For the reasons detailed in the above, therefore, selenographers consider that the attempt to show that Linné has not changed has entirely failed, but that in the case of this crater we have an instance of real physical change on the moon. With regard to the nature of this change little definite has as yet been ascertained, and it will require a long examination of this region with powerful telescopes to determine what change really has occurred. From numerous observations the explanation agreeing best with the present conditions of the surface is that the walls of the old crater have collapsed and fallen into the interior. By this means the interior would be nearly completely filled up, leaving, however, a sort of rough, cone-like, small crater towards the centre. Under exceptionally favourable atmospheric conditions, with the assistance of a powerful telescope, the surface immediately around the small crater has been seen to present the appearance of being rough and irregular. Round what would have been the border of the old crater are numerous mounds and rough blocks, and on the east is one if not two low hills or peaks, presenting the appearance of being portions of the old wall. The difficulty of making these observations is very great, and they are only possible in the very finest atmospheric conditions. A prolonged examination of this region, however, would amply repay the labour.

There is one other point on which a remark must be made, and that is with reference to an apparently generally spread misconception of the change which Linné is supposed to have undergone. Thus, in the interesting article on this subject in the "Quarterly Journal of Science" for October, 1873, Mr. R. A. Proctor says:-" Mr. Browning, after considering the evidence afforded by his own observations, considered in connection with those made by others, arrives at the conclusion that there is scarcely any ground for supposing that any change has occurred in this small but celebrated crater.'" Mr. Proctor then adds-" These remarks appear to me to contain the gist of the whole matter. We see that Linné has a surface so constituted that as the sun is rising there, and so pouring his rays very obliquely, there is a continual change of aspect precisely resembling that which can be recognised when certain kinds of rock surfaces, and especially crystalline formations, are viewed under oblique illumination. We know that in such cases the tints vary, not only absolutely but relatively, insomuch

With an aperture of 91 inches, and a power of 600.

that a part which is darker than another with one oblique illumination will be lighter under another and but slightly different illumination.

"It appears to me that no other explanation can reasonably be suggested, because, in point of fact, we have to choose between the theory that there has been a definite change of surface on this part of the moon, or that the change is only apparent. Now, if there has been a definite change at any time, fresh changes must have restored, either from time to time or definitely, the former condition of the surface. But this seems extremely unlikely, whilst such a change as Sir John Herschel considered to afford the best explanation of Schmidt's observation may be regarded as one which no subsequent process could so modify as to restore, or nearly restore, the original appearance of the region." (This explanation of Sir John Herschel's referred to was that the crater of Linné had been filled up to overflow with viscous lava.) "Such a change would doubtlessly account well for the observed appearances, but it leaves the subsequent restoration of the crater unexplained."(Pp. 500, 501.)

From the above quotation Mr. R. A. Proctor would appear to consider that the entire known observations that have been made of Linné can be explained by supposing the surface to vary in tint with differences of illumination, and he also seems to suppose that of late years Linné has been seen to present the same appearance and possess the same characters as it was described as having before the supposed change occurred. But it has been already shown that this is entirely a misconception: whatever change occurred in Linné was definite, and since 1867 Linné has never been seen to accord with the description of the early selenographers. To suppose the minute crater-like formation now existing on the site of Linné can be in any manner identical with, or even similar to, the twenty times larger formation that was observed, drawn, and measured by Lohrmann, and Beer and Mädler, is out of the question. Nor will any selenographer allow that differences of illumination, of any kind or on any surface, are capable of explaining the difference in appearance of the formation at the two different epochs. No alteration in illumination whatever could make an object where Linné is placed look at one period like a considerable and deep crater, and at another as a small, scarcely visible crater. The means by which Mr. R. A. Proctor would explain the supposed change in Linné are perfectly inadequate. Nor will the effect of variation in illumination even account for the minor variations which it has been supposed have

occurred since 1866, such as, for example, the alteration in the size of the small crater within Linné from a diameter of barely one-third of a mile in January and February, 1867, to a diameter of five times as great in July, 1867. It is true the reality of many of these supposed changes is doubtful, but alterations in the angle of illumination are inadequate to explain them per se. In fact, so far as the known observations extend, the alterations undergone by Linné in its present condition appear to be perfectly normal, and exactly similar to the changes undergone by all other analogously placed and constituted formations. The changes are striking, but in no manner strange or unexpected to the experienced selenographer.

The facts about Linné may be therefore summed up very briefly. According to three or more independent selenographers, the most experienced and eminent that Science has seen, the object named Linné was a conspicuous crater of large diameter and great depth. Now in its place all that exists is a tract of uneven ground, containing a small, scarcely visible, insignificant, crater-like object. It is impossible that one could ever be systematically taken for the other. It is inconceivable how our three greatest selenographers could have systematically and independently made the same blunder, and that one blunder only. For in no other case do we find any error of this nature. Their description must therefore be held to truly describe the nature of the formation at their epoch (1820-1845). The object is no longer of the same size and description. A real physical change on the moon's surface must therefore have occurred at this point. This, then, is the conclusion that selenographers as a body have arrived at, yet, despite the strong evidence on which it rests, it is not generally recognised by astronomers.

