.144 rOPULAR LECTURES AND ADDRESSES. 



arc spaces in volcanic regions occupied by liquid 

 lava ; but whatever portion of the whole mass is 

 liquid, whether the waters of the ocean, or melted 

 matter in the interior, these portions arc small in 

 comparison with the whole ; and we must utterly 

 reject any geological hypothesis which, whether 

 for explaining underground heat or ancient up- 

 heavals and subsidences of the solid crust, or 

 earthquakes, or existing volcanoes, assumes the 

 solid earth to be a shell of 30, or 100, or 500, or 

 i,OOO kilometres thickness, resting on an interior 

 liquid mass. 



If the inner boundary of the imagined rigid 

 shell of the earth were rigorously spherical, the 

 interior liquid could experience no processional 

 <>r mitational influence from the pressure on its 

 hounding surface, and therefore if homogeneous 

 rould have no precession or nutation at all, or if 

 heterogeneous only as much precession and nuta- 

 tion as would be produced by attraction from 

 without in virtue of non-sphericity of its surfaces 

 of equal density, and therefore the shell would 

 have enormously more rapid precession and nuta- 



