tions, that the others, not thus acquainted with these features, refuse to accept. The purport of the present paper is to detail instances of what selenographers consider to be real physical changes in the moon, together with some of the reasons which they have for supposing them to be such. From an opposite point of view the subject has been already ably handled in these pages by one of our most eminent astronomers, Mr. R. A. Proctor, who in his own department of the science stands still unrivalled.*

The most prominent instance of a supposed change on the surface of the moon is indubitably that of Linné, because it is an instance which was brought before the entire astronomical world, and engaged for nearly a year the attention of almost every astronomer in Europe. The leading facts of the case may be briefly stated as follows:-On the north-west quadrant of the moon is a large tract of nearly level surface, of a pale grey, almost greenish colour, and this region-which is in extent about 430 miles in diameter -contains near the centre a moderate-sized bright crater, called Bessel, nearly 14 miles in diameter, and with a circular wall rising 4000 feet above the interior and about 1600 feet above the surrounding surface. Scattered about this plain are a few small crater-like objects, about 2 miles in diameter and with walls rising about 300 feet high. Now, near the eastern centre of this great tract of level ground, or the Mare Serenitatis as it is called, one of the most eminent selenographers, Lohrmann, had placed a distinct bright crater, subsequently called Linné, and this crater he described as being about 5 miles in diameter, and, after Bessel, the most conspicuous object on the Mare Serenitatis. Ten years later our greatest selenographer, Baron von Mädler, confirmed Lohrmann's description, and named the crater Linné, which he described as deep and very distinct in oblique illumination. He measured its diameter, which was 63 miles, or about half the size of the great crater Bessel, and selected it as the most distinct object in its region, for one of the measured units in his great trigonometrical lunar survey. Now, had Lohrmann's account not been correct, Mädler would certainly have been struck at once by the fact; but Mädler's work, which was entirely independent of his predecessor, entirely confirms the account. And had Mädler's account been incorrect, the instance would have been so evident that it could not have escaped the attention of Gruithuisen and Schmidt, who were about this time making

* Quarterly Journal of Science, January, 1873, and October, 1873.

lunar observations, and especially the latter, as he repeatedly drew this particular part of the moon. In the drawings of Schmidt, however, whenever this crater Linné is shown, it is as a deep crater, entirely in accordance with the description of Lohrmann and Mädler.

In October, 1866, however, Schmidt was startled by finding no trace of Linné, when it was in a position when it should have been most conspicuous. Instead of the deep wide crater with its interior filled with intense black shadow, all he could detect was a faint, indistinct, cloudy marking, about 5 miles in diameter. During the subsequent evenings it retained this aspect, though at times it was so indistinct as to be barely visible. Schmidt immediately announced this startling circumstance, and during the end of 1866 and the commencement of 1867 nearly every astronomer in Europe directed his attention to this now-celebrated spot. Since then this portion of the moon has been repeatedly examined by selenographers, but Linné has never been seen of the size and character ascribed to it by Lohrmann, Mädler, and Schmidt. The conspicuous deep crater has utterly disappeared.

In what manner is this change to be explained, and how is it that apparently so unmistakable an instance of physical change has been rejected as not established? These questions are best answered by describing what has been seen in the place formerly occupied by the deep crater. Where Linné was, it has been already mentioned, an indistinct white cloud-like mark was seen; but powerful telescopes and assiduous examination soon revealed something else. When first the sun illuminated the region containing Linné a small conical mountain peak was detected, this mountain being about 100 to 200 feet high, and casting a short black shadow, and was by some mistaken for a small crater. Subsequently other observers detected a minute crater cone. When first seen it was estimated by Schmidt to be about a quarter of a mile in diameter, and by Secchi, of Rome, and Buckingham as about one-third of a mile across; this crater lying to the west of the small hill. If the estimates of these earlier observers are to be trusted, and Schmidt at least was not likely to be mistaken, a most important circumstance must have occurred; for when next seen the crater was nearly three times as large, or nearly 1 mile in diameter, according to D'Arrest of Copenhagen, Schjellerup, Huggins, and Wolf. In July, 1867, the crater was sufficiently large and distinct to permit of its being measured, and according to Huggins it was rather under 2 miles in diameter-a result confirmed,

during the next month, by Buckingham and Knott. It must be remembered, however, that this crater was only seen when the region was very obliquely illuminated by the sun, at sunrise or sunset in fact. At any other period all that was seen was a white cloudy marking, about 8 miles in diameter. Since 1868, it is only on three or four occasions, when the atmosphere has been exceptionally good, that the small crater within Linné has been seen, and then only in powerful telescopes. It does not seem ever to have been seen as a distinct crater in telescopes as small as those employed by the earlier selenographers, who only knew the crater as a fine, distinct, conspicuous object, with a black interior, nearly five times the diameter of that of the present crater. The present diameter of the crater opening of the cone on the site of Linné is about 1 miles, the total diameter of the cone at the summit about half as much again, and at the base about 2 miles. The height above the surface is about 200 feet at most.

As a crater, in fact, Linné is now one of the most difficult to see on the entire Mare Serenitatis. Under illumination, when every other crater stands out boldly as distinct objects, Linné is, with rare exception, either invisible as an elevation or else appears as an insignificant hill. It is questionable whether the earlier selenographers could have seen it at all under these conditions. It should be mentioned, however, that between 1867 and 1869 several observers considered that they could detect traces of the ring of a very shallow crater around the crater cone in the position of Linné, and in size slightly larger than the crater of Beer and Mädler. The actual existence of this ring is doubtful; it has not been seen since 1869, and the appearance noticed probably arose from a number of ridges and mounds near the position of Linné.

Considering, then, the facts as stated above, it would appear that there could be no question but that a real physical change had occurred on this portion of the surface of the moon. The large crater independently described by Lohrmann, Beer and Mädler, and Schmidt, as existing in this portion of the Mare Serenitatis, unquestionably no longer exists, and in its place appears a white cloudy marking, containing a small crater cone, with an opening scarcely one-twentieth of the area of the former crater. What reasons have therefore been advanced against any change having taken place, that astronomers in general should favour this view? The principal reason why astronomers are so strongly against allowing that any change can

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

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 as g. 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.