on the Motion of Glaciers. 407 



although they professed to be reasoning from the same pre- 

 mises, and although in the merely deductive or mathematical 

 part no error was supposed or admitted on either side. 



Such investigations, though thorny and often unfruitful, are 

 valuable as steps to better ; but it is vei'y rarely that the foun- 

 dation or primary establishment of a great physical theory has 

 been laid by them. It would be an inversion of the order of 

 discovery were it so. It would be discovery by deduction in- 

 stead of induction. The deductive part of the theory of gla- 

 ciers will come, I trust, in time, but not until the cause of 

 glacier motion has been generally admitted and understood. 



I must do Mr. Hopkins the justice to state, that as he has 

 remoulded the expression of his views several times*, in each 

 leaving out something which rendered his views tangibly di- 

 stinct from mine, and introducing expressions which might ad- 

 mit of an interpretation coincident with them, so he has also 

 employed a simpler style of mathematical reasoning, and ap- 

 pears now to be convinced that any considerations of value 

 employed in his earlier papers were reducible to most simple 

 propositions of geometry and common mechanics. Were I 

 to enter into a controversy point by point with Mr. Hopkins, 

 I should have to show by what a formidable process he de- 

 monstrates the elementary truth, that a sliding body supported 

 by one resisting point presses upon it with the force of its entire 

 weight multiplied into the sine of the angle of inclination f. I 

 should also have to show that the investigation of the tempe- 

 rature of the bottom of a glacier J is a ponderous display of 

 dexterity, where the physical conditions of the problem are so 

 entirely left out of sight that it conveys no positive information 

 whatever on the subject. And as to the other "solutions" 

 which Mr. Hopkins's two quarto papers contain — such as a 

 mode of finding the curvature of a crevasse after any given 

 time, and the actual inclination of its bed, — they can only be 

 regarded as mathematical practice intended for his pupils, 

 without any application, however remote, to the actual illus- 

 tration of the theory of glaciers. 



3. Mr. Hopkins, in his keenness to controvert the plastic 

 theory, has been unfortunate enough to misstate that theory, 

 and even the facts on which it, or any other, must be based. 



First, he has misrepresented my theory ; not willingly, I 

 assume; but because, until lately it is evident that he had 

 not read it so as to understand it, even approximately. In 

 his second memoir in the Cambridge Transactions, Mr. Hop- 



* In his First memoir in the Cambridge Transactions, in his Second 

 memoir, and in his late papers in the Philosophical Magazine, 

 t First 4to memoir. % Ibid, and Phil. Mag. January 1845. 



