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THE POPULAR SCIENCE MONTHLY.— SUPPLEMENT. 



ity is increased that the moon never had an at- 

 mosphere of considerable density, even if she 

 ever had widely-extended oceans. 



The importance of this point will be seen 

 when it is remembered that if the moon ever had 

 oceans, and an atmosphere such as the earth has, 

 it is certain that the moon must have reached a 

 condition of extreme planetary old age, since 

 otherwise the oceans and atmosphere could not 

 have disappeared so completely as they have; 

 whereas, on the other hand, if the moon never 

 had widely-extended oceans, or an atmosphere of 

 considerable density, we need not necessarily 

 assume that all vulcanian activity has disappeared. 



If, then, it should appear that craters of con- 

 siderable size may be formed in the moon's crust 

 even now, it would follow of necessity — 1. That 

 the moon is not utterly decrepit ; and, 2. That she 

 can never have had oceans of great extent or an 

 atmosphere of considerable density. It will be 

 seen that I consider the a priori evidence as 

 somewhat favoring the second of these conclu- 

 sions, though not therefore the first. For it by no 

 means follows that because, if the moon is not now 

 in the latest stage of planetary life, she can never 

 have had a deep atmosphere and large oceans, 

 therefore if she never had a deep atmosphere and 

 large oceans she cannot now be in the latest stage 

 of planetary existence. A thousand millions of 

 years hence the moon will probably present much 

 the same appearance as at present, although, sup- 

 posing her not now in her final stage, she would 

 be in her final stage then. On the whole, while 

 It seems to me on a priori grounds exceedingly 

 probable that very little water and only air of 

 extreme tenuity ever existed on the moon, it ap- 

 pears to me even more probable (speaking always 

 of a priori evidence only) that the moon has 

 reached the latest stage of planetary life. 



Nevertheless it must be admitted that these 

 questions can only be satisfactorily resolved by 

 a posteriori evidence. The considerations ad- 

 duced may lead us to look with somewhat more 

 caution on observed evidence of change than we 

 should do if antecedent probabilities led us to 

 expect change ; but, if we should find unmistakable 

 evidence of change, we must conclude that the 

 opinion we had based on antecedent probabilities 

 was incorrect. 



which have been attributed to the former existence of 

 water on the moon, as the color of the lower lunar 

 levels, the supposed signs of glacial action, etc., may 

 not all be equally well explained by referring them to 

 the time when the moon's crust was first formed, the 

 lower levels representing the regions occupied by the 

 portions of the surface which last remained fluid. 



The first case to be considered is that of the 

 lunar crater Linnaeus or Linne. For two reasons 

 this case is more satisfactory, as will presently 

 appear, than that of the new crater supposed to 

 have recently appeared. 



In the lunar plain, called the Sea of Serenity 

 (probably because we have no reason to suppose 

 it is exceptionally serene, while we are certain it 

 is not a sea), there was once a deep crater, about 

 six and a half miles across. It was very distinct 

 when the sun's rays fell obliquely on it — that is, 

 shortly after the time of sunrise, and shortly be- 

 fore the time of sunset there. But when fully 

 illuminated the crater was not a well-defined ob- 

 ject; the tint of its floor is indeed markedly 

 lighter than that of the surrounding plain, but 

 the light tint merges gradually into that of the 

 Sea of Serenity. Thus Madler : but Lohrmann 

 described the crater as only about four and a half 

 miles in diameter. Both these observers agree 

 in describing the crater as deep, and having steep 

 walls. Now, in November, 1866, Schmidt, of 

 Athens, announced that this crater was missing. 

 To understand the importance of this announce- 

 ment, if the crater originally existing had been 

 filled up, let it be noted simply that the quantity 

 of matter necessary to fill that crater would be 

 at least equal to that which would be required to 

 form a mountain covering the whole area of Lon- 

 don, to a height of two miles ! Naturally, as- 

 tronomers were greatly interested by Schmidt's 

 discovery, and during the years 1867 and 1868 

 many observations of Linne were made with tel- 

 escopes of great power. 



The result of these observations was to show 

 first that the area originally occupied by the cra- 

 ter and its outer slopes still presented a whitish 

 aspect under the illumination of a high sun. Li 

 extent, then, the region of the crater had not 

 changed. Secondly, within the white region a 

 shallow circular depression, about seven miles in 

 diameter, was recognized with sloping sides (on 

 the inside as well as the outside), so that at the 

 bottom the depression bad a diameter of only 

 about three miles, the depth of the depression 

 being about a third of a mile. Thirdly, within 

 this shallow depression a small crater about half- 

 a mile in diameter on the inside, and of consider- 

 able but unknown depth, was detected. 



There could be no doubt whatever that a 

 great and' important change had taken place in 

 Linne — a change compared with which the most 

 tremendous volcanic action on our own earth 

 within historic times would be almost as nothing 

 — were it not that some old drawings by Schroter 



