Part 17

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.

Precisely as in such a case as we have just considered, if we were obliged to test the water by so inexact a method, we should make experiments with one hand only, and carefully consider the condition of that hand during the progress of the experiments, so in the case of the floor of Plato, we must exclude as far as possible all effects due to mere contrast. We must examine the tint of the plain, at lunar morning, mid-day, and evening, with an eye not affected either by the darkness or brightness of adjacent regions, or adjacent parts of the same region. This is very readily done. All we have to do is to reduce the telescopic field of view to such an extent that, instead of the whole floor, only a small portion can be seen. It will then be found, as I can myself certify (the more apparently because the experience of others confirms my own), that the supposed change of tint does not take place. One or two who were and are strong believers in the reality of the change do indeed assert that they have tried this experiment, and have obtained an entirely different result. But this may fairly be regarded as showing how apt an observer is to be self-deceived when he is entirely persuaded of the truth of some favourite theory. For those who carried out the experiment successfully had no views one way or the other; those only failed who were certainly assured beforehand that the experiment would confirm their theory.

The case of the lunar crater Linne, which somewhere about November 1865 attracted the attention of astronomers, belongs to a very different category. In my article on the moon in the "Contemporary Review" I have fully presented the evidence in the case of this remarkable object. I need not therefore consider here the various arguments which have been urged for and against the occurrence of change. I may mention, however, that, in my anxiety to do full justice to the theory that change has really occurred, I took Maedler's description of the crater's interior as "very deep," to mean more than Maedler probably intended. There is now a depression several hundred yards in depth. If Maedler's description be interpreted, as I interpreted it for the occasion in the above article, to mean a depth of two or three miles, it is of course certain that there has been a very remarkable change. But some of the observers who have devoted themselves utterly, it would seem, to the lively occupation of measuring, counting, and describing the tens of thousands of lunar craters already known, assert that Maedler and Lohrman (who uses the same description) meant nothing like so great a depth. Probably Maedler only meant about half a mile, or even less. In this case their favourite theory no longer seems so strongly supported by the evidence. In some old drawings by the well-known observer Schroeter, the crater is drawn very much as it now appears. Thus, I think we must adopt as most probable the opinion which is, I see, advanced by Prof. Newcomb in his excellent "Popular Astronomy," that there has been no actual change in the crater. I must indeed remark that, after comparing several drawings of the same regions by Schroeter, Maedler, Lohrman, and Schmidt, with each other and with the moon's surface, I find myself by no means very strongly impressed by the artistic skill of any of these observers. I scarcely know a single region in the moon where change might not be inferred to have taken place if any one of the above-named observers could be implicitly relied upon. As, fortunately, their views differ even more widely _inter se_ than from the moon's own surface, we are not driven to so startling a conclusion.

However, if we assume even that Linne has undergone change, we still have no reason to believe that the change is volcanic. A steep wall, say half a mile in height, surrounding a crater four or five miles in diameter, no longer stands at this height above the enclosed space, if the believers in a real change are to be trusted. But, as Dr. Huggins well remarked long ago, if volcanic forces competent to produce disturbance of this kind are at work in the moon, we ought more frequently to recognize signs of change, for they could scarcely be at work in one part only of the moon's surface, or only at long intervals of time. It is so easy to explain the overthrow of such a wall as surrounded Linne (always assuming we can rely upon former accounts) without imagining volcanic action, that, considering the overwhelming weight of _a priori_ probability against such action at the present time, it would be very rash to adopt the volcanic theory. The expansions and contractions described above would not only be able to throw down walls of the kind, but they would be sure to do so from time to time. Indeed, as a mere matter of probabilities, it may be truly said that it would be exceedingly unlikely that catastrophes such as the one which may have occurred in this case would fail to happen at comparatively short intervals of time. It would be so unlikely, that I am almost disposed to adopt the theory that there really has been a change in Linne, for the reason that on that theory we get rid of the difficulty arising from the apparent fixity of even the steepest lunar rocks. However, after all, the time during which men have studied the moon with the telescope--only two hundred and sixty-nine years--is a mere instant compared with the long periods during which the moon has been exposed to the sun's intense heat by day and a more than arctic intensity of cold by night. It may well be that, though lunar landslips occur at short intervals of time, these intervals are only short when compared with those periods, hundreds of millions of years long, of which we had to speak a little while ago. Perhaps in a period of ten or twenty thousand years we might have a fair chance of noting the occurrence of one or two catastrophes of the kind, whereas we could hardly expect to note any, save by the merest accident, in two or three hundred years.

To come now to the last, and, according to some, the most decisive piece of evidence in favour of the theory that the moon's crust is still under the influence of volcanic forces.

