ADDRESS-SECTION A, B.A. 1876. 245 



tion than it actually has forty times as much, 

 for instance, if the thickness of the shell is 60 

 kilometres. A very slight deviation of the inner 

 surface of the shell from perfect sphericity would 

 suffice, in virtue of the quasi-rigidity due to vortex 

 motion, to hold back the shell from taking sen- 

 sibly more precession than it would give to the 

 liquid, and to cause the liquid (homogeneous or 

 heterogeneous) and the shell to have sensibly the 

 same precessional motion as if the whole con- 

 stituted one rigid body. But it is only because 

 of the very long period (26,000 years) of pre- 

 cession, in comparison with the period of rotation 

 (one day), that a very slight deviation from 

 sphericity would suffice to cause the whole to 

 move as if it were a rigid body. A little further 

 consideration showed me : 



(i) That an ellipticity of inner surface equal to 

 0(j000 1 x 3( .. would be too small, but that an ellip- 

 ticity of one or two hundred times this amount 

 would not be too small to compel approximate 

 equality of precession throughout liquid and 

 shell. 



