102 Influence of the Earth's Structure on the Length of the Day, 



sion another is immediately deduced, which is of fundamental 

 importance in connexion with the question now under considera- 

 tion. It follows that the shell and nucleus must rotate together 

 very nearly as if one mass, for otherwise, according to a result 

 obtained by Mr. Hopkins*, the annual precession of the equi- 

 noxes would greatly differ from that which is observed. Hence 

 it follows that great friction and pressure must exist at the sur- 

 face of contact of the nucleus and shell, and probably also even 

 between the particles of the fluid. Such a state of things should 

 indeed be expected a priori from the highly crystalline structure, 

 and probably unequal surface of the shell, as well as the viscous 

 nature of the fused materials of the earth, so far as we can judge 

 from those coming under our notice f. The existence of great 

 pressure has been already indicated in connexion with the gradual 

 tendency of the nucleus to expand its volume. 



While the strata of equal density in the shell would thus in- 

 crease in oblateness from the outer to the inner surface, the 

 strata of equal pressure in the nucleus would always follow the 

 opposite law, of decreasing in oblateness from the surface to the 

 centre. I have shown that this result, combined with that which 

 has been just mentioned, would lead to another directly con- 

 nected with the question of the earth's rotation, namely that the 

 difference between the greatest and least moments of inertia of 

 the earth continually tends to increase during the process of 

 solidification, and consequently that the stability of the earth's 

 axis of rotation, so far from being disturbed by that process, is 

 increased during its successive stages. This point was further 

 developed in a letter to Sir John Lubbock J, replying to some 

 communications with which he had favoured me with reference to 

 a short paper he had inserted in the Journal of the Geological 

 Society of London, wherein he had endeavoured to show the 

 possibility of a change in the position of the earth's axis of rota- 

 tion from the effect of physical changes in its structure. 



The increase of the difference between the greatest and least 

 moments of inertia of the earth would be chiefly due to an abso- 

 lute increase of the former. As this corresponds to the present 

 axis of rotation, it follows, that not only would the stability of 

 the earth's axis of rotation be more completely assured, but also 

 that its velocity of rotation would be diminished. This con- 

 clusion, combined with that already arrived at from a general 

 consideration of the influence of the process of solidification, 

 seems to establish the existence of a tendency to increase the 

 length of the day. At the same time the slow cubical contrac- 



* Phil. Trans. 1840, p. 207 ; see also Phil. Trans. 1851, p. 546. 

 t See Berzelius in PoggendorfF's Annalen, vol. xliii. 

 i Proceedings of the R^yal Society, February 1852. 



