AT- THE earth's SURFACE. 425 



surface of the land, we shall find an intensity of heat in the in- 

 terior, compared with which the heat necessary to melt moun- 

 tains of quartz, formerly supposed to present so great a diffi- 

 culty, is a mere atom in the scale, scarcely affording even a 

 point of comparison. 



Some idea may be formed of this, by recurring to the illu- 

 stration of the iron bar, with a decreasing temperature, ma- 

 king the most liberal allowance in favour of the Huttonian 

 hypothesis, with regard to the respective portions of space. 

 Thus the bar, being one thousand inches in length, if its tem- 

 perature at the one extremity be 50°, and if within five inches 

 of this it is at a white heat, then the heat increasing at the 

 same rate, through every succeeding five inches, what must 

 be its intensity at the other extremity ? No effort of the 

 imagination can form the most remote conception of it, nor 

 can any argument be wanting to prove, that no such heat can 

 exist in the interior of the earth. . 



If to avoid the difficulty, a less rapid decrement of tempera- 

 ture be supposed, then, from a heat of that intensity which 

 must be assumed to exist at the bottom of the ocean, to pro- 

 duce the effects ascribed to it, the decrease in the short space 

 between that and the surface cannot be such as to brine the 

 tempex'ature within that which is at all compatible with the 

 established economy of nature. The difficulty is, therefore, 

 insurmountable ; it must occur on the one hand or on the 

 other ; and it is not merely connected with Mr Playfair's 

 argument, but as that argument is founded on a law perfectly 

 just with regard to the diffusion of temperature, it is a difficul- 

 ty which necessarily follows from the assumption of a central 

 heat, or of any internal heat such as that which must be assu- 

 med in the Huttonian Theory, — a heat which is to operate a- 

 round the whole circumference of the globe, continue its operar 

 tions for such immense periods, and renew it for indefinite time. 



Leaving 



