IT was long ago observed that when stars are occulted by the moon, they disappear and reappear, not gradually, but instantaneously. That is, the star, in passing behind the moon, does not grow dim until it fades from sight altogether : it vanishes at once, and its reappearance, at the farther edge of the moon’s disk, is equally abrupt. From this fact it was rightly inferred that the moon has no enveloping atmosphere. More recently spectroscopic observation has verified this inference, besides establishing the parallel fact that there is no appreciable quantity of water on the lunar surface.

This absence of air and water from the exterior of a planet so similar to the earth in its general aspect has always been regarded as a phenomenon needing to be accounted for ; and very queer have been some of the hypotheses by which it has been sought to explain it. It has even been hinted that all the lunar air and oceans may have been carried off by a comet ; or that, owing to a slight displacement of the moon’s centre of gravity, the air and water may have entirely retreated to that hemisphere of the planet which is always turned away from us. At the present day neither of these suggestions is worthy of serious consideration. The first is simply ridiculous, in view of what we now know about comets ; and the second, though not intrinsically incredible, will not bear examination. Any one, says Mr. Proctor, “ who will draw a cross-section of the moon (in a plane passing through the earth), and endeavor to assign such a position to an atmosphere of moderate extent that even during the moon’s extreme librations no signs of the atmosphere could be perceptible from the earth, will at once see that the theory is untenable.”

A much more probable explanation supposes the lunar atmosphere and oceans to be frozen solid. The entire cessation of volcanic activity upon the lunar surface indicates that the planet has nearly or quite lost its primitive stock of internal heat, and this is what might be expected from the small size of the planet. The degree of cold implied by the solidification or “ rigidification ” of the moon’s nucleus immeasurably exceeds anything within terrestrial experience; and it may well have been great enough to freeze all the lunar oceans, and even to liquefy, or perhaps to solidify, the gases of the lunar atmosphere. The moon is indeed subjected at each rotation to the fierce noontide heat sent from the sun ; but although this may scorch and blister the rocky surface, it can exercise but little melting power. From the airless surface of the moon, the solar radiance must be immediately reflected into space, as from the surface of a polished mirror. Just as on the summits of the Himalayas, where the atmosphere is so rare, the huge snow-masses remain through centuries unmelted, in spite of the sun’s blazing heat, so upon the surface of the moon the air and water once frozen must remain frozen forever.

This explanation, however, does not give a satisfactory account of the disappearance of the lunar atmosphere. Granting the disappearance of the atmosphere, the maintenance of a more than arctic cold in spite of the utmost intensity of solar radiation may readily be admitted. But in this explanation the absence of a surface atmosphere is presupposed rather than accounted for. A far more thorough-going hypothesis was propounded some years since by M. Sæmann, in a paper on the unity of geological phenomena throughout the solar system translated by Professor Sterry Hunt, and published in the American Journal of Science, January, 1862. In his excellent pamphlet on the “ Geology of the Stars,” 1 Professor Winched, of the University of Michigan, has newly called attention to M. Sasmann’s hypothesis, and shown that it gives a complete account of the facts in the case.

Observe first that the former existence of air and water on the lunar surface is not a mere inference from analogy. The moon having been originally a portion of the earth’s equatorial zone, it is difficult to suppose that it does not contain materials which have from the oldest times constituted so large a portion of the earth’s exterior. But besides this, the vast plains on the moon which the old astronomers supposed to be seas, and named as such, have now been found to be areas underlaid by sedimentary rocks, thus attesting the former presence of water. Hence, as Professor Winched sensibly argues, there must in all probability have been winds to excite the erosive movements of the water which caused this sedimentation. For tidal action upon the moon cannot be regarded as a considerable factor in the erosion, unless we go back to that enormously remote period when the earth’s tidal pull was still dragging the moon’s rotation into synchrony with its revolution.

Since, then, we have plain indications of the former existence of air and water on the surface of the moon, how does M. Sæmann account for their disappearance ? They have been drunk up by the thirsty rocks. On our own globe the tendency of the surface water is constantly to percolate through the soil of the land or sea-bottom, and thence through the rocks, downward towards the centre of the earth. Yet with our present supply of internal heat, it is not probable that any water can reach more than one fiftieth part of the distance towards the earth’s centre, without becoming vaporized and thus getting driven back towards the surface. In this way there is kept up a circulation of water throughout the peripheral portions of the earth’s crust. But as the earth becomes cooler and cooler, the water will be enabled to circulate at greater and greater depths, thus materially lowering the level of the ocean. In this way, long before the centre has become cool, all the surface-water of the earth will have been sucked into the pores of the rocks, and the same will afterwards take place with the atmosphere. M. Sæmann shows that by the time the earth had reached complete refrigeration, the pores of the rocks would absorb more than one hundred times the amount of all the oceans on the globe, “ and that the unfilled pores would more than suffice for the retirement of the atmosphere.”

