504- Geological Society : — Anniversary o/" 1839. 



from that which the solid spheroid woukl give ? What if it ap- 

 peared that the precession and nutation thus calculated for a fluid 

 interior agreed better with observation than the result hitherto ob- 

 tained by supposing the earth solid ? If this were so, we should 

 have evidence of the earth's interior fluiditj', evidence, too, of a per- 

 fectly novel and most striking natui'e. But to answer these ques- 

 tions is far from an easy task ; the precession of the solid earth is a 

 problem in which Newton erred, and in which the greatest mathe- 

 maticians of modern times have not found their greatest strength 

 superfluous. Yet how incomparably more difficult in all cases is the 

 mechanics of fluid than of solid bodies ! It may, therefore, require 

 more than one trial before any satisfactory solution of the problem 

 can be obtained. Mr. Hopkins has attacked it by the aid of cer- 

 tain hypotheses, and the result is, so far, not favourable to the de- 

 cisiveness of this test of the interior condition of the earth ; but not- 

 withstanding this state of things, I venture to say on your behalf. 

 Gentlemen, that an idea so full of promise of that which we so 

 much desire, and which seems to be so utterly out of our reach, 

 the knowledge of tiie condition of the centre of the earth, — that 

 such an idea is not to be lightly abandoned*. 



IM. Necker, of Geneva, offered an addition to the causes of con- 

 vulsions of the earth, which are contemplated by our Geological 

 Dynamics, in a paper in Avhich he ascribed the earthquakes which 

 took place in the southern provinces of Spain, in 1829, to the falling in 

 of strata, the subjacent gypseous and saliferous masses being washed 

 out by subterraneous currentsf. Without denying all influence to 

 such a cause, we may observe that it does not appear likely that 



* The following are the results at which Mr. Hopkins has avrived, sup- 

 posing the earth to consist of a homogeneous spheroidal shell filled with 

 a fluid mass of the same density as the shell : — 



1. The precession will be the same, whatever be the thickness of the 

 shell, as if the whole earth were solid. 



2. The lunar nutation will be the same as for the solid spheroid, to such 

 a degree of approximation, that the difference would be inappreciable to ob- 

 servation. 



3. The solar nutation will be sensibly the same as for the solid spheroid ; 

 unless the thickness of the shell be very nearly of a certain value, some- 

 thing less than one fourth the earth's radius, in which case this nutation 

 might become much greater than for the solid spheroid. 



4. In addition to the above motions of precession and nutation, the pole 

 of the earth would have a small circular motion, depending entirely on the 

 internal fluidity. The radius of the circle thus described would be the 

 greatest when the thickness of the shell should be least ; but the inequality 

 thus produced would not, for the smallest tliickness of the shell, ex- 

 ceed a quantitv of the same order as the solar nutation ; and for any but 



the most inconsiderable thickness of the shell, would be entirely inappre- 

 ciable to observation. 



Mr. Hopkins intends hereafter to consider the case of variable density. 



[See our present volimie, p. 3fi-l. — Edit.] 



[t An abstract of M. Nccker's paper has appeared in llie present volume, 

 p. .370.— Edit.] 



