234 Prof. H. Hennessy on the 



rical computations. The heterogeneous character of the rocks 

 composing the Earth's crust, and the probably varied nature 

 of the matter composing 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 better expressed them in recent times than Professor 

 M. E. Wadsworth*. 



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 phy- 

 sical and mechanical properties of its materials which come 

 under our notice. Of these properties the most important 

 are density, compressibility, contraction or dilatation from 

 changes of temperature. Newton and other philosophers 

 have already adopted the same principle to a limited extent, 

 when assuming for the mass of iluid composing the Earth in 

 its primitive condition those specific properties which have 

 been assigned to all kinds of fluids observed at the surface. 

 It is impossible to frame any statement more erroneous and 

 misleading than that I have endeavoured to render the ques- 

 tion more hypothetical than it was. On the contrary, I have 

 discarded the invariable assumption of mathematicians who 

 treated the question, namely, the hypothesis of the invaria- 

 bility of positions of the particles composing the solidifying 

 earth. The speculations of all rational inquirers, upon the 

 Earth's internal structure must necessarily start from the same 

 general principle as above. Some investigators have disre- 

 garded that principle, and made the problem thereby a purely 

 mathematical exercise. 



In order to reason upon the Earth's figure, we must assume 

 that the laws of fluid equilibrium apply to the inner portions 

 of the fluid as well as the outer. There is nothing hypothe- 

 tical in reasonings as to the formation of the solid shell and 

 the law of increase of ellipticity of its inner surface as a re- 

 sult of the transition of the formerly fluid matter to the state 

 of solidity. On the contrary, the assumptions of Mr. Hopkins 

 and other mathematicians, that this transition created no 

 change in the law of density of the matter composing the 

 Earth, and in the ellipticity of the strata of equal pressure, are 

 not merely hypothetical ; they are directly opposed to well 

 established physical and mechanical laws. 



On the other hand, those who have concluded that nothing 

 can be known of the form of the fluid nucleus, seem to deny 



* "Lithological Studies." Memoirs of Harvard College Museum, vol. i. 

 p. 3 ; and ' American Naturalist/ June 1884, p. 587, 



