ADDRESSSECTION A, B.A. 1876. 251 



logical hypothesis of a thin rigid shell full of 

 liquid. 



But interesting in a dynamical point of view as 

 Hopkins's problem is, it cannot afford a decisive 

 argument against the earth's interior liquidity. It 

 assumes the crust to be perfectly stiff and unyield- 

 ing in its figure. This, of course, it cannot be 

 because no material is infinitely rigid ; but, com- 

 posed of rock and possibly of continuous metal in 

 the great depths, may the crust not, as a whole, be 

 stiff enough to practically fulfil the condition of 

 unyieldingness ? No, decidedly it could not ; on 

 the contrary, were it of continuous steel and 500 

 kilometres thick, it would yield very nearly as 

 much as if it were india-rubber to the deforming 

 influences of centrifugal force and of the sun's and 

 moon's attractions. Now although the full problem 

 of precession and nutation, and, what is now 

 necessarily included in it, tides, in a continuous 

 revolving liquid spheroid, whether homogeneous or 

 heterogeneous, has not yet been completely worked 

 .out, I think I see far enough towards a complete 

 solution to say that precession and nutations will 



