24' Prof, lleunessy on the Tkickncss ami 



fused to the solid condition. These results have been pub- 

 lished in some of the most widely circulating scientific collections 

 of Europe. Some of them have been referred to by Alexander 

 von Humboldt* and by Sir Charles Lyellf; and all have been 

 quoted by M. Elic de Beaumont in his Systemes de MontagnesX, 

 as well as by other well-known investigators. In venturing to 

 base a great portion of ray reasonings upon such results, I felt 

 that in the present state of our knowledge no facts could be 

 quoted which could outweigh those deduced from the experi- 

 ments of such careful observers, and that it would therefore be 

 impossible for me to overlook the very definite conclusions to 

 which these gentlemen have been independently led. 



The foregoing remarks will in some measure answer Arch- 

 deacon Pratt's objection to my views regarding the superior con- 

 traction of the fluid matter composing the nucleus compared to 

 that of the solid shell. The general laws of contraction of fluids 

 and solids for similar changes of temperature are admitted to be 

 such as to give a greater contraction to the former than to the lat- 

 ter. On this account Humboldt and ]M. Elie de Beaumont have 

 gone still further, by maintaining the possibility of a tendency to 

 separation between the shell and nucleus, a conclusion from which 

 I have dissented, in accordance with some of the I'esults of 

 Mr. Hopkins combined with some of my own. While I hope 

 that these remarks will satisfy Archdeacon Pratt as to the legi- 

 timacy of the assumptions which he has called in question, I 

 trust that Professor Haughton§ will see that there are some 

 students of nature who not only hold the opinion that the crust 

 of the earth is denser than the fluid from which it was formed, 

 but who have based that opinion upon the results of exact expe- 

 rimental inquiry. 



The results obtained by M. Plana and myself, show that Mr. 

 Hopkins's formula, quoted by Ax-chdeacon Pratt, must be per- 

 fectly nugatory for the determination of the thickness of the 

 earth's crust. The shape in which it is finally presented by its 

 author is 



P'-P,= 



-<-y(-^)'" 



where P, denotes the precession of a solid homogeneous spheroid 

 of which the cllipticity = ep and P' the precession of the earth 

 supposed to consist of a solid shell enclosing a fluid heterogene- 



* Kosinos vol. i. section on Volcanos. 

 t Principles of Geology, 9th ed. p. 173. 

 X p. 1230-.31. 

 § See Phil. Mag. for June. 



