The Library Magazine of Select Foreign Literature / All volumes

by Various

We may, therefore, continue to use the numbers resulting from Bischoff's calculation, even though we admit the probability that they differ widely from the true values of the periods we are considering.

Setting the moon, then, as about two hundred and fifty millions of years in advance of the earth in development, even when we overlook all the eras preceding that considered by Bischoff, and the entire sequent interval (which must be long, for the earth has no longer a surface one hundred degrees Centigrade hotter than boiling water), let us consider what is suggested by this enormous time-difference.

In the first place, it corresponds to a much greater interval in our earth's history. During the two hundred and fifty millions of years the moon has been cooling at her rate, not at the earth's. According to the conclusion we deduced from the moon's relative mass and surface, she has aged as much during those two hundred and fifty million years as the earth will during the next fifteen hundred million years.

Now, however slowly we suppose the earth's crust to be changing, it must be admitted that in the course of the next fifteen hundred millions of years the earth will have parted with far the greater part, if not with the whole, of that inherent heat on which the present movements of her surface depend. We know that these movements at once depend upon and indicate processes of contraction. We know that such processes cannot continue at their present rate for many millions of years. If we assume that the rate of contraction will steadily diminish—which is equivalent, be it noticed, to the assumption that the earth's vulcanian or subterranean energies will be diminished—the duration of the process will be greater. But even on such an assumption, controlled by consideration of the evidence we have respecting the rate at which terrestrial contraction is diminishing, it is certain that long before a period of fifteen hundred millions of years has elapsed, the process of contraction will to all intents and purposes be completed.

We must assume, then, as altogether the most probable view, that the moon has reached this stage of planetary decrepitude, even if she has not become an absolutely dead world. We can hardly reject the reasoning which would show that the moon is far older than has been assumed when long stages of her history and our earth's have been neglected. Still less reasonable would it be to reject the conclusion that at the very least she has reached the hoar antiquity thus inferred. Assuming her to be no older, we yet cannot escape the conviction that her state is that of utter decrepitude. To suppose that volcanic action can now be in progress on the moon, even to as great a degree as on the earth, would be to assume that measurable sources of energy can produce practically immeasurable results. But no volcanic changes now in process on the earth could possibly be discernible at the moon's distance. How utterly unlikely does it seem, then, that any volcanic changes can be now taking place on the moon which could be recognized from the earth! It seems safe to assume that no volcanic changes at all can be in progress; but most certainly the evidence which should convince us that volcanic changes of so tremendous a character are in progress that at a distance of two hundred and sixty thousand miles terrestrial telescopists can discern them, must be of the strongest and most satisfactory character.

Evidence of change may indeed be discovered which can be otherwise explained. The moon is exposed to the action of heat other than that which pervaded her own frame at the time of her first formation. The sun's heat is poured upon the moon during the long lunar day of more than a fortnight, while during the long lunar night a cold prevails which must far exceed that of our bitterest arctic winters. We know from the heat-measurements made by the present Lord Rosse, that any part of the moon's surface at lunar mid-day is fully five hundred degrees Fahrenheit hotter than the same part two weeks later at lunar midnight. The alternate expansions and contractions resulting from these changes of temperature cannot but produce changes, however slowly, in the contour of the moon's surface. Professor Newcomb, indeed, considers that all such changes must long since have been completed. But I cannot see how they can be completed so long as the moon's surface is uneven, and at present there are regions where that surface is altogether rugged. Mighty peaks and walls exist which must one day be thrown down, so unstable is their form; deep ravines can be seen which must one day be the scene of tremendous landslips, so steep and precipitous are their sides. Changes such as these may still occur on so vast a scale that telescopists may hope from time to time to recognise them. But changes such as these are not volcanic; they attest no lunar vitality. They are antecedently so probable, indeed, while volcanic changes are antecedently so unlikely, that when any change is clearly recognised in the moon's surface, nothing but the most convincing evidence could be accepted as demonstrating that the change was of volcanic origin and not due to the continued expansion and contraction of the lunar crust.

And now let us see how stands the evidence in the few cases which seem most to favour the idea that a real change has taken place.

We may dismiss, in the first place, without any hesitation, the assertion that regular changes take place in the floor of the great lunar crater Plato. According to statements very confidently advanced a few years ago, this wide circular plain, some sixty miles in diameter, grows darker and darker as the lunar day advances there until the time corresponding to about two o'clock in the afternoon, and then grows gradually lighter again till eventide. The idea seems to have been at first that some sort of vegetation exists on the floor of this mighty ring-shaped mountain, and that, as the sun's heat falls during the long lunar day upon the great plain, the vegetation flourishes, darkening the whole region just as we might imagine that some far-extending forest on the earth would appear darker as seen from the moon when fully clothed with vegetation than when the trees were bare and the lighter tints of the ground could be seen through them. Another idea was that the ground undergoes some change under the sun's heat corresponding to those which are produced in certain substances employed in photography; though it was not explained why the solar rays should produce no permanent change, as in the terrestrial cases adduced in illustration. Yet another and, if possible, an even stranger explanation, suggested that, though the moon has no seas, there may be large quantities of water beneath her crust, which may evaporate when that crust becomes heated, rising in the form of vapour to moisten and so darken the crust. Certainly, the idea of a moistening of the lunar crust, or of portions thereof, as the sun's rays fall more strongly upon it, is so daring that one could almost wish it were admissible, instead of being altogether inconsistent, as unfortunately it is, with physical possibilities.

But still more unfortunately, the fact supposed to have been observed, on which these ingenious speculations were based, has not only been called in question, but has been altogether negatived. More exact observations have shown that the supposed darkening of the floor of Plato is a mere optical illusion. When the sun has lately risen at that part of the moon, the ringed wall surrounding this great plain throws long shadows across the level surface. These shadows are absolutely black, like all the shadows on the moon. By contrast, therefore, the unshadowed part of the floor appears lighter than it really is; but the mountain ring which surrounds this dark grey plain is of light tint. So soon as the sun has passed high above the horizon of this region, the ring appears very brilliant compared with the dark plain which it surrounds; thus the plain appears by comparison even darker than it really is. As the long lunar afternoon advances, however, black shadows are again thrown athwart the floor, which therefore again appears by contrast lighter than it really is. All the apparent changes are such as might have been anticipated by anyone who considered how readily the eye is misled by effects of contrast.

To base any argument in favour of a regular change in the floor of Plato on evidence such as this, would be as unwise as it would be to deduce inferences as to changes in the heat of water from experiments in which the heat was determined by the sensations experienced when the hands were successively immersed, one hand having previously been in water as hot as could be borne, the other in water as cold as could be borne. We know how readily these sensations would deceive us (if we trusted them) into the belief that the water had warmed notably during the short interval of time which had elapsed between the two immersions; for we know that if both hands were immersed at the same moment in lukewarm water, the water would appear cold to one hand and warm to the other.