424 The Rev. S. Haughtou on the Thickness 



of crust deduced of comparatively little value. If the consi- 

 deration of friction be introduced, and Laplace^s law of density 

 be abandoned, the thickness of the earth's crust inight be sup- 

 posed 10 miles as easily as 800 miles. 



Secondly. ]\Ir. Hennessy has found a thickness for the crust 

 of the earth on the following hypotheses : — 



A. Laplace's law of density for the fluid parts of the earth. 



B. The specific gravity of the solidified fluid not the same 

 as that of the fluid, and an assumption of their ratio from some 

 experiments. 



C. That the outer surface of the earth is not perpendicular to 

 gravity, and that the small difference between its surface and 

 that perpendicular to gravity is a real difference. 



By combinations of the foregoing hypotheses, Mr. Hennessy 

 obtains various values for the thickness of the earth's crust, all 

 of which of course have no higher value than the hypotheses on 

 which they rest. The first and third of these hypotheses I do 

 not admit; but I think that much important information might 

 be obtained by well-directed experiments made with reference to 

 the second hypothesis. 



Thirdly. My own investigations on the thickness of the earth's 

 crust are essentially sceptical, and were published with the view 

 of showing that the entire subject requires so many hypotheses, 

 that it is of little., if any value in aid of geological theory. I 

 am quite willing, however, to admit that this defect would be 

 I'cgarded by some geologists as no disadvantage, as it would fall 

 in the better with the wild and unfounded speculations which 

 are the standing reproach of their science. 



The result at which I arrived from the mathematical exa- 

 mination of this problem was, that three hypotheses are neces- 

 sary for its solution, and that these three liypotheses are arbi- 

 trary in the present state of our knowledge. I think it is easy 

 to show that ]\Ir. Hopkins and Mr. Hennessy have each made 

 three such hypotheses. 



Mr. Hopkins's hypotheses. 



1. Laplace's law Denied. 



2. Perfect fluidity Lnprobable. 



3. Law of density of crust and nucleus^ y i , , 



the same j ^ 



3Ir. Hennessy's hypotheses. 



1. Laplace's law in fluid part . . . Denied. 



2. Law of density of crust .... Unknown. 



3. Law of ellipticity of crust. . . . Unknown. 



It is quite true that T added an exami)ie of the mode of 