On May 19, 1877, Dr. Hermann J. Klein, of Cologne, observed a crater more than two miles in width, where he felt sure that no crater had before existed. It was near the centre of the moon's visible hemisphere, and not far from a well-known crater called Hyginus. At the time of observation it was not far from the boundary between the light and dark parts of the moon: in fact, it was near the time of sunrise at this region. Thus the floor of the supposed new crater was in shadow--it appeared perfectly black. In the conventional language for such cases made and provided (it should be stereotyped by selenographers, for it has now been used a great many times since Schroeter first adopted the belief that the great crater Cassini, thirty-six miles in diameter, was a new one) Dr. Klein says, "The region having been frequently observed by myself during the last few years, I feel certain that no such crater existed in the region at the time of my previous observations." He communicated his discovery to Dr. Schmidt, who also assured him that the region had been frequently observed by himself during the last few years, and he felt certain that no such crater, &c., &c. It is not in the maps by Lohrman and by Beer and Maedler, or in Schroeter's drawings, and so forth. "We know more," says a recent writer, singularly ready to believe in lunar changes; "we know that at a later period, with the powerful Dorpat telescope, Maedler carefully re-examined this particular region, to see if he could detect any additional features not shown in his map. He found several smaller craterlets _in other parts_" (the italics are mine), "but he could not detect any other crater in the region where Dr. Klein now states there exist a large crater, though he did find some very small hills close to this spot." "This evidence is really conclusive," says this very confident writer, "for it is incredible that Maedler could have seen these minute hills and overlooked a crater so large that it is the second largest crater of the score in this region." Then this writer comes in, of course, in his turn, with the customary phrases. "During the six years, 1870-1876, I most carefully examined this region, for the express purpose of detecting any craters not shown by Maedler," and he also can certify that no such crater existed, etc., etc. He was only waiting, when he thus wrote, to see the crater for himself. "One suitable evening will settle the matter. If I find a deep black crater, three miles in diameter, in the place assigned to it by Dr. Klein, and when six years' observation convinces me no such crater did exist, I shall know that it must be new."

Astronomers, however, require somewhat better evidence.

It might well be that a new crater-shaped depression should appear in the moon without any volcanic action having occurred. For reasons already adduced, indeed, I hold it to be to all intents and purposes certain that if a new depression is really in question at all, it is in reality only an old and formerly shallow crater, whose floor has broken up, yielding at length to the expansive and contractive effects above described, which would act with exceptional energy at this particular part of the moon's surface, close as it is to the lunar equator.

But it is by no means clear that this part of the moon's surface has undergone any change whatever. We must not be misled by the very confident tone of selenographers. Of course they fully believe what they tell us: but they are strongly prejudiced. Their labours, as they well know, have now very little interest unless signs of change should be detected in the moon. Surveyors who have done exceedingly useful work in mapping a region would scarcely expect the public to take much interest in additional information about every rock or pebble existing in that region, unless they could show that something more than a mere record of rocks and pebbles was really involved. Thus selenographers have shown, since the days of Schroeter, an intense anxiety to prove that our moon deserves, in another than Juliet's sense, to be called "the inconstant moon." In another sense again they seem disposed to "swear by the inconstant moon," as changing yearly, if not "monthly, in her circled orb." Thus a very little evidence satisfies them, and they are very readily persuaded in their own mind that former researches of theirs, or of their fellow-pebble-counters, have been so close and exact, that craters must have been detected then which have been found subsequently to exist in the moon. I do not in the slightest degree question their _bona fides_, but a long experience of their ways leads me to place very little reliance on such stereotyped phrases as I have quoted above.

Now, in my paper in the "Contemporary Review" on this particular crater, I called attention to the fact that in the magnificent photograph of the moon taken by Dr. Louis Rutherfurd on March 6, 1865 (note well the date) there is a small spot of lighter colour than the surrounding region, nearly in the place indicated in the imperfect drawing of Klein's record which alone was then available to me. For reasons, I did not then more closely describe this feature of the finest lunar photograph ever yet obtained.

The writer from whom I have already quoted is naturally (being a selenographer) altogether unwilling to accept the conclusion that this spot is the crater floor as photographed (not as seen) under a somewhat higher illumination than that under which the floor of the crater appears dark. There are several white spots immediately around the dark crater, he says: "which of these is the particular white spot which the author" (myself) "assumes I did not see?" a question which, as I had made no assumption whatever about this particular writer, nor mentioned him, nor even thought of him, as I wrote the article on which he comments, I am quite unable to answer. But he has no doubt that I have "mistaken the white spot" (which it seems he can identify, after all) "for Klein's crater, which is many miles farther north, and which never does appear as a white spot: he has simply mistaken its place."

I have waited, therefore, before writing this, until from my own observation, or from a drawing carefully executed by Dr. Klein, I might ascertain the exact place of the new crater. I could not, as it turned out, observe the new crater as a black spot myself, since the question was raised; for on the only available occasion I was away from home. But I now have before me Dr. Klein's carefully drawn map. In this I find the new crater placed not nearly, but _exactly_ where Rutherfurd's crater appears. I say "Rutherfurd's crater," for the white spot is manifestly not merely a light tinted region on the darker background of the Sea of Vapours (as the region in which the crater has been found is called): it is a circular crater more than two miles in diameter; and the width of the crescent of shadow surrounding its eastern side shows that in March 1865, when Rutherfurd took that photograph, the crater was not (for its size) a shallow one, but deep.

Now, it is quite true that, to the eye, under high illumination, the floor of the crater does not appear lighter than the surrounding region; at least, not markedly so, for to my eye it appears slightly lighter. But everyone knows that a photograph does not show all objects with the same depth of shading that they present to the naked eye. A somewhat dark green object will appear rather light in a photograph, while a somewhat light orange-yellow object will appear quite dark. We have only to assume that the floor of the supposed new crater has a greenish tinge (which is by no means uncommon) to understand why, although it is lost to ordinary vision when the Sea of Vapours is under full illumination, it yet presents in a photograph a decidedly lighter shade than the surrounding region.

I ought to mention that the writer from whom I have quoted says that all the photographs were examined and the different objects in this region identified within forty-eight hours of the time when Dr. Klein's letter reached England. He mentions also that he has himself personally examined them. Doubtless at that time the exact position of the supposed new crater was not known. By the way, it is strange, considering that the name Louis Rutherfurd is distinctly written in large letters upon the magnificent photograph in question, that a selenographer who has carefully examined that photograph should spell the name Rutherford. He must really not assume, when on re-examining the picture he finds the name spelled Rutherfurd, that there has been any change, volcanic or otherwise, in the photograph.