Earth's Internal Structure on the Length of the Day. 101 



not only the external form, but in several cases their internal 

 structure, shows that these bodies have revolved in their aerial 

 course. One of these bombs when broken through presented 

 the following* appearances : — 1. The central part was highly cel- 

 lular, the cells decreasing in size from the centre towards the 

 exterior. 2. These cells terminated at a shell-like case of com- 

 pact stone about a third of an inch in thickness. 3. Outside 

 this case was a crust of finely cellular lava. Mr. Darwin ex- 

 plains these phsenomena very rationally, by saying that the outer- 

 most crust cooled rapidly into the state in which it came under 

 his observation ; then the enclosed fluid was pressed against the 

 shell so formed by centrifugal force arising from the rapid rota- 

 tion of the mass, thus producing the compact stony casing; 

 lastly, the action of the same force relieving the pressure of the 

 fluid at its central portions, the expansive tendency of the in- 

 cluded elastic vapours would ultimately produce the coarse cel- 

 lular structure at the centre. It might be added^ that the pass- 

 age of the fluid into the solid state, when forming the compact 

 case of stone, being necessarily accompanied by contraction, 

 would allow some space for the operation of the expansion of 

 the remaining fluid. 



If the removal of matter from the interior towards the exterior 

 portions of the earth took place equally along every radius drawn 

 from the centre to the surface, all of the earth^s moments of 

 inertia would be augmented, and the length of the day, so far as 

 it could be affected by this cause, would of course be increased. 

 The contraction of the solidifying surface of the nucleus upon 

 the inner surface of the shell, in passing from the fluid to the 

 solid state, taking place from within outwards, its effect could 

 not be that suggested by M. Delesse*, in terminating his valu- 

 able remarks on the crystalline contraction of rocks, namely a 

 diminution of the earth^s radius, and a consequent increase in 

 the velocity of rotation, but precisely the reverse. 



On the old assumption of mathematical investigators, that the 

 particles composing the earth retained the same positions on 

 passing from the fluid to the solid state, the oblateness of the 

 strata of equal density of the solidified mass would correspond to 

 the surfaces of equal pressure in the fluid, and would be less and 

 less oblate in going from the outer to the inner surface of the 

 shell. By discarding this assumption, and admitting the influ- 

 ence of the physical changes that take place in the passage of 

 the fluid matter of the nucleus to the state of solidity, I have 

 shown that the process of soliditication already briefly described 

 would tend to augment the oblateness of the strata of the shell 

 in going from its outer to its inner surface. From this conclu- 

 * Comptes Rendus, vol. xxv. p. 545. 