The next instance of supposed physical change on the surface of the moon is one of the most peculiar in the entire range of astronomical observations, and is that in the case of the crater named Messier by Beer and Mädler. In the equatorial region of the moon, on the Mare Facunditatis, the westernmost of the great lunar plains, close to one another are two small crater plains, according to Beer and Mädler about 9 miles in diameter. These two formations lie isolated on the open plain, and are surrounded by only some very low ridges, and mounds, and some rounded depressions like craters. From the easternmost of the two, Messier A, extend two long, slightly diverging streaks, of a pale grey colour, which in full moon give the formation

much the appearance of a bright comet with a long double tail. Schröter, of Lilienthal, discovered this object, and drew it carefully, making the western formation of the two -or Messier itself-slightly the larger. He also suspected that its appearance was variable. In consequence of the observation of Schröter, Beer and Mädler paid particular attention to these formations, examining them most carefully on than three hundred distinct occasions, between 1829 and 1837. They were thus enabled to declare with certainty that the two formations, Messier and Messier A, were exactly alike in every manner, not the slightest difference in any particulars being detectable. Both were circular crater plains, 9 miles in diameter, with 7 bright greyish white walls, surrounding a yellowish grey interior only 3° bright. On the walls, which were of the same height, were several wall peaks, in both formations. situated in the same position with regard to the formation. In fact, as Beer and Mädler draw particular attention to, their own observations are decisive proof that the two formations were exactly equal in every respect, and that in diameter, form, height of their walls above the surrounding surface, and depth of the interior beneath the crest of the walls, colour of the interior and of the walls, and the position of the surrounding wall peaks, Messier and its twin formation Messier A were completely alike. And it is impossible that any difference in these respects would have escaped the attention of the greatest selenographer of our day, during a course of over three hundred observations in a space of nine years.

Some years after this, Gruithuisen-a most zealous and keen-sighted, though fanciful, observer-detected a slight dissimilarity between the form of the two craters, but this fact received little, if any, attention. In November, 1855, the Rev. T. W. Webb, one of the best living iunar observers. who was examining this region with a telescope of similar dimensions to that of Beer and Mädler's, observed that the eastern crater plain appeared the larger of the two. On again looking at the formations on March 11th, 1856, he at once detected that not only was the western crater plain, or Messier the smaller of the two, but that it was elliptical, with its greatest diameter extending from east to west. This fact was confirmed by subsequent observations and from drawings made by him in 1857, whilst Messier A, the eastern formation, appears to have remained unchanged, still being a circular crater plain, with a diameter of 9 miles, the western crater plain, Messier itself, had an elliptical

form, with a long diameter of about 10 to II miles and a short diameter of about 7 to 8 miles. The most rigid equality between the form and dimensions of these two formations, so strongly asserted to exist by Beer and Mädler, then no longer existed in 1857, according to the observations of the Rev. T. W. Webb.

This fact was of very great selenographical importance, and should have attracted general attention, because whatever uncertainty may be attached to the observations of Linné made by Beer and Mädler, it cannot be seriously urged that Beer and Mädler could have failed to recognise the dissimilarity between Messier and Messier A discovered by the Rev. T. W. Webb, when we consider that for over three hundred times they most carefully examined the two formations for the express purpose of detecting any difference. Nor can that potent agent, the lunar librations, be invoked to account for this difference, for in this particular instance it is entirely without sensible influence. For Messier lying very close to the moon's equator, and in longitude 47° W., no variation in the libration of the moon can sensibly affect the meridianal apparent diameter of the formation, and it is in this last that the diminution has been observed. If it was established, therefore, that the two formations were no longer alike, but differed markedly in any respect, the very strongest evidence of actual physical change would be the consequence.

Although therefore it is thus important that further observations of these formations should be obtained, the matter attracted little attention until the period 1870 to 1875. During these years Messier and Messier A have been repeatedly examined with the aid of most powerful telescopes, and the present dissimilarity between Messier and Messier A has been placed beyond the possibility of doubt. From some measures during the past year the long diameter of Messier appears to be 12'2 miles, and the short diameterwhich is nearly, though not quite meridianal--is 6.9 miles. The difference between the form and dimensions of the two formations is now obvious in the smallest astronomical telescope, for it is unmistakable with a power of 150 on a telescope with an aperture of a little over 2 inches. With the fine Frauenhofer equatorial of Beer and Mädler, with an aperture of nearly 4 inches and a power of nearly 300, the difference now existing between the two formations would have instantly arrested the attention of any selenographer. It is inconceivable that the two formations could have possessed their present form in Beer and Mädler's time, and