186 Mr Whewell's Address to the 



have not found their greatest strength superfluous. Yet how incompara- 

 bly more diflicult in all cases is the mechanics of fluid than of solid bo- 

 dies ! It maj', therefore, require more than one trial before any satisfac- 

 tory solution of the problem can be obtained. Mr Hopkins has attacked 

 it by the aid of certain hypotheses, and the result is, so far, not favour- 

 able to the decisiveness of this test of the interior condition of the earth ; 

 but, notwithstanding this state of things, I venture to say on j"our behalf, 

 gentlemen, that an idea so full of promise of that which we so much de- 

 sire, and which seems to be so utterly out of our reach, the knowledge 

 of the condition of the centre of the earth, — that such an idea is not to be 

 slightly abandoned.* 



M. Necker, of Geneva, offered an addition to the causes of convulsions 

 of the earth, which are contemplated by our Geological Dj-namics, in a 

 paper in which he ascribed the earthquakes which took place in the 

 southern provinces of Spain, in 1829, to the falling in of strata, the sub- 

 jacent gypseous and saliferous masses being washed out by subterraneous 

 currents. Without denying all influence to such a cause, we may ob- 

 serve, that it does not appear likely that there would be thus produced, 

 simultaneously, any greater efltcts than those which are known to have 

 occurred from the falling in of unsupported mines ; and these have never 

 approached in their scale to any except the smallest earthquakes. 



AVhile geologists are thus looking in all directions for causes which 

 may produce the phenomenon which they study, it is natural that the 

 powerful, but as yet mysterious, influences of electricity should draw 

 their attention. Mr Robert Were Fox has endeavoured to shew, that 

 by voltaic agency, a laminated structure, and deposits of metals in cracks, 

 resembling metallic veins, may be produced in masses of clay. The ex- 

 periments are of an interesting kind, and it can hardly be doubted that 



• The following are the results at which Mr Hopkins has arrived, supposing 

 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 

 decree 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; un- 

 less the thickness of the shell be very nearly of a certain value, something 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 inter, 

 nal 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 thickness of the shell, exceed a quantity of the same 

 order as the solar nutation ; and for any but the most inconsiderable thickness 

 of the shell, would be entirely inappreciable to observation. 



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



