Theory of Volcanic Energy. 303 



the heat calculated to be evolved out of crushing one volume 

 would fuse about one tenth of an equal volume*. 



Let us, then, consider a prism of rock of any length. This 

 is itself a part of the earth's crust, and by its resistance has, 

 up to the moment of its giving way, resisted, and so permitted, 

 the necessary accumulation of the pressure which eventually 

 causes it to yield. Conceive that in this prism there are portions 

 situated here and there which are weaker than the average, 

 and that these weak portions when crushed allow of the prism 

 being shortened at the places where they are situated by the 

 quantities u lt oc 21 ct s , &c. respectively. It is clear that the weaker 

 places will yield first and under a less pressure, and by the 

 relief so afforded delay the crushing of the others, because the 

 pressure must accumulate afresh. But, for argument's sake, 

 we will suppose all to yield together, and the pressure throughout 

 the action to be equal to the value it had at the first yielding. 



Now suppose that when the prism has yielded the whole of 

 it becomes shortened by the length a. If, then, P be the 

 pressure which caused it to yield, Va will be the whole work 

 done upon the prism. The length a is made up of the portions 

 a,, a 2 , « 3 , &c, by which the weak portions have been shortened; 

 while Pa x , Pa 2 , P« 3 , &c. are the portions of work done at these 

 places. And these taken together make up P«, since aj + a 2 + 

 « 3 + &c. = a. We see, then, that the work must be confined to 

 these places; for if there were work done elsewhere we should 

 have more work than P#, which is impossible. Hence the work 

 convertible partially into heat takes place at all these places, and 

 at each in proportion only to the yielding, and nowhere else ; 

 so that it cannot be localized at any one place, It seems to me, 

 therefore, that, unless the heat got out of crushing any portion 

 of rock is sufficient to fuse that particular portion, none will be 

 fused ; and this led me to my second objection (put first by Mr. 

 Mallet) j that, if crushing rock could fuse it, the cubes experi- 

 mented upon ought to have been fused ; for I certainly under- 

 stood Mr. Mallet to rely upon his experiments for showing how 

 much heat might be obtained by that means. 



The supposed weak parts of the prism answer to the joints 

 of ashlar stones, as instanced by Mr. Mallet in his late paper 

 in this Magazine. 



In reply to my objection last mentioned, Mr. Mallet enters 

 upon the consideration of the circumstances under which crushing 

 may be thought to occur deep down in the earth's crust, and holds 



* § 188. " It follows from equation (6) that 1 cubic mile of crushed 

 mean rock will thus heat 0262 cubic mile of the heated material, or will 

 fuse 0*108 cubic mile of the fused material, the specific heat of all being 

 taken as the same." 



