174 On the Dtffusion of Hiai 



not discharge more than it receives from the other heavenly 

 bodies, if it discharge even this, the argument recurs, 1 helieve, 

 in its original form, as sufficiently established, that if an interior 

 heat exist, it must be diffused through the substance of the earth, 

 and an equihbrium of temperature be at length established, in- 

 compatible with those operations ascribed to its action in tlie. 

 Huttonian theory. 



But further, though the heat were not retained, — though it 

 were granted that it is propagated into the regions of space, this 

 does not render the argument I have urged less conclusive. The 

 strength of that argument lies in the circumstance of the dif- 

 fusion of heat from the interior, not in the accinnulation of that 

 heat at the surface. Tlie latter is a contingent event, which may 

 or may not happen, without being -of essential importance in th^ 

 conclusion to be drawn. The former is a necessary result, which 

 must prove subversive of the arrangement, the assumption of 

 which constitutes the leading 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, with- 

 out evidence, merely to obviate an argument, the force of which 

 cannot otherwise be avoided, no hypothesis, even the most ex- 

 travagant, could ever be overturned. Heat cannot originate in 

 nothing ; and if we assume its constant discharge from the cir- 

 cumference of this planet, what cause is it possible to assign for 

 sts constant supply from the interior ? Admit even its existence 

 in any degree of intensity, still it is obvious that it must be in 

 limited quantity. If we speak of a spring of heat, as conveying 

 the idea of an unlimited supply, we deceive ourselves by the use 

 of a term to which no definite signification can be affixed, but 

 what is subversive of the reasoning it is designed to support. A 

 source or spring means nothing more than a hidden reservoir 

 connected with external supplies. There can be no reservoir of 

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

 bv constant su[)ply, shall continue to furnish it, to be discharged 

 from the circumference through indefinite time into uidimited 

 space. And it is not possible to imagine any circulation by 

 which it shall be restored. 



It is, therefore, 1 conceive, of little importance in the discus- 

 sion, whether the heat supposed to be conveyed from the centre 

 to the circumference is accumulated there, or is discharged into 

 the regions of space. Its propagation to the surface must be 



sub- 



