ON TOE THYSICAL STRUCTURE OF THE EARTH.* 



By Henry Hennessy, E. R. S. 



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

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

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

 brilliant results obtained during the present and i)rcceding ceutury by 

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

 'have beeu brought within the grasp of dynamical laws may have led 

 to the notion that by similar methods mauy obscure problems relating 

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

 the general configuratiou of the Earth aud 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 make 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 iu general, under the complex nature of the 

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

 the symbolical methods of analysis. Iu 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 uature 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 attentiou to<iuestions of a general 

 uature, aud no one has beeter expressed them in recent times than 

 Prof. M. E. Wadsworth.t 



The principle upon which I 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 and October, 188(i, vol. 

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



t " Litholo<;ical Studies." Memoirs of Harvard College Museum, vol. I, p. 3, and 

 Americati Nalurulist, June, 1HH4, p. 587. 



201 



