ADDRESSSECTION A, B.A. 1876. 249 



be considered as absolutely proved with reference 

 to the interior solidity of the earth from pre- 

 cession and nutation ; but now think of the solar 

 semiannual and the lunar fortnightly nutations 

 The period of each of these is less than 300 days. 

 Now the hydrodynamical theory shows that, irre- 

 spectively of the thickness of the shell, the nuta- 

 tion of the crust would be zero if the period of 

 the nutational disturbance were 300 times the 

 period of rotation (the ellipticity being othj-) 5 if 

 the nutational period were anything between this 

 and a certain smaller critical value depending 

 on the thickness of the crust, the nutation would 

 be negative ; if the period were equal to this 

 second critical value, the nutation would be in- 

 finite ; and if the period were still less, the nuta- 

 tion would be again positive. Further, the 183 

 clays period of the solar nutation falls so little 

 short of the critical 300 days that the amount 

 of the nutation is not sensibly influenced by the 

 thickness of the crust, and is negative and equal 

 in absolute value to %\ (being the reciprocal of 

 i ! "~ times what the amount would be were 



