142 . Mr, Hopkins on the Density of the Earth, Sfc, [May 13, 



with the view of ascertaining how far the observed amount of preces- 

 sion might be consistent with the existence of a fluid nucleus. The 

 result was, that such could only be the case provided the thickness of 

 the solid shell were much greater than that which, as above stated, has 

 been supposed by many geologists. The numerical result was, that 

 the least admissible thickness of the crust must be about one-fifth of 

 the earth's radius ; but without assigning any great importance to an 

 exact numerical result, Mr. Hopkins had a full confidence in the 

 investigation, as showing that the thickness of the crust could not be so 

 small as 200 or 300 miles, and consequently that no geological theory 

 can be admitted which rests on the hypothesis of the crust being nearly 

 as thin as it has been frequently assumed to be. 



The influence of the interior fluidity on the precessional motion 

 above described, is due to the difierence between the motions which the 

 attractions of the sun and moon tend to produce on a solid mass in one 

 case, and a fluid mass on the other. It has been recently stated, as an 

 objection to this investigation, that the interior fluid mass of the earth 

 may move in the same manner as if it were solid. The only reply 

 which could be given to such an objection was, Mr. Hopkins conceived, 

 that it was mechanically impossible that these motions should be the 

 same, though the resulting precessional motion for the solid crust, 

 under certain conditions, to be determined only by the complete mathe- 

 matical solution of the problem, might be the same as if the whole mass 

 were solid. The effect of the attractions of the sun and moon also 

 depends on the ellipticity of the inner surface of the solid shell ; and it 

 has been said that since that ellipticity depends on the law of the 

 earth's density, which can only be imperfectly known, no result can be 

 depended on which involves that ellipticity. This was not a correct 

 statement of the problem. It was assumed in the solution referred to, 

 that the ellipticity of the inner surface would depend partly on the law 

 of density, and partly on the forms of the isothermal surfaces. Mr. 

 Hopkins had supposed it possible, at the time he was engaged in this 

 investigation, that a surface of equal solidity might approximate to a 

 surface of equal pressure ; he has now experimental reasons for believ- 

 ing that it must approximate much more nearly to an inte^pal surface 

 of equul temperature. Now for depths greater, probably much greater, 

 than those which have often been supposed to correspond to the thick- 

 ness of the earth's solid crust, there is no doubt that the internal iso- 

 thermal surfaces have a greater ellipticity than the external surface 

 itself; a conclusion which is independent of the law of density. 

 Hence, a like conclusion will hold with reference to the internal 

 surface of the shell, if it approximate sufficiently to the surface, of 

 equal temperature ; and this is the conclusion most unfavourable to the 

 thin shell supposed by some geologists. Restricting the interpretation, 

 then, of Mr. Hopkins's results to the question, whether the earth's 

 solid shell be as thin as some geologists have supposed, or at least 

 several hundred miles in thickness? (and this is the only question of 

 geological importance) —Mr. Hopkins denied the validity of either of 

 the objections above stated. 



