THE QUARTERLY

JOURNAL OF SCIENCE.

JANUARY, 1877.

I. PHYSICAL CHANGES UPON THE SURFACE OF THE MOON.

By EDMUND NEISON, F.R.A.S.,

Author of "The Moon, and the Condition and Configuration of its Surface."

T

HE present condition of the surface of the moon is one of the most interesting and important questions within the whole range of Astronomy; and being a subject of considerable difficulty, and requiring a thorough acquaintance with more than one branch of science to be properly dealt with, it is not surprising that widely different views on the question are held by different astronomers. The question is, however, of importance to others besides astronomers, and many eminent physicists and geologists have recognised that the study of the present condition of the surface of our satellite promises to throw much light on many vexed geological and physical problems. The opinion of that eminent geologist the late Prof. Phillips on this subject is well known, for he often expressed his conviction that the lunar surface presented the best field for the study of many of the more difficult problems of geology, and under this conviction he devoted many years to the study of selenography. Considering, in fact, the close relationship between the earth and its satellite, and the general analogy that selenographers maintain exist between their respective classes of primitive formations, it is unquestionable that the study of lunar surface must throw light on many obscure points in the past history of the earth, and probably in fact on the general history of the planets.

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The question of the present condition of the surface of the moon has engaged, therefore, the attention of some of the most eminent astronomers, and much has been done towards obtaining the information necessary for the proper consideration of the problem. But though much has been done in this direction, very much more remains to be done before this question can be held to admit of decisive settlement; for though the general nature of the lunar formations and surface has been made out fairly well, the details, which are of the utmost importance, are only very partially understood. The paucity of material available on which astronomers in general may found an opinion is not widely understood, for, with the exception of a few pages in the great work of Beer and Mädler's "Der Mond," and a small work by another great selenographer (Schmidt, of Athens), also entitled "Der Mond," until lately no published information may be said to exist accessible to astronomers in general. Hitherto information, with regard to the details of the structure of the formations on the surface of the moon, has had to be obtained directly by the careful study of the lunar surface, a work necessarily occupying years, and only to be acquired by assiduous labour. These particulars, though not at first sight of importance, must not, however, be overlooked, for it will subsequently appear that they go far to explain the position now occupied by the question-What is the present condition of the lunar surface?

It is a remarkable circumstance, in relation to this question, that whereas those astronomers who have devoted much time and labour to the study of the moon's surface, and to whom astronomers in general are mainly indebted for our present knowledge of the surface of our satellite, hold in general one view as to the present condition of the lunar surface, astronomers as a body hold a very different opinion. To take a striking instance,-scarcely any astronomer known to have devoted time to the study of selenography doubts that many processes of actual lunar change are in progress, and it is doubtful if there is one who could not promptly instance one or more such cases. Yet the general opinion of astronomers appears to be against any such physical changes having occurred. And another instance, almost as striking, exists in connection with the subject of the lunar atmosphere: whilst all selenographers appear to have detected instances where the existence of this atmosphere is revealed, astronomers in general appear to question almost the possibility of its existence, and this in face of the absence of any evidence

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 popular 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.