ADDRESS SECTION A, B.A. 1876. 247 



periodic speed of the disturbance just too great 

 to allow the motion of the whole to be sensibly 

 that of a rigid body, the deviation first sensible 

 renders the precessional or nutational motion of 

 the shell smaller than if the whole were rigid, 

 instead of greater, as I expected. The amount 

 of this difference bears the same proportion to 

 the actual precession or nutation as the fraction 

 measuring the periodic speed of the disturbance 

 (in terms of the period of rotation as unity) bears 

 to the fraction measuring the interior ellipticity 

 of the shell ; and it is remarkable that this result 

 is independent of the thickness of the shell, as- 

 sumed, however, to be small in proportion to the 

 earth's radius. Thus in the case of precession 

 the effect of interior liquidity would be to dim- 

 inish the precessional motion, that is to say the 

 periodic speed of the precession, in the propor- 

 tion stated ; in other words, it would add to the 

 precessional period a number of days equal to 

 the number whose reciprocal measures the ellip- 

 ticity. Thus, in the actual case of the earth, if we 

 still take -^^ as the ellipticity of the inner bound- 



