52 , -- @EOGNOSY, 

such astrain upon the substance of the globe, that 1t seems im the 
highest degree improbable that the planet could maintain its shape 
as it does unless the supposed crust were at least 2000 or 2500 miles 
in thickness. That the solid mass of the earth must yield to this 
strain is certain, though the amount of deformation is so slight as to 
have hitherto escaped all attempts to detect it.2 Had the rigidity 
been even that of glass or of steel, the deformation would probably 
have been by this time detected, and the actual phenomena of 
precession and nutation, as well as of the tides, would then have been - 
very sensibly diminished. The conclusion is thus reached that the 
mass of the earth “is on the whole more rigid certainly than a 
continuous solid globe of glass of the same diameter.” * 7 
(b.) Argument from the tides—The phenomena of the oceanic | 
tides are only explicable on the theory that the earth is either solid to 
the centre, or possesses so thick a crust (2500 miles or more) as to give 
to the planet practical solidity. Sir William Thomson remarks that 
“were the crust of continuous steel, and 500 kilometres thick, it 
would yield very nearly as much as if it were india-rubber to the 
deforming influences of centrifugal force, and of the sun’s and moon’s 
attractions.” It would yield, indeed, so freely to these attractions 
“that it would simply carry the waters of the ocean up and down 
with it, and there would be no sensible tidal rise and fall of water 
relatively to laud.” ° Mr. George H. Darwin in the series of papers 
already referred to, has investigated mathematically the bodily tides — 
of viscous and semi-elastic spheroids, and the character of the ocean 
tides on a yielding nucleus.° His results tend to increase the force 
of Sir William Thomson’s argument, since they show that “no very 
considerable portion of the interior of the earth can even distantly — 
approach the fluid condition,” the effective rigidity of the whole globe 
being very great. Y 
(c.) Argument from relative densities of melted and solid rock.— 
The two preceding arguments must be considered decisive against 
the hypothesis of a thin shell or crust covering a nucleus of molten 
matter. It has been further urged, as an objection to this hypothesis, 
that cold solid rock is necessarily more dense than hot melted rock, 
and that even if a thin crust were formed over the central molten 
globe it would immediately break up and the fragments would sink 
towards the centre.’ Undoubtedly this would happen were the 
material of the earth’s mass of the same density throughout. But, 
as has been already pointed out, the specific gravity of the interior 
is at least twice as much as that of the visible parts of the crust. If 
this difference be due, not merely to the effect of pressure, but to the — 
Thomson, Proc. Roy. Soc. April, 1862. | 
See Association Irangaise pour ? Avancement des Sciences, v. p, 281. 
Thomson, loc. cit, 
Thomson, Trans. Ioy. Soc, Edin. xxiii. p, 157. 
* Thomson, Brit. Assoc. Rep. 1876, Sections, p. 7. 
Phil. Trans. 1879, Part I. 
. This objection has been repeatedly urged by Sir William Thomson, Sco Trans. 
Rey. Soc. Edin. xxiii. p. 157; and Brit. Assoe. Ttep. 1870, Sections, p, 7. 
ke 
* &© & 
_ S&S 
