244 Prof. H. Hennessy on the 



Let the outer strata of equal density be successively removed, 

 so as to leave a succession of free fluid surfaces, until a spheroid 

 is reached in which the difference of density is insensible. It 

 is manifest that, with each successive removal of the upper 

 stratum of liquid, the compression in the remaining strata be- 

 comes reduced, and also the variation in density from surface 

 to centre, until this variation becomes altogether extinguished. 

 With the same velocity of rotation, the ellipticities of the sur- 

 faces of liquid thus successively exposed would increase up to 

 the limiting value, J m. 



If at any time of the Earth's solidification we suppose 

 a nucleus of fluid to be enclosed within the solid shell, 

 the successive increasing of thickness of the shell, from the 

 congelation of the fluid matter of the nucleus, must be accom- 

 panied by the removal of successive outer strata from the 

 nucleus. From what has been seen already, the nucleus will 

 tend to acquire an increase of ellipticity, and therefore to mould 

 the semifluid pasty matter about to pass into a solid state into 

 a shape different from what it would have if no change what- 

 ever in the positions of the particles had taken place. As the 

 nucleus is supposed to be in a state of fusion from heat, the 

 successive additions to the inner surface of the shell from the 

 matter of the nucleus must proceed at a very slow rate. The 

 congelation of the surface-stratum of the nucleus must be a 

 process of the same order of slowness as the flow of heat 

 through the shell; and the mathematical theory of conduc- 

 tion established by Fourier shows that this cannot proceed 

 otherwise than slowly. The changes in shape of the surface 

 of the nucleus would be correspondingly slow and gradual. 

 When once a comparatively rigid outer crust had been formed, 

 the process of moulding additional strata of solidified matter 

 against the inner surface of the crust from the nucleus would 

 proceed in a slow and gradual order, so that the resulting 

 solid strata would conform to the shape impressed upon them 

 by the moulding forces. A remarkable illustration of the way 

 in which fused matter ejected from the Earth's interior may, 

 while turning on its centre and at the same time cooling, 

 mould itself against a solid crust formed upon it has been ad- 

 duced by Charles Darwin, and has been already quoted by me 

 on a former occasion. From these considerations I have been 

 led to conclude, that the ellipticity of the shell's inner surface 

 may exceed, but cannot be less than, the ellipticity of its outer 

 surface*; and, referring to the same question, Plana used the 

 words, " La loi des ellipticites a subi dans le passage de l'^tat 



* See the subjoined representation of a section of the shell and 

 nucleus. 



