422 1 ON THE DIFFUSION OF HEAT 



mulation of that heat at the surface. The latter is a con- 

 tingent event, which may or may not happen, without being of 

 essential importance in the conclusion to be drawn. The 

 former is a necessary result, which must prove subversive of 

 the arrangement, the assumption of which constitutes the lead- 

 ing principle of Dr Hutton's System. 



Mr Playfair, accordingly, in the general case which he 

 takes for the illustration of his argument, assumes a constant 

 supply of heat at the centre, as well as a constant escape of it 

 from the circumference. But in applying the argument, where 

 is the proof of such a supply of heat at the centre of the 

 earth ? It can scarcely be necessary to remark, that none is 

 attempted to be given. And were it allowable to make an 

 assumption, without evidence, merely to obviate an argument, 

 the force of which cannot otherwise be avoided, no hypothesis, 

 even the most extravagant, could ever be overturned. Heat 

 cannot originate in nothing ; and if we assume its constant dis- 

 charge from the circumference of this planet, what cause is it 

 possible to assign for its constant supply from the interior ? 

 Admit even its existence in any degree of intensity, still 

 it is obvious, that it must he in limited quantity. If we 

 speak of a spring of heat, as conveying the idea of an unli- 

 mited supply, we deceive ourselves by the use of a term to 

 which no definite signification can be affixed, but what is sub- 

 versive of the reasoning it is designed to support. A source 

 or spring means nothing more than a hidden reservoir, con- 

 nected with external supplies. There can be no reservoir of 

 heat in the centre of the earth, which, without being recruited 

 by constant supply, shall continue to furnish it, to be dischar- 

 ged from the circumference through indefinite time into unli- 

 mited space. And it is not possible to imagine any circula- 

 tion by which it shall be restored, 



It 



