250 POPULAR LECTURES AND ADDRESSES. 



the earth solid throughout. Now this amount, as 

 calculated in the Nautical Almanac, makes o"'55 

 and o"'5i the semiaxes of the ellipse traced by 

 the earth's axis round its mean position ; and if 

 the true nutation placed the earth's axis on the 

 opposite side of an ellipse having o //< 86 and o"'8i 

 for its semiaxes, the discrepance could not pos- 

 sibly have escaped detection. But, lastly, think 

 of the lunar fortnightly nutation. Its period is 

 .,',, of 300 days, and its amount, calculated in the 

 Nautical Almanac on the theory of complete 

 sol id it\-, is such that the greater scmiaxis of the 

 approximately circular ellipse described by the 

 pole is o"'O325. Were the crust infinitely thin 

 this nutation would be negative, but its amount 

 nineteen times that corresponding to solidity. 

 This would make the greater semiaxis of the 

 approximately circular ellipse described by the 

 pole amount to 19 X o"'O<S<S5, which is \""j. It 

 would be negative and of some amount between 

 I "7 and infinity, if the thickness of the crust wen; 

 anything from y.cro to I2O kilometres. This con- 

 ion is absolutely decisive against the geo- 



