and their Connexion with Volcanos. 67 



There is, however, another hypothesis, which, for distinction's 

 sake, I shall denominate the mechanical one, founded on the as- 

 sumption now generally embraced, as to the interior of the earth 

 enjoying throughout a higher temperature than that of its sur- 

 face, independently of any chemical or electrical agencies, by 

 which it may in certain parts be affected. 



On the mode, however, in which this central heat is to ope- 

 rate, in bringing about the phenomena of volcanos, a great divi- 

 sion of opinion exists. One set of philosophers, for example, 

 content themselves, with tracing these effects simply to the con^ 

 traction of the external crust of the earth upon its contents ; and 

 in this way they imagine some of the melted matter within, to be 

 from time to time expressed through those portions of the sur- 

 face which present the least resistance, somewhat in the same man- 

 ner,_I suppose, in which the juice is forced by pressure through 

 the rind of an orange. 



Others, on the contrary, with rather more attention, as it ap- 

 pears to me, to the facts of the case, conceive the phenomena to 

 arise primarily from water being brought into contact with this 

 incandescent material, and in consequence l)ecoming volatilized 

 in. the form of steam, to the elasticity of which they attribute 

 some of the more remarkable effects. 



But it cannot be too constantly kept in mind, that the conco- 

 mitants and sequelae of an eruption are various and complicated^ 

 and that no theory ought to be admitted, which does not em- 

 brace an explanation of them all. Now, of these two hypothe- 

 ses, the former takes into the account only one single effect, 

 namely, the emission of lava-currents *, overlooking entirely the 



no means so ; and, besides, that in calculating their specific gravity in the in- 

 terior of the globe, we ought to take into account the increase of density 

 produced by the immense compression. Indeed, if Professor Leslie is to be 

 believed, the difficulty seems to lie the other way ; for, in his opinion, the 

 specific gravity of the earth's would be increased so much beyond that of the 

 mean density of the globe, owing to this cause, that bethinks it necessary to 

 consider its centre as hollow, or as filled merely with light, the rarest sub- 

 stance known. 



• In an article in tlie Foreign Quarterly Journal, which has appeared 

 since this memoir was written, headed " Fourier on Heat," it is remarked: 

 ** The phenomena of volcanos, hot springs, and earthquakes, are exphiined 

 with singular felicity on this hypothesis (viz. that of a central heat), they ap- 



E 2 



