SECT. XXV. VOLCANIC THEORIES. 235 



returned at upwards of ten thousand. These terrible engines 

 of ruin, fitful and uncertain as they may seem, must, like all 

 durable phenomena, have a law which may in time be discovered 

 by long-continued and accurate observations. 



The shell of volcanic fire that girds the globe at a small depth 

 below our feet has been attributed to different causes. By some 

 it is supposed to originate in an ocean of incandescent matter, 

 still existing in the central abyss of the earth. Some conceive 

 it to be superficial, and due to chemical action, in strata at no 

 very great depth when compared with the size of the globe. The 

 more so as matter on a most extensive scale is passing from old 

 into new combinations, which, if rapidly effected, are capable of 

 producing the most intense heat. According to others, electricity, 

 which is so universally diffused in all its forms throughout the 

 earth, if not the immediate cause of the volcanic phenomena, at 

 least determines the chemical affinities that produce them. It 

 is clear that a subject so involved in mystery must give rise to 

 much speculation, in which every hypothesis is attended with 

 difficulties that observation alone can remove. 



But the views of Mr. Babbage and Sir John Herschel on the 

 general cause of volcanic action, and the changes in the equili- 

 brium of the internal heat of the globe, accord more with the 

 laws of mechanics and radiant heat than any that have been pro- 

 posed. The theory of these distinguished philosophers, formed 

 independently of each other, is equally consistent with observed 

 phenomena, whether the earth be a solid crust encompassing a 

 nucleus of liquid lava, or that there is merely a vast reservoir or 

 stratum of melted matter at a moderate depth below the super- 

 ficial crust. The author is indebted to the kindness of Sir 

 Charles Lyell for the perusal of a most interesting letter from Sir 

 John Herschel, in which he states his views on the subject. 



Supposing that the globe is merely a solid crust, resting upon 

 fluid or semi-fluid matter, whether extending to the centre or 

 not, the transfer of pressure from one part of its surface to 

 another by the degradation of existing continents, and the forma- 

 tion of new ones, would be sufficient to subvert the equilibrium 

 of heat in the interior, and occasion volcanic eruptions. For, 

 since the internal heat of the earth is transmitted outwards by 

 radiation, an accession of new matter on any part of the surface, 

 like an addition of clothing, by keeping it in, would raise the 



