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LXIV. On the Thickness of the Earth's Crust. By the Rev. 

 Samuel Haughton, F.R.S., Professor of Geology in the Uni- 

 versity of Dublin. 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 



IN your May Number, the Archdeacon of Calcutta has pub- 

 lished an interesting Paper " On the Thickness of the Crust 

 of the Earth,'^ on which I would wish to offer a few observations. 



In noticing my own investigations on this subject, he has 

 accused me of a fallacy in reasoning, and adds that consequently 

 " my conclusions do not prove anything whatever regarding the 

 proportion of the solid to the fluid parts." 



My object, in the paper commented on, was to show that no 

 investigations on this subject were of any value, inasmuch as 

 they all rested on arbitrary hypotheses, among which I include 



the law of density [ ^J, and all guesses as to the ratio of 



the specific weights of the solid crust and liquid nucleus, and 

 friction between them. At the close of my paper, I have given 

 an example of the mode of calculation to be employed, if we had 

 the requisite knowledge of the specific gravities and law of den- 

 sity of the crust and nucleus. As my object was merely to give 

 an example, I have chosen a sufficiently absurd hypothesis : viz. 

 specific gravity of outer shell 2"75, and of whole earth 5"50, and 

 the whole composed of two homogeneous parts ; required to find 

 the thickness of the shell. The answer to this problem comes 

 out to be 768 miles, which Archdeacon Pratt, by some strange 

 misconception of my meaning, takes to be my deliberate con- 

 clusion as to the thickness of the crust of the earth. 



The fallacy in reasoning of which I am accused, is deliberate, 

 and consists in assuming the same law of ellipticity and density 

 to hold both for the crust and nucleus. 



I do not know whether it does or not ; but no one else knows 

 anything positive about it, and I am therefore entitled to deny 

 the validity of any determination of thickness of the earth's crust, 

 based upon any arbitrary assumption of a law of density, of which 

 I know nothing. It has always appeared to me that the specu- 

 lations of mathematicians respecting the interior of the globe 

 were as unfounded and unreal as their speculations respecting 

 the supposed luminiferous medium. We are equally ignorant 

 in both cases ; and our calculations must be only regarded as so 

 much useless and learned labour in vain. 



I believed, when I wrote my paper on the original and 

 actual fluidity of the earth and planets, and still believe, that 

 the thickness of the crust of the earth, for anything we know 



