22 CONSTITUENT MATERIALS OF THE EARTH, 



with the annular eminence, beyond which again vast boulder- 

 like masses are in some instances seen scattered. \Vhat, how- 

 ever, most strikes the senses of an observer, is the vast pro- 

 fundity of some of the pits between the ring and the inner 

 mount ; in one case, this is reckoned to be not less than 22,000 

 feet, or twice the height of ^Etna. 



These characteristics of the moon forbid the idea that it can 

 be at present a theatre of life like the earth, and almost seem 

 to declare that it never can become so. But it is far from 

 unlikely that the elements which seem wanting may be only 

 in combinations different from those which exist here, and may 

 yet be developed as we here find them. Seas may yet fill the 

 profound hollows of the surface ; an atmosphere may spread 

 over the whole. Should these events take place, meteoro- 

 logical phenomena and all the phenomena of organic life, will 

 commence, and the moon, like the earth, will become a green 

 and inhabited world. 



It is unavoidably held as a strong proof in favour of any 

 theory, when all the relative phenomena are in harmony with 

 it. This is eminently the case with the Laplacian cosmogony, 

 for here the associated facts cannot be explained on any other 

 supposition. 1 It remains that a few words should be said of 

 the well-known hypothesis of a central heat. The immediate 

 surface of the earth exhibits only the temperature which might 

 be expected to be imparted to such materials by the heat of 

 the sun. There is a point a very short way down, but varying 

 in different climes, where all effect from the sun's rays ceases. 

 Then commences a temperature from an entirely different 

 cause, one which evidently has its source in the interior of the 

 earth, and which regularly increases as we descend to greater 

 and greater depths, the rate of increment being, in general, 

 about one degree Fahrenheit for every fifty feet ; and of this 

 high temperature there are other evidences, in the phenomena 

 of volcanoes and thermal springs, as well as in what is ascer- 

 tained with regard to the density of the entire mass of the 

 earth. This approximates five and a half times the weight of 

 water ; but the actual weight of the principal solid substances 

 composing the outer crust is as two and a half times the weight 

 of water ; and this, we know, if the globe were solid and cold, 

 should increase greatly towards the centre, water acquiring 

 the density of quicksilver at 362 miles below the surface, and 



1 See Proofs, Illustrations, &c., No. 2. 



