whatever that there is no atmosphere of the nature supposed.

It would be an interesting inquiry to ascertain in what manner arose this direct conflict of opinion on this subject, between those who have systematically studied the appearances presented by the moon, and those who have not in the same systematic and assiduous manner examined the lunar surface. It appears to have originated in a short summary by Mädler of the appearances presented by the moon, wherein the differences between the condition of the moon and earth were forcibly stated, and where he pointed out the impossibility of the view that was held in the time of the earlier astronomers,-that the moon might be a mere copy of the earth, containing a dense atmosphere, large oceans, abundant vegetation, and animal life, or even human inhabitants. In a condensed and more unqualified form these remarks crept into all astronomical text-books, and were the main basis of the views commonly held by

astronomers.

The present condition of this question is therefore not surprising; with the views of those astronomers who have devoted many years to the study of the moon, but very im perfectly known, and in fact inaccessible to most astro nomers, the general body of observers are left to draw their own conclusions from their own acquaintance with the lunar

The subject of the phenomena which would be presented by the moon were it surrounded by an atmosphere of any given density is one of very considerable difficulty. It cannot be treated in a popular-and therefore necessarily superficial-manner, because it involves subjects demanding a tolerably extensive acquaintance with mathematical physics to enable their real bearing to be understood. The mathematical treatment of the subject, though involving no point of any particular difficulty, is nevertheless complex and very laborious, from its extent. It is not surprising, therefore, that when this subject is regarded in a popular manner ideas should be arrived at that a more elaborate and careful examination shows, by mathematical demonstration, to be mistaken; and it was in this manner that Bessel, in his investigation of a portion of this subject, arrived at results which-though they have been since shown to be wrong-have exerted the greatest influence on the progress of selenography. In saying that there is no real evidence whatever that the moon does not possess the atmosphere which has been ascribed to it by the author, it must not be supposed that no attention has been paid to the various circumstances which have from time to time been pointed out as proving the impossibility of the moon possessing an atmosphere of a surface density far less than this. But on submitting these objections to the careful examination that they demand, and in particular by calculating with the aid of the powers of mathematical analyses what would be the real dimensions of these phenomena which it is supposed would reveal the existence of the lunar atmosphere, it is found that, for the atmosphere supposed, they would be insensible. Most of these objections have been already considered in my recent work on the Moon, or in the "Quarterly Journal of Science" (October, 1874). The whole subject involves such difficulties that statements founded on merely a pooular or general consideration of the problem it involves require therefore to be regarded with caution.

surface. Approaching the study of the moon with strongly preconceived notions that the surface is a mere arid desert, nearly red-hot at times and almost immeasurably cold at others, without water, air, or life, the real condition of the moon is not such as to dissipate these views at the first glance. Its cold, still, apparently unchangeable surface, so utterly unlike what the earth might be supposed to appear as seen from the moon, convinces the casual observer that the world he then sees is utterly unlike the world he knows. He looks for immense cloud-masses floating in a dense atmosphere, and sees none; for wave-tossed seas and winding broad rivers, and there are none; searches for luxuriant forests and green prairies, and they are absent. This is enough, and he retires from further contemplation of the "airless, waterless, lifeless, volcanic desert" of the textbooks. During subsequent periods he may again take a casual glance of our satellite, look at lunar occultations, or watch the grand changes in appearance that the face of the moon presents as the sun sweeps across its heavens and slowly illuminates in turn its varied formations. But study the surface, endeavour to piece together bit by bit the different features that are revealed, until he thoroughly comprehends the details and nature of the formations he sees, is what the general observer and the great majority of astronomers do not even attempt to do.* To properly become acquainted with the nature of the lunar surface requires that the study of the moon should be made the primary object of one's observations; and those who have done this, and devoted years to the assiduous study of the moon, constitute those astronomers who-as previously mentioned-hold different views as to the present condition of the lunar surface to astronomers in general.

The previous observations are, it will be seen, of importance in connection with the subject of the present article, for they show how selenographers and astronomers in general may hold very different opinions on questions connected with the moon,-one are led to conclusions from the careful study of the minuter details of the lunar forma

An interesting instance, illustrating the condition of the general knowledge of selenographical subjects amongst astronomers, is the prevailing opinion that no retardation of occultation of stars by the moon has ever been observed, but that stars approach the limb of the moon, and disappear behind it at the instant they theoretically should. Yet it has been stated repeatedly that the only three series of reductions of the observation of occultation of stars show that a very considerable retardation does take place-a retardation of from 5 to 10, and often even 15 seconds of time. Different opinions may be held as to the cause of this retardation of occultation, but of the fact there can be no question, and it has been known for nearly twelve years.

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 6 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