Mr. Hopkins on the Mechanism of Glacial Motion. ^237 



212° F. During the evaporation of a solution of spec. grav. 

 1*146 and 1*15, at a temperature below 120°, lie never de- 

 lected any loss *. 



Chloride of ammonium may be evaporated in a water-bath 

 without perceptible loss. 



XXXV. On the Mechanism of Glacial Motion. Third Letter. 

 By W. Hopkins, Esq., M.A., F.R.S., ^-c. 



7b the Editors of the Philosophical Magazine and Journal, 



Gentlemen, 



1 PROCEED to offer some critical remarks on the mecha- 

 nical reasoning employed by Prof. Forbes to account for 

 phaenomena connected with the motion of glaciers. 



1. Let us first consider the reasoning by which he deter- 

 mines the direction of maximwn tension. It appears to me 

 embodied in what he has said on the subject in the note of 

 page 378 of his Travels. He observes, " The experiment 

 on a model, described in this note, is more strictly analogous 

 to the glacier phaenomena, than those of a more striking kind 

 described in the text, page 379, where the succession of co- 

 lours naturally gives to the mind the impression of a primitive 

 structure near the origin of the glacier, which is mechanically 

 deformed into these conchoidal surfaces. They strikingly re- 

 call, however, this important fact, that the direction of maxi- 

 mum distension of the particles must be, not parallel to the 

 length of the glacier, but in the direction of the branches of 

 these elongated loops, since their elongation is the simple re- 

 sult of the mechanical tension to which they are subjected." 

 The loops here spoken of are those formed in models of plas- 

 ter of Paris, described by Prof. Forbes when the descending 

 stream was composed of "alternate doses of white and blue 

 fluid poured in successively." They are identical, I conceive, 

 with the loop P' Q' represented in fig. 5. of my last letter, as 

 that into which a transverse element, originally rectilinear, 

 would be converted by the more rapid motion of the central 

 part of the glacier. 



Again, the Professor observes in the same note, " The 

 least distance which can ever exist between a side and a ceti- 

 tral particle of a canal-shaped glacier, is half the breadth of 

 the glacier. But the unequal motion of the centre and sides 

 tends continually to separate them wider apart, and to distend 

 the row of particles which connects them. The structural 

 bands are, therefore, perpendicular to the line of greatest ten- 

 sion, and hence crevasses will naturally occur, crossing the 

 ♦ On Nitrous Oxide, p. 5]. ed. 1839. 



