IMAGINATIVE writers have frequently entertained themselves by picturing the conditions
of life in the moon. Mr. Wells's brilliant story and Jules Verne's entertaining romance are familiar to all readers. Less known, perhaps, to the present generation is the tantalising sketch of the things that he might have told us which Poe gave in one of the longest • sentences that he ever wrote :—
"I have much to say of the climate of the planet ; of its wonderful alternations of heat and cold; of unmitigated and burning sunshine for one fortnight, and more than polar frigidity for the next; of a constant transfer of moisture, by distillation like that in vacuo, from the point beneath the sun to the point the farthest from it ; of a variable zone of running water ; of the people themselves ; of their manners, customs, and political institutions ; of their peculiar physical construction ; of their ugliness; of their want of ears, those useless appendages in an atmosphere so peculiarly modified; of their consequent ignorance qf the use and properties of speech; of their substitute for speech in a singular method of inter-communication; of the in- comprehensible connection between each particular individual in the moon with some particular individual on the earth—a con- nection analogous with, and depending upon that of the orbs of • The Moon. By William H. Pickering. 100 Illustrations. London John Murray. [42s. net.] the planet and the satellite, and by means of which the lives and destinies of the inhabitants of the one are interwoven with the lives and destinies of the inhabitants of the other ; and above all, if it so please your Excellencies—above all, of those dark and hideous mysteries which lie in the outer regions of the moon— regions which, owing to the almost miraculous accordance of the satellite's rotation on its own axis with its sidereal revolu- tion about the earth, have never yet been turned, and, by God's mercy, never shall be turned, to the scrutiny of the telescopes of man."
Unfortunately, Poe never developed this theme,—probably he felt, with his usual skill in the literary uses of horror, that a stronger effect was produced by leaving these things to the imagination of the reader. Science, as represented in Mr. Pickering's admirable photographic atlas of the moon, with the brief but lucid letterpress that accompanies it, tells us equally wonderful things, though of a different nature. It gives a decided negative, indeed, to those who still speculate upon the possibilities of life in our satellite. Life—at any rate, conscious life—is inconceivable in the moon. We know with absolute certainty that neither air nor water can exist there: and without air and water no life imaginable by us is possible higher than that of some bacteria and equally low forms of vegetable existence. The absence of air is proved by Dr. Johnstone Stoney's remarkable discovery of the law of planetary atmospheres, which is based on the kinetic theory of gases. We know that all gases consist of mole- cules in very rapid motion, which has a definite speed for every gas, rising in the case of hydrogen to an extreme limit of seven miles a second. Now there is a limiting velocity for every planet, easily deduced from its mass by the laws of dynamics, such that a projectile fired at this speed straight upwards from its surface would not be arrested by gravitational attraction, but would wend its way into the void of space, never to return. In the ease of the earth this limiting velocity is about seven miles a second. Hence we can explain the absence of free hydrogen in our atmosphere. Every now and then one of its molecules would happen to be travelling away from the earth at this limiting velocity, and would therefore go off into space. In the moon's case the limiting velocity is only one and a half miles per second. The average speed of oxygen molecules is a quarter of a mile per second, and every now and then they attain an extreme of seven times this speed, or one and three-quarter miles. Clearly, then, in the lapse of ages the moon must have lost all the free oxygen which its atmosphere once possessed, either by its com- bining with other elements to form solid compounds, or by its flying away into space. The other gases which are found in our atmosphere—nitrogen, carbonic acid gas, and water- vapour—would disappear from the moon with nearly equal or greater facility. Thus we may be quite certain that the moon has no appreciable atmosphere : any which it possesses can only consist of the gases that are still being given off from its interior. Analogy with the earth persuades us that these can only be water-vapour and carbonic acid. Further, another physical law tells us that the water-vapour can never appear in a liquid form, since the pressure of the possible atmosphere is not sufficient to liquefy the water-vapour in the daytime, though recent observations make it almost certain that the intense cold of the lunar night does precipitate it in a solid form, as hoar frost. Thus we may bid a fond farewell to all dreams of sentient life upon the moon.
It is, however, a fascinating speculation to wonder whether life did ever exist there. We are pretty sure that the moon has passed, long ago, through the various stages of evolution which the earth is still undergoing. The warrant for this belief is found in our knowledge of the history of the moon, which we owe to Professor G. H. Darwin, and which is one of the most notable results of recent astronomical research. It is a curious fact, as showing the interdependence of all lines of scientific study, that a study of the tides first indicated the true solution of the moon's historical problem. It has long been known that the tides act as a brake on the earth, and are very gradually retarding the speed of its revolution. The loss of speed is almost insignificant; the day is at present about one eighty-fourth part of a second longer than it was at the beginning of the Christian era. But the retardation has to be reckoned with, and in the vast spans of geological time, in comparison with which a thousand years and a day are much the same thing, this secular loss of our chief timepiece plays a great part. For causes on the earth. The effect on our satellite is that she is continually moving faster, and therefore the size of her orbit is increasing,—she is moving farther away from the earth. Now, if we consider the operation of these causes in the past, we see that there was once a time when the earth was spinning far more rapidly than now, whilst the moon was much closer to it. Calculation shows that when the length of our day was only three hours, the moon was almost in contact with the earth. Now it can also be shown that, if the earth was revolving once in three hours, the centrifugal force due to rotation would just about balance the retaining force of gravitation. The birth of the moon at once flashes upon us. As the earth cooled and contracted from its original nebulous condition, the speed of its rotation must have in- creased until, when the day was shortened to three hours,—
" A catastrophe occurred, a catastrophe of such magnitude as has never been seen upon the earth before or since—five thousand million cubic miles of material left the earth's surface never again to return to it. Whether it all left at once or whether the action was prolonged we do not know, but we may try in vain to imagine the awful uproar and fearful volcanic phenomena ex- hibited when a planet was cleft in twain and a new planet was born into the solar system.'' An ingenious conjecture has been made as to the actual place of the moon's birth. A glance at the map shows a re- markable similarity between the eastern and western coast- lines of the Atlantic : one side would fit into the other with only a little humouring. It has been suggested that the
great depression of the Pacific Ocean shows where the moon was torn away from the earth, and that the Old and New Worlds, once forming a single vast continent, were rent asunder by that fearful cataclysm. This is a mere conjecture, insusceptible of proof, but by no means unlikely to be true. What is quite certain is that the moon originated as a part of this planet. Therefore it seems safe to conclude that when it started its independent existence it passed through a similar course of evolution to that of the earth ; and why should not
life have appeared in due season ? This is only a "devout imagination," but it has some basis of analogy. Though the moon, by reason of its smaller size, was bound to lose its atmosphere, it must have taken millions of years to do so,
and there may have been time for the cycle of life, from the primeval germ up to sentient beings and down again to the hardiest lingering plant-cells, to run its full circle.
Probably we shall never know ; but it is quite permissible to look at the full moon with some of the awe-struck sentiment that it produced in our ancestors, not because of its magical influences, but because it is a dead world, ever foreshadowing our own ultimate doom, like the mummy at Egyptian banquets.
CRIME AND ITS PUNISHMENT.*
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