34 On the Constitution and Construction of the 



force. Does this fluid matter compose a large or a small part 

 of the entire mass ? Is it situated near the surface, or at a 

 vast depth \ The disturbing action of the moon will not be 

 the same upon a globe all solid, and upon one nearly all fluid ; 

 will not be the same upon a globe in which the solid shell 

 forms one-half of the mass, and another in which it forms only 

 one-tenth. 



These statements will convey a general idea of the condi- 

 tions of the problem which Mr Hopkins had to solve ; and he 

 seems to have been careful to examine it under all its various 

 forms. 



The conclusion to which his researches have conducted him 

 is thus announced : — " Upon the whole, then, we may venture 

 to assert, that the minimum thickness of the crust of the globe, 

 which can he deemed consistent with the observed amount of 

 precession, cannot be less than one-fourth or one-fifth of the 

 earth^s radius^ That is from 800 to 1000 miles. 



Let it be observed that this is the minimum thickness con- 

 sistent with the known precession. The actual thickness may 

 be much greater. The globe may even be solid to the centre, 

 and this, too, without very materially altering the conditions 

 of the problem ; for if the shell is 1000 miles thick, it consti- 

 tutes /<9w/*-5e?;e^i^5 of the bulk of the globe ; and though the 

 remaining three-sevenths may have a higher density, the action 

 of disturbing forces upon them from without is greatly lessened 

 by their central position. Two other elements are yet want- 

 ing to give us complete information on the points in question. 

 These are the eff'ect of pressure in producing condensation in 

 the matter of the globe, and the eff^ect of heat in resisting it. 

 Professor Leslie made some experiments to ascertain the rela- 

 tion of density to pressure, from which he inferred, that at the 

 depth of 400 miles, or one-tenth of the semi-diameter, marble 

 would have its density increased nearly one-half, while water 

 would have its density more than quadrupled, and would, in 

 fact, be heavier than marble. He hence concluded, that the 

 density of the globe must increase so rapidly in the interior, 

 that if it consisted either of solid or fused matter to the centre, 

 the mean density would greatly exceed five and a half times 

 the weight of water ; and, therefore, that it must consist of a 

 hollow shell, the cavity of which is probably filled with some 



