416 Prof. Forbes's Rcplij to Mr. Hopkins 



sideration of the movement of fluids possessing viscosity and 

 that of plastic bodies (which is an extension of the same case) 

 under their own weight. Let him, for instance, consider the 

 lines of greatest tension in a viscous fluid extending itself in a 

 sloping trough when a sluice has been withdrawn, and the 

 tendencies to separation by sliding of the parts over one 

 another. Such I presume is the kind of mathematical in- 

 vestigations which Dr. Whewell means to recommend in 

 his First Letter to Mr. Taylor*. The application to very 

 viscous bodies will then not be difficult. It will at best be a 

 " popular " view ; nor will the mathematical expression of the 

 forces and motions increase our knowledge (in the first in- 

 stance) beyond what experiment, guided by general mechani- 

 cal principles, has already unfolded or may unfold. The ma- 

 thematician who values facts only, "pour donner prise au 

 calcul," must be glad if his results can be shaped so as to re- 

 present what is already known to be experimentally true. But 

 he must not hope to predict phaenomena, or to consider that 

 his symbolizing adds much weight to a mechanical theory. 

 That must be the office of the inductive philosopher who can 

 see his way in advance of the operations of his analytic tool. 

 Nevertheless, by viewing the subject on more sides than one, 

 something will be gained, and it may be hoped that the vis- 

 cous theory will then be fairly treated and impartially com- 

 pared with the facts of glaciers. 



Thus Mr, Hopkins will satisfy both himself and me; — him- 

 self, by putting the glacier theory in a more mathematical form 

 than 1 have given it ; and me, by his becoming thenceforward 

 (as I predict he will) a stanch supporter of the viscous theory, 

 even should his support be coupled with the reservation that 

 conviction being the result of his own proofs and not of mine, 

 he claims in full the honours of a flrst discoverer. 

 Edinburgh, March 24, 1845. 



Postscript. 



When the preceding remarks were written, I believed that 

 Mr. Hopkins had concluded, for the present, the exposition 

 of his views ; but as in the April Number of the Phil. Mag. 

 he has taken special notice of the plastic models alluded to in 

 the foregoing pages, I will state very shortly what this addi- 

 tional paper requires me to remark. 



Mr. Hopkins has repeated my experiments with plastic 

 bodies urged hy gravity down narrow channels, in which, pri- 

 marily, innumerable fine lines of separation due to the differ- 



* Phil. Mag., February 1845. 



