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III. Mallet's Volcanic Theory "tested" by the Rev. 0. 

 Fisher. By Robert Mallet, F.R.S* 



IN July and August last the Rev. 0. Fisher, in asking me 

 for some explanations in reference to my paper " Rock- 

 crushing and its Consequences " (Phil. Mag. for July 1875), 

 with which I supplied him, informed me that he was preparing 

 a further paper, in which he proposed to test my theoryjof the 

 origin of volcanic heat and energy by some application of ma- 

 thematical reasoning. I venture to subjoin an extract from 

 my letter in reply addressed to him on the 30th of August 

 last : — " I have no wish to dissuade you from any criticism of 

 my volcanic views which may occur to you as important to 

 make ; but at the same time allow me to remark that those 

 views of mine are greatly more dependent upon a large num- 

 ber of physical considerations than upon any mathematical 

 ones, and I do not think that symbols or arithmetic are likely 

 to throw any additional light upon the subject, however they 

 may tend to confuse it. I replied to Mr. Hilgard's and your 

 own objections because it seemed necessary that I should 

 fill up a lacuna purposely left in my original paper, and not 

 in the spirit of controversy ; nor do I wish or intend to engage 

 in any further controversy as to any objections that may be 

 made to any of my views. 



" Bacon has, I think, somewhere said that, with respect to 

 large and complex questions, it is best to lay aside instant 

 discussion and allow time and rumination to wisen us upon 

 the subject. Time and the advance of science in the future 

 will no doubt afford surer tests of the truth or falsehood of my 

 views than we now possess; but in the existing state of terres- 

 trial physics, partial objections, even if well founded, seem to 

 me of little value or use." 



Mathematical reasoning is an admirable and potent instru- 

 ment for the discovery of truth when the data upon which it 

 is founded are exact, sufficient, and such as we are sure exist 

 in nature ; but all its validity depends upon these data. 



We know almost nothing as to the nature of the interior of our 

 globe; and it is only in a very imperfect way that we can even 

 imagine the conditions, highly complex as these undoubtedly 

 must be, under which mechanical strains act upon it even 

 within a few miles of the surface, where we may reasonably 

 infer its materials and their arrangement to be highly compli- 

 cated, and differing from point to point in their chemical and 



* Communicated by the Author. 

 C2 



