ON THE PHYSICAL STRUCTURE OF THE EARTH.* 



By Heney Hennessy, F. R. S. 



The structure of the Earth, as a mechauical and physical question, is 

 closely connected with the origin and formation of its satellite, and of 

 the planets and satellites belonging to the same solar system. The 

 brilliant results obtained during the present and preceding century by 

 the aid of mathematical analysis, whereby the motions of those bodies 

 have been brought within the grasp of dynamical laws may have led 

 to the notion that by similar methods many obscure problems relating 

 to the planet we inhabit might be accurately solved. But although 

 the general configuratiou of the Earth and planets has been treated 

 mathematically, with results which leave little to be desired, when 

 applications of analytical methods are attempted to questions of de- 

 tail in terrestrial structure, the complication of the conditions is so 

 great as to impose the necessity on some investigators of so altering 

 these conditions as to maiie their results perfectly inapplicable to the 

 real state of the Earth. Physical geology presents problems the solu- 

 tion of which undoubtedly calls for mechanical and physical considera- 

 tions ; but these may in general, under the complex nature of the 

 l)henomena, be often better reasoned out without the employment of 

 the symbolical methods of analysis. In most cases the conditions are 

 totally unlike those above alluded to, which admit of precise numerical 

 computations. The heterogeneous character of the rocks composing 

 the Earth's crust, and the probably varied nature of the matter compos- 

 ing its interior, render mathematical applications rarely possible, and 

 sometimes misleading Such views seem to be gradually gaining 

 strength among geologists who pay attention to questions of a general 

 nature, and no one has beeter expressed them in recent times than 

 Prof. M. E. Wadsworth.t 



The principle upon which 1 have ventured to found all my researches 

 on terrestrial physics is this : to reason on the matter composing the 

 globe from our knowledge of the physical and mechanical properties 



* From the L. E. D. Philosophical Magazine, September aud October, 1886, vol. 

 XXII, pp. 233-251 and 328-331. 



t " Litbologlciil Studies." Memoirs of Harvard College Museum, vol. i, p. 3, and 

 American Naturalist, June, 1884, p. 587. 



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