have occurred appears to be the existence of a strong prejudice against the very possibility of such changes occurring. The probable origin of this feeling has been already adverted to. This of itself, however, would not have been sufficient, of course, to bring about this result; it remains, therefore, to consider the evidence against the reality of any change in Linné having occurred that has from time to time been brought forward.

Immediately after Schmidt had announced the supposed change in Linné, reference was made to the lunar drawings in the "Selenotopographische Fragmente" of Schröter, of Lilienthal, the earliest of the great selenographers. Amongst the very earliest drawings was found one of the Mare Serenitatis, made on November 5th, 1788, with powers of 95 and 161, on a reflector of 6 inches aperture and 7 feet focus. On this drawing Schröter does not draw Linné as a crater, but not very far from its place he draws a white spot on a ridge which he marks v, and a larger dark spot which he marks Schmidt considered this white spot v to be Linné, and this view has been strongly urged by Huggins, and has been generally accepted as correct by astronomers. It has been urged, therefore, that as Schröter drew this formation in a manner not unlike its present appearance no change can have occurred, but that Lohrmann, Beer and Mädler, and Schmidt must all have been entirely mistaken as to what they thought they saw. It was also found that, on reference to a map made by Lahire during the seventeenth century, no trace of Linné is to be found. It may be, however, at once observed that this last fact is absolutely valueless; the map of Lahire is perfectly untrustworthy in these minute details, and Hevelius, Riccioli, and Cassini are little superior. From all the maps made about this date numerous craters are omitted, far larger than Linné ever was supposed to be, so that the fact of Linné not having been drawn proves nothing. These maps were made principally by full-moon drawings, where Linné would not be visible as a crater. Even, however, had this not been so, the value of this negative evidence from Lahire's map is entirely destroyed by the direct evidence furnished by Riccioli's map, which shows Linné as a distinct crater; and it must be remembered that the present crater on the site of Linné could not possibly have been seen by Riccioli with the optical means by which the materials for his map were obtained.

The only real evidence, then, that no change has taken place in Linné is this single drawing of Schröter's, which is

relied on as proving that the subsequent maps and drawings of Lohrmann, Beer and Mädler, and Schmidt must be entirely wrong in this point. It is a remarkable circumstance that in every other case of a discrepancy between the drawings of Schröter and Beer and Mädler, one alone of the above authorities, the drawings of Schröter, have been rejected as entirely unworthy of comparison with those of Beer and Mädler. In this particular instance, however, one of the earliest drawings of Schröter, made with his most imperfect and least powerful instruments, on an occasion. when the definition from his own account must have been very inferior, has been brought forward to prove the incorrectness of the drawings and micrometric measures of his great successors. Now, no selenographer acquainted with Schröter's works will allow that this is permissible. Schröter's early drawings are never to be trusted when they differ from his successors in the more minute features. Whenever a discrepancy exists, it will be found that Lohrmann or Beer and Mädler are correct, and Schröter wrong. It is true the same does not hold with Schröter's later drawings, and especially when he was in possession of his great reflectors of 13 feet and 26 feet focus respectively; but this particular drawing of Schröter's is one of his very first. It must also be remembered that Schröter did not draw the portion containing Linné with the same fulness or accuracy as the rest, but that, while the western portion is fairly correctly drawn, the eastern portion, where Linné should be, is misplaced and imperfect. Mr. Birt, our best English selenographer, doubts in fact whether this white spot, v, really is meant for Linné, and has made out a very strong case for supposing that the dark grey spot, g, farther south, really represents Linné as seen by Schröter. If this is really the case it entirely upsets the view commonly held by astronomers that' Schröter's drawing proves no change to have occurred in Linné. For Schröter describes this spot g, which was only faintly illuminated, from its being near the dark portion of the moon, as being very similar to, and probably of the same nature as, another smaller formation a little to the west of it. Now this smaller formation, r, is really a fine distinct crater, Bessel m, about 3 to 3 miles in diameter and 600 or 700 feet in depth. If, then, the spot g is Linné, as seen by Schröter, it would have been a large crater, in tolerable accordance with the description of Lohrmann and Beer and Mädler. Further, as Mr. Birt points out in relation to the western and best shown portion of the drawing of Schröter, g falls exactly into the position which Linné should occupy.

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 9 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 Fæcunditatis, 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.