the Crust of the Earth. 44:7 
tom. v. p-48). Mr. Hopkins and Archdeacon Pratt haye adopted 
this law, although, as it appears to me, it is utterly inconsistent 
with the little we do know of the interior of the globe. 
It is in the highest degree probable that the specific gravities 
of the successive layers of the globe depend almost altogether 
on their chemical composition, which is very varied, and only in 
a very slight degree on the pressure to which they are subject ; 
and that, consequently, a theory like that of Laplace, which 
supposes the chemical composition uniform, and the density to 
depend on an assumed law of compressibility, must be rejected, 
as a matter of course, by every mathematician who wishes to have 
a positive basis of fact for his speculations. For this reason, [ 
believe the charge which Archdeacon Pratt has brought against 
me, of having attempted “an algebraical, not a physical problem 
of densities,” might with more fairness be brought against his 
own unauthorized assumption of Laplace’s law, which he con- 
siders ‘in itself a very probable law.” 
From a consideration of the igneous rocks of various ages of 
the crust of the earth, many geologists have come to the conelu- 
sion that the two outer layers of that crust are composed of 
siliceo-felspathic rocks and ferro-calciferous eruptive rocks, having 
average specific gravities of 2°55 and 3:00. The difference in 
specific gravity of these layers is evidently due to the presence 
of iron in the latter, and has no relation whatever to the pressure 
to which they have been subjected. Such facts as this are com- 
pletely ignored by the merely mathematical assumption that the 
whole earth is composed of a homogeneous mass of fluid following 
a supposed law of compressibility. I believe, therefore, that I 
am entitled to deny, as a matter of fact, that we possess any 
positive knowledge of the interior of the earth; and I shall re- 
tain my conviction that such knowledge 1s beyond our reach, 
until it is acquired by some process more legitimate than un- 
founded hypotheses, which are contradicted by the few facts that 
actually do come under our observation. 
Before leaving this subject, it is worth while observing that 
Archdeacon Pratt’s logic is as peculiar as his mathematics ; for 
while he supposes that he has disposed of my sceptical argument 
by the detection of a supposed fallacy in my mathematics, he 
omits to perceive that, if I were really guilty of the fallacy, it 
would strengthen my argument against our positive knowledge 
of the interior of the globe, as it is plaim that if I am not en- 
titled to use the equation (B) to establish a relation between the 
ellipticity and density of the fluid portion of the globe, I must 
make an additional hypothesis, and therefore be forced to 
discuss equations involving four unknown quantities instead of 
three. 
