REMARKS UPON MR. MALLET^ THEORY OF VOLCANIC ENERGY. 469 



34. Remarks upon Mr. Mallet's Theory of Volcanic Energy. 

 By the llev. 0. Fisher, M.A., F.G.S. (Read May 12, 1875.) 



The theory of Mr. Mallet, F.R.S., contained in his now celebrated 

 paper which was read before the Royal Society in 1872, has already 

 been much discussed ; but there seems to be still room for the fol- 

 lowing remarks, which I hope I shall not be thought presumptuous 

 in laying before you. 



The treatise is of such considerable dimensions that the simplest 

 plan will be to take the sections of it in order as they occur, passing- 

 over those which approve themselves, and discussing only those which 

 appear to be open to question. 



There is nothing contained in the first eleven sections but what 

 entirely commends itself to my judgment. Indeed some of the 

 remarks of the distinguished author were anticipated in a paper of 

 mine read at Cambridge in 1868, of which Mr. Mallet had no 

 knowledge at the time when his theory was given to the world. 



In § 12 the opinion that the crust of the earth rests on a liquid 

 nucleus is mentioned, but not with favour. Whatever its condition 

 may be at a very great depth, the view which at present seems 

 probable to me is, that there is, at any rate, a substratum in a state 

 of igneous fusion beneath the crust. 



In § 15 Mr. Hopkins's investigation upon precession, according to 

 which he believed himself to have shown that the crust of the earth 

 is at present from 800 to 1000 miles thick, is referred to, but not 

 with approval. 



General Barnard, of the United-States Army, has satisfied himself 

 that he has proved that Hopkins's investigation was vitiated by an 

 oversight, and that no reliance can be placed upon his result*. 

 There remain, however, the investigations of Sir. W. Thomson f, 

 derived from considerations regarding the production of tides in the 

 interior of the earth. These investigations are, I conclude, irrefra- 

 gable as far as the mathematical part of the investigation is concerned. 

 But there are some points in his argument on which something may 

 nevertheless be said in favour of a liquid substratum beneath the 

 crust of our globe. 



The idea of a tide, which we derive from the flux and reflux of 

 the sea upon our coasts, causes the popular conception of a tide to 

 be different from the true one with which we have to deal in the 

 case before us. What we have to imagine is, that the earth, which 

 we may for the moment conceive to be a sphere, is drawn out into a 

 prolate spheroid towards the attracting body, be it moon or sun. 



Sir W. Thomson's argument, in the main, is this f. Unless the 

 earth is immensely rigid, it will be sensibly drawn out in the manner 



* " Problems of Eotary Motion." Smithsonian Contributions, No. 240, New 

 Addendum, p. 42. 

 t Phil. Trans. 1864. 

 } " On the Rigidity of the Earth," Phil. Trans. 1864. 



