3S4 Mr. Hopkins's Reply to Dr. Whewell's 



ing as the mass is semifluid^ or possesses at least a certain 

 degree o^ solidity. The curves of dislocation have, in both 

 cases, their convexity turned in the same direction, but in the 

 former case the loop is much more elongated than in the lat- 

 ter. In the more elongated curves the tangent at a point near 

 the side will be inclined to the axis at an angle of about 15°, 

 whereas in the other curves the corresponding angle will be 

 at least 45°. Now it is unquestionable that this last system of 

 dislocations affords the accurate type of the transverse fissures 

 of a glacier; but what glacial phaenomena correspond to the 

 other system ? I challenge any one to point out in a canal- 

 shaped glacier transverse fissures approximating to the elon- 

 gated loops of that system; nor will the lines of structure 

 suffice. There are, in fact, no phaenomena hitherto observed 

 which represent the loops in question, and hence I maintain 

 ihsii glacial motion, cannot be correctly represeiited by that ofa 

 semifluid mass. 



The experimental results I have now described are so clear 

 and determinate, and appear to me to corroborate so distinctly 

 the theoretical results I had previously obtained, that I trust 

 I may be allowed to add this account of them to my previous 

 communications, as the conclusion of the exposition of my 

 views respecting the mechanism of glacial motion. 

 1 am, Gentlemen, 



Your obedient Servant, 



Cambridge, March 19, 1845. W. HoPKINS. 



L. Mr. Hopkins's Reply to Dr. Whewell's Remarks on 

 Glacier Theories. 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 



AVING completed the exposition of my own views re- 

 specting the motion of glaciers, I shall now beg permis- 

 sion to offer a few remarks in reply to the letters addressed to 

 you by Dr. Whewell on the same subject. 



With respect to the definitions which your correspondent 

 has given of such terms as solidity, Jtexibility^ &c., every one, 

 I apprehend, will agree with him, and in so doing must allow 

 the justice of the observation at the close of my first letter, 

 which appears to have called forth these definitions, viz. that 

 Prof. Forbes had used such terms too indiscriminately. He 

 has spoken of having proved the plasticity of glacial ice by 

 observations which only proved a small degree o'i Jlexibility"^, 



* In the last paragraph of my First Letter, p. 1 6. 



H 



