302 The Rev. O. Fisher on Mr. Mallet's 



It appears both from § 18" and from the above, that A or 

 even A' was scarcely deep enough to give the normal rate for 

 their great wave-lengths. It also appears that for the shorter 

 system of C the depth 185 is insufficient. From § 18 it seems 

 that the maximum or normal rate is reached. We may take the 

 nearly identical numbers for B and E for the valuation of the 

 mean constant, which is 1785*84 (in metre- minute measurement). 



[To be continued.] 



XXXVI. Mr. Mallet's Theory of Volcanic Energy tested. 

 By the Rev. 0. Fisher, F.G.S.* 



IN the July Number of this Magazine Mr. Mallet has replied 

 to certain remarks upon his theory of volcanic energy made 

 by Professor Hilgardf and by myself J, and containing some- 

 thing in common. Professor Hilgard suggested an objection 

 to the theory which he thinks " must strike every reader of the 

 original memoir/' and assumes not to require proof — that " the 

 maximum temperature resulting from the crushing to powder 

 of the hardest rock is something over 217° Fahr." as determined 

 by Mr. Mallet. But inasmuch as Mr. Mallet in his memoir 

 appeared to rely upon the localization of the heat produced in 

 crushing the whole of a certain quantity of rock for fusing one 

 part of it, I attempted to show that such localization of the 

 heat was not possible, and that consequently, if crushing 

 rock could fuse it, all the rock so crushed ought to be fused; 

 and, inasmuch as for any thing that appeared to the contrary, 

 Mr. Mallet relied upon the result of his experiments for giving 

 the full amount of heat obtainable in that way, I concluded 

 that his argument required that the cubes experimented upon 

 ought to have been themselves fused. 



In his reply Mr. Mallet commences by suggesting that the 

 heat evolved by crushing rock in the depths of the earth's crust 

 may be much greater than what he inferred by calculation from 

 his experiments; for it must be borne in mind that the heat 

 was not measured, but calculated. However, I will begin by 

 commenting on his answer to my first objection (which he has 

 taken second), viz. that the heat produced by crushing could 

 not be localized ; for that it could be so is obviously essential 

 to the argument of the original memoir, in which he shows that 



* Communicated by the Author. 



t American Journal of Science, 3rd Series, vol. vii. no. 42, Art. L. 

 p. 535 ; Phil. Mag. for July 1874, p. 45. 



X Quart. Journ. Geol. Soc. vol. xxxi. p. 469. 



