20 Mr. R. Mallet on his Volcanic Theory. 



physical natures ; of the material and its arrangement at much 

 greater depths we know absolutely nothing. 



The theories propounded of the descent of glaciers present 

 examples, now familiar to many, of mathematics misused, 

 because as yet our knowledge of the physical nature of ice is 

 so imperfect. This is still more true where mathematical cal- 

 culation is attempted to be applied under conditions such as 

 affect the interior of our globe (as compared with which the 

 motions of glaciers are simplicity itself), as is done by the Rev. 

 0. Fisher in his paper entitled " Mr. Mallet's Theory of Vol- 

 canic Energy tested," which appears in the Philosophical 

 Magazine for October last. 



I shall adhere to my resolution expressed in the above ex- 

 tract not to engage in any mathematical controversy in sup- 

 port of my views as to volcanic activity, but to leave it to the 

 issue of advancing knowledge, when in time to come more 

 extended and new forms of observation or experiment shall 

 have afforded more certain physical data than we at present 

 possess whereon to base our conclusions. In abstaining thus 

 it must not be supposed that I admit the validity of the Rev. 

 0. Fisher's conclusions, or that the pretentious title of his 

 paper is in any wise justified by them. Passing by the earlier 

 parts of his paper, as to which all that need be said may already 

 be found in my paper in the Philosophical Magazine for July, 

 the connected argument by which he professes to test my 

 views commences at page 309. The physical data upon which 

 it^is founded have no real or probable existence in nature, and 

 are in some instances in conflict with each other, while some 

 of the numerous hypotheses involved in his calculation are not 

 warranted by any thing set forth by me in my original paper 

 (Phil. Trans, for 1873). For his own purpose he takes my 

 small paper subsequently read (addition &c, read May 1874), 

 in which upon certain hypotheses and suppositions I assigned 

 limits of thickness to the earth's solid crust, as though it were 

 part of my original paper and resting upon an equally assured 

 base. 



The Rev. 0. Fisher assumes a rigid crust and rigid nucleus, 

 that these are in contact at a definable spherical surface, that 

 at this surface they adhere or stick together, and that when 

 this adhesion is broken by tangential forces originating in 

 contraction of the nucleus, the surfaces of contact drag over 

 each other with an enormous resistance, which he supposes at 

 the moment of rupture equal to the whole weight of the crust. 

 This fanciful coefficient of adhesion and friction (/u, and yJ in 

 the author's formulae) he professes to take from me ; but what 

 I have assumed for sake of illustration only at pages 8-9 



