﻿" Densities in the Earth's Crust." 413 



and the distance of the point of application of this force from 

 the middle line of the rectangle is 



la. 



Comparison of this hypothetical result, with observation, in 

 respect both to the magnitude of the force and its point of 

 application, will, I hope, form the subject of a future com- 

 munication. 



Eastern Telegraph Company's Cable-ship 'Elcetra/ 

 Athens to Genoa, Sept. 3 ... 6, 1894. 



XLVIII. " Densities in the Earth's Crust." 

 By Rev. J. J. Blake, M.A.,F.G.S.* 



MR. OSMUND FISHER having been unfortunately called 

 upon to categorically assent to or answer my criticisms 

 of a portion of his work on the ' Physics of the Earth's 

 Crust/ has attempted the latter alternative in the April 

 number of this magazine. I had hoped that he would have 

 adopted the former on more careful consideration of the 

 subject ; but, as it is not so, I hope I may be excused if I 

 point out still more clearly the gist of my objections to his 

 method. 



In several places Mr. Fisher does not seem to recognize 

 that it is not his conclusions but his mathematics that are 

 discussed, for if the mathematics are wrong there are no con- 

 clusions to discuss. Thus he begins by stating that if I had 

 more fully mastered his " results," I should not have stated 

 that "the argument . . . seems to depend on the greater density 

 of the superficial layer in continental than in oceanic areas," 

 which he observes is the exact opposite of his " conclusion ; " 

 but, as ocean water is certainly of less density than rock, 

 this is not a conclusion at all, but a datum — the mathematical 

 problem being this: — Given that the attraction of a sphere on 

 a particle at any point on its surface is constant for all such 

 points— but that the superficial layer in one part is less dense 

 than in another — find the relations between the densities and 

 thicknesses of the underlying layers. 



Again, he says it would be absurd to assume any other law 

 of attraction than the Newtonian for the case of nature, — very 

 likely ; but the objection is that, if the method were correct, 

 the same results might be deduced even from an absurd law. 

 It is for this reason that I call the functions he speaks of (/(#) 

 &c.) " unknown." 



Again, he answers my objection that in his solution " it is 



* Communicated by the Author. 



