POPULAR LECTURES AND ADDRESSES. 



ary of the supposed rigid shell, the effect would 

 be to augment by 300 days the preccssional period 

 '.600 years, or to diminish by about -g-V t- nc 

 annual precession of about 51", an effect which 

 I need not say would be wholly insensible. But 

 on theMunar nutation of i<S - 6 years period, the 

 effect of interior liquidity would be quite sensible : 

 i.S'6 years being twenty-three times 300 days, the 

 effect would be to diminish the axes of the ellipse 

 which the earth's pole describes in this period 

 each by J.. of its own amount. The sem faxes of 

 this ellipse, calculated on the theory of perfect 

 rigidity from the very accurately known amount 

 t>l" precession, and the fairly accurate knowledge 

 which we have of the ratio of the lunar to the 

 -olar part of the preccssional motion, arc (f'22 

 and 6"*86, with an uncertainty not amounting to 

 one-half per cent, on account of want of perfect 

 accuracy in the latter part of data. If the true 

 values were ch by J.. of its (>wn amount, 



the disc-vpancc might have escaped detection, or 

 might not have escaped detection ; but certainly it 

 could be found if looked for. So far nothing can 