According to M. Sæmann, this state of things, which is by and by to be realized on the earth, is already realized on the moon. Being forty-nine times smaller than the earth, the moon has cooled down forty-nine times as rapidly, and its geologic epochs have been correspondingly short. “ Its zoic age,” says Professor Winched, “ was reached while yet our world remained, perhaps, in a glowing condition. Its human period was passing while the eozoön was solitary occupant of our primeval ocean.” More careful reflection will probably convince us that, with such a rapid succession of geologic epochs, the moon can hardly have had any human period. For the purposes of comparative geology, the earth and the moon may be regarded as of practically the same antiquity. Now, supposing the earliest apelike men to have made their appearance on the earth during the Miocene epoch, some five million years ago, we must remember that at that period the moon must have advanced in refrigeration very far beyond the earth. Supposing organic evolution to have gone on with equal pace in the two planets, it would seem a probable conclusion that the moon would be rapidly becoming unfit for the support of organic life at about the time when man appeared on the earth. Still more, it is a strictly logical inference from the theory of natural selection, that upon a small planet there is likely to be a slower and less rich and varied evolution of life than upon a large planet. Grouping together all these considerations, it does not seem at all likely that the moon can ever have given rise to organisms nearly so high in the scale of life as human beings. Long before it could have attained to any such point, its surface must have become uninhabitable by air-breathing organisms. With its rapid refrigeration, its surface air and water must have sunk into its interior and left it the mere lifeless ember that it is, — a type, nevertheless, of the ultimate condition of every one of the radiating and cooling members of the solar system. The moon would thus appear to be not merely an extinct world, but a partially aborted world, just as the still smaller asteroids and meteorites would seem to be totally aborted worlds ; the quantity of planetary matter being so small in the latter case that there is an apparent incongruity in speaking of these bodies as worlds in any sense. Nevertheless, from the earth down to the moon, from the moon down to the asteroid, and from the asteroid down to the meteorite, the differences are at bottom only differences of degree ; though the differences in result may range all the way from a world habitable by civilized men down to a mere dead ball of planetary matter.

Here we are introduced to an interesting series of reflections on the continuity of cosmic phenomena, concerning which we hope to say more next month. For the present we would change the subject, and allude to the discoveries recently made by Mr. Calvert, which seem to point to the existence of human beings in the Miocene period. Sir John Lubbock writes to Nature, of March 27th, that he has learned by letter certain results obtained near the Dardanelles by Mr Frank Calvert, which are of striking significance for the antiquity of man. Mr. Calvert has found a fragment of a bone, either of a mastodon or of a dinotherium, “ on the corner side of which is engraved a representation of a horned quadruped, with arched neck, lozengeshaped chest, long body, straight fore-legs, and broad feet.” Along with this are traces of other figures, partly obliterated. In the same stratum Mr. Calvert has found “a flint flake, and several bones broken as if for the extraction of marrow.”

Of course these statements await verification, and to draw a positive conclusion from them at present would be in the highest degree unwarrantable. It can only be said that if these data are verified, and if it turns out that Mr. Calvert is not mistaken in the character of the stratum which lie has been examining, the antiquity of the human race will have to be computed in millions of years rather than, as heretofore, in hundreds of thousands. But in this there need be nothing to surprise us. The non-existence of human remains in any Tertiary strata (save possibly in the uppermost Pliocene) has been an assumption based on purely negative evidence, like the older assumption as to the non-existence of fossiliferous rocks below the Silurian. We must be prepared at any moment, on the reception of positive evidence, to extend our conceptions of the antiquity of man, as well as of the number and duration of geologic epochs.

The immense antiquity of the human race, even as at present established at something like a million of years, affords very powerful confirmatory proof of the derivation of man from some lower form belonging to the order of primates. Since the period during which man has possessed sufficient intelligence to leave a traditional record of himself is but a minute fraction of the period during which he has existed upon the earth, it is but fair to conclude that during those long ages of which none but a geologic record of his existence remains, he was by slow increments acquiring that superior intelligence which now so widely distinguishes him from other animals. Throughout an enormous period of time, his brain-structure and its correlated intellectual and emotional functions must have been gradually modified by natural selection and by direct adaptation, while his outward physical appearance has undergone few modifications ; even the most striking of these being directly or indirectly associated with increase in brain-structure, These inferences are in harmony with the beautiful principle announced by Mr. Wallace, that so soon as the intelligence of an animal has, through ages of natural selection and direct adaptation, become so considerable that a slight variation in it is of more use to the animal than any variation in physical structure, then such variations will be more and more constantly selected, while physical variations, being relatively of less vital importance to the species, will be more and more neglected. Thus while the external appearance, and his internal nutritive and muscular apparatus, may vary but little in many ages, his intellectual and moral attributes and his cerebral structure will vary with comparative rapidity. Thus we may understand why man differs so little in general physical structure and external appearance from the other higher primates, while in the special point of cerebral structure and accompanying intelligence he differs so widely from his nearest living congeners.

  1. * Half-Hour Recreations in Popular Science. No. 7. The Geology of the Stars. By Prof. A. Winchell, of the University of Michigan. Boston : Estes and Lauriat. 1873.