Mr. Mallet's Theory of Volcanic Energy. 139 



any physical question. It has been long understood, however, 

 that an approximate solution of almost any problem in the 

 ordinary branches of natural philosophy may be easily ob- 

 tained by a species of abstraction, or rather of limitation of the 

 data, such as enables us easily to solve the modified form of 

 the question, while we are well assured that the circumstances 

 so modified affect the result only in a superficial manner "-'*. 



This is the method I have taken ; and if Mr. Mallet desires 

 seriously to impugn my conclusions, he must either show that 

 my formulas are insufficient to give the full amount of heat 

 developable under his theory, or else he must substitute in 

 them such admissible values of the quantities involved as will 

 support his views better than those that I have taken out of 

 his own writings. His "lofty point of thermodynamics," 

 from which he so complacently assails the conclusions of pre- 

 vious geologists, will not serve him now ; for I occupy a 

 station on the same range of heights, and one which com- 

 mands his own. If he will do what I have suggested, we 

 shall know whose house is, what he calls mine, " a house of 

 cards." 



Although Mr. Mallet disclaims attempting a refutation of 

 my arguments, he has attacked them in those places where he 

 evidently thinks them most assailable. Naturally, " for my 

 own purposes " (that is, for the purpose of my argument 

 ■ — neither an unfair nor unreasonable act) I took his " small " 

 (quarto) paper, which I did not represent to be part of his 

 original memoir, and used it. It was from this paper, pub- 

 lished in the ' Transactions,' that I took those " peascods " 

 and put them into my " mathematical mill." They were Mr. 

 Mallet's own " peascods ;" and if he has any " wheat " in store 

 which he holds back from the world, I shall be happy to put 

 it also into the mill, and to see what sort of flour it will yield. - 

 The quantity which I have represented in my formulas 

 by fij p' is not, as Mr. Mallet supposes, a coefficient of fric- 

 tion. True, the letter is commonly used for that purpose ; and 

 probably that led Mr. Mallet to suppose that it was so used 

 here. But he might have observed that in such a case the 

 dimensions of the equation would have been incorrect, and 

 we should have had a length equal to an area. Nevertheless 

 it is true that fi and fi will depend respectively upon friction 

 and adhesion, and increase and decrease with them. Now, if 

 any one will take the trouble to look at Mr. Mallet's paper in 

 this Magazine " On Bock-crushing &c," p. 8, he will see it is 

 stated that the coefficient of friction has been put as high as 

 | of the pressure ; and " should this coefficient increase pro- 

 * Natural Philosophy, § 438. 

 L2 



