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LX. On the Thickness of the Crust of the Earth. By the Rev. 
Samvet Haveuton, F.R.S., Fellow of Trinity College, and 
Professor of Geology in the University of Dublin. 
To the Editors of the Philosophical Magazine and Journal. 
GENTLEMEN, 
[’ the April Number of the Philosophical Magazine, Arch- 
deacon Pratt replies to my communication published im 
December, 1859. 
So far as the controversy is personal between us, it turns on 
a very simple mathematical question, which I am quite willing 
to leave for the decision of mathematicians. I regret that my 
statements on this subject have not been sufficiently clear, though 
I endeavoured to make them so, and must therefore beg the 
favour of a few lines on the matter, before entering on the 
other question raised by the present controversy. 
I. The supposed “fallacy” in my reasoning. 
For the convenience of reference, I shall call the two equations 
in dispute (A) and (B). Archdeacon Pratt makes the following 
statements :— 
May, 1859. 
“ Equation (B) does not follow 
Srom equation (A) by differentiation. 
In fact equation (B) assumes that 
the law of density and ellipticity is 
continuous throughout the whole 
April, 1860. 
“ Professor Haughton... . re- 
plies to my reasoning by showing 
that he has differentiated equation 
(A) right. This I never called in 
question.” 
mass, solid and fluid, the solid 
parts lying in strata of the form 
and density they would have if they 
were wholly fluid.” 
Archdeacon Pratt now admits that equation (B) may be ob- 
tained from equation (A) by differentiation, but he has omitted 
to see that I expressly state that equation (B) can only be ap- 
plied to the fluid nucleus of the earth, and that I so apply it in 
order to diminish by one, the total number of unknown quantities, 
which must become known before the thickness of the earth’s 
crust can be determined. My words are,— 
[Equation B] “determines the relation which necessarily exists 
between the law of density and ellipticity of the fluid portions 
of the earth*.” 
To prevent further misconception, I shall here briefly repro-. 
duce my argument, intended to show that our speculations on 
the thickness of the earth’s crust, if it have a crust at all, are 
essentially hypothetical. 
If the earth have a solid crust, containing a liquid nucleus 
* Transactions of the Royal Irish Academy, vol. xxii. Science, p. 265. 
