1884,] On the Evidence that the Earth’s Interior is Solid. 589 
tion in a similar manner to the portions first cooled. As each 
cooled portion of the fluid descends, the three following causes 
= would therefore impede its descent : 
_ Ist. From the arrangement already indicated of the denser 
-strata about the center of the mass, and from the nature of the 
law of density of the strata, each stratum into which the cooled 
fluid would descènd, would be denser than the preceding. : 
2d. If the general effect of the refrigeration be to increase the 
density of the cooled matter, each stratum would have its den- 
sity augmented by the passage through it of the cooler matter 
above 
3d. The descending portions will have their densities dimin- 
ished by the increase in their temperature. 
- With an earth thus constituted for his starting point, Hennessy 
concluded that its crust could not be less than eighteen miles 
oF more than 600 miles. He further claimed that owing 
to friction, viscidity, and pressure’ it was probable that the fluid 
_ interior and the solid exterior would rotate nearly as if the earth 
_ "as solid from its center, thus denying the validity of Hopkins’ 
Conclusions, 
“ Professor Samuel Haughton, starting from the assumed prem- 
ni that the earth was “ composed of a solid shell, having the 
density of the rocks at the surface and a fluid homogeneous 
: mi? also having a fluid heterogeneous one, deduced 768 
a e the probable thickness in both cases ; 
The conclusions of Hopkins were sustained by Sir William 
He starts with the hypothesis that the earth is com- 
Posed of a thin shell which passes abruptly into a homogeneous 
‘ie ible fluid, mobile like water, which forms the interior. 
also assumes that this liquid interior is heterogeneous and 
“a Taking for his Starting points globes suchas these, he con- 
‘ti that the earth is more rigid than a homogeneous globe of 
26 4nd probably than one of steel. : | 
tanh = further claimed that it was demonstrable that the 
ù x solidified from the center, because melted rock contracted 
soon. we? and that hence the crust would sink to the center 
Soon as formed2 
> 1851, XXII, 251-273. 
1864, XXIII, 157-169; Phil. Mag., 1863 (4), XXV, I-14, 
3» PP- 573-582; Thomson and Tait’s Natural Philosophy, 
Geol. Soc. Glas., 1878, v1, 38-49. 
