darkest towards lunar midday. Lastly, as the lunar afternoon progresses, the floor of Plato gets gradually lighter again. The midday darkening was attributed to some process of vegetation or else to chemical 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 midday, 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 advocated 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 favor 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 admitting of actual demonstration can be 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 lingering 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 cosmical death. And, so far as our power of understanding or even of conceiving the universe is concerned, it seems as though this tendency of every individual body in the universe towards death involved the tendency 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 conclusions 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, conclusion. 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, be the material substance of a universe next below ours, while below that are lower and lower orders of universe absolutely without limit? And, as the seemingly 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 recognize, 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 ourselves 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 inscrutable 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 one music as before,
* But vaster;
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,
* Rising, 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.
* ↑ See Milton's "Areopagitica."
* ↑ 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 mathematicians 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 the 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.
* ↑ We here use the words "thrown off" as equivalent to "left behind." The theory that the moon was thrown off by the earth, or the earth by the sun, is altogether inconsistent with mechanical possibilities.
* ↑ The idea was that the moon, though nearly spherical, is somewhat 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 elevation the seas would be gathered together, and the air would he 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 of 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 "The 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 quarter 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 exceedingly 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! Conceive a world, one half of whose surface is required as an abode for its malefactors!
* ↑ The present writer, in the Spectator for June 24, 1876.
* ↑ The work called "The Unseen Universe," presents a portion of the evidence to this effect, hut unfortunately the style of that work is not sufficiently lucid to bring its reasoning within the range of the general non-scientific reader.