11 
“ These conclusions, drawn from a consideration of the 
necessary order of cooling and consolidation, according to 
Bischoff’s results on the relative specific gravity of solid and 
melted rocks, are in perfect accordance with §§ 832 — 849, 
on the present condition of the earth’s interior ; that it is not, 
as commonly supposed, all liquid within a solid crust from 
thirty to one hundred miles thick, but is, on the whole, more 
rigid than a solid globe of glass of the same diameter, and 
probably than one of steel.” 
The investigation here alluded to seems to me decisive 
against the doctrine of the earth’s central fluidity, and carries 
to a further point the conclusion of Mr. Hopkins, thirty years 
ago, from the phenomena of nutation and precession. It 
accords with my own inference from an hypothesis wholly 
distinct. But while I think that Sir William has disproved 
the notion of the central fluidity of the earth, and justly 
rejects the notion of geological uniformity for many hundred 
millions of years, I wholly dispute the soundness of his doc- 
trine, that the date of the formation of the crust can be defined 
by “ Fourier’s Theorems ” on conducted heat, or that the 
waste of solar heat is in constant excess over the fresh supply. 
In fact, the doctrine of uniformity would be equally untrue, 
whether the light and heat of the sun have increased or 
diminished sensibly in the course of a million years. 
IV. — The Translation Theory. 
18. Another view has been suggested by Poisson, to account 
for past changes in the earth’s climate, and warm and glacial 
periods, — the earth’s translation through hotter and colder 
regions of space. This does not need to detain us long, as 
there seem to be very simple and decisive reasons against it. 
Mr. Croll has thus given them briefly and clearly in a few 
words. 
“ This is not .a very satisfactory hypothesis. . . . Space is not a substance 
which can possibly be either hot or cold. If we adopt this hypothesis, we 
must assume that the earth, during hot periods, was in the vicinity of some 
other great source of heat and light beside the sun. But the proximity of a 
mass of such magnitude as would be able to affect to any great extent the 
earth’s climate, would, by its gravity, seriously disarrange the mechanism of 
the solar system. If it had ever, in a former period, come into the vicinity 
of such a mass, the orbits of the planets ought to afford evidence of it. But 
again, to account for a cold period, like the glacial epochs, we must 
assume the earth to have come near a cold body. And recent discoveries 
with regard to interglacial periods are wholly irreconcilable with this 
theory.” 
