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 Linné, 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 Mädler's description of the crater's interior as "very deep," to mean more than Mädler probably intended. There is now a depression several hundred yards in depth. If Mädler'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 Mädler and Lohrman (who uses the same description) meant nothing like so great a depth. Probably Mädler 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 Schröter, 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 Schröter, Mädler, 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 Linné 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 Linné (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 Linné, 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 Schröter 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 Mädler, or in Schröter'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, Mädler 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 Mädler 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 Mädler," 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 Schröter, 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."