A COMPARISON OF THE FEATURES OF THE EARTH AND THE MOON. 49 



been found necessary to account for the maria by such action lends a certain 

 countenance to this view. Yet it seems to me safer to suppose that the moon 

 has, as a whole, had essentially the same experience in space as our earth. As 

 before noted, the earth, since its organic life, at least in the present series of 

 forms, began to exist, has evidently had no such impacts of foreign bodies as 

 formed the maria. It is, of course, among the possibilities that the earth has been 

 subjected to invasions of large meteoric bodies, as the moon appears to have 

 been, and that an ancient organic period was not only destroyed but the records 

 of its existence entirely effaced. There is, however, no other known evidence on 

 which to found such a conjecture, except what we find on the moon. 



As regards the failure of the moon to exhibit the marks of adjustment of its 

 crust, which first hardened, to an interior diminished by the loss of heat, it may 

 at first appear that as the value of gravity is only about one-sixth what it is on the 

 surface of the earth the stress which would impel the superficially cooled section 

 to accommodate itself to the lessened bulk of the interior would be proportion- 

 ately smaller, so that the outer shell might remain unsupported while the inner 

 portion shrunk away from it. This view seems inadmissible, for the reason that 

 in the case of the earth, as has been well reckoned, a shell less than a mile thick 

 would, if unsupported, crush and fall in of its own weight, so that in the moon 

 the crust would in a like manner crush at less than six miles of depth. It is thus 

 evidently necessary to form some other hypothesis which will account for the 

 lack of adjustment. I have essayed several of these, which I will now briefly set 

 forth with the reasons why they seem adequate or otherwise. 



At first it seemed possible that the aggregate wrinkling and crushing exhib- 

 ited in the larger ridges and furrows, as well as in the host of small ridges which 

 are seen with the greater telescopes, might have been sufficient to provide for 

 the necessary contraction through the buckling and shoving of the crust. Yet 

 on carefully examining selected areas of the crust where these features are best 

 shown it does not seem possible that the accommodation or " take up " thus 

 effected can amount to many miles of length. Moreover, the phenomena are 

 not those which would be produced by the folding of a thick crust, as it sank 

 upon a diminished nucleus, but only such as superficial strains would induce on 

 a thin outer layer. It appeared conceivable that for some reason an accommo- 

 dative folding might have taken place on the portion of the moon which is never 

 seen, but this supposition is supported by no evidence whatever ; all we see on 

 the extreme margin of the visible surface leads to the conclusion that the hidden 

 side is essentially like that we behold. Again, it appeared possible that the whole 

 mass of the satellite remained in the boiling condition until it had been brought 

 to a state where the cooling quickly induced rigidity throughout the sphere, all 

 parts down to the center having attained somewhere near the same temperature. 

 In this way we could explain the small amount of internal contraction which has 

 apparently occurred since the most ancient features on the lunar surface, the 

 larger vulcanoids, were formed. 



Although in a general way we know the law of cooling bodies, we are not 



