﻿M. A. Heim on Glaciers. 497 



be, on the average, equal in amount in the upper, middle, and 

 lower parts of the glacier, which is not the case. As in no part 

 of the glacier, certainly not at the lower end, has any (even the 

 least) upward motion ever been observed at certain times, the 

 line which, as a base, remains relatively fixed must, when the 

 temperature falls, lie at the upper end of the glacier ; when the 

 temperature rises, at the lowest point. That with rise of tem- 

 perature the lower end would remain stationary, and drag after 

 it the whole long glacier-tongue (which cannot descend by its 

 own weight), without its being rent transversely into single in- 

 dependent fragments, I cannot conceive. The surface of the 

 glacier undergoes more and greater variations of temperature 

 near its margins than in the centre — now from reflection of heat 

 from the sides of the valley, then from their shade, but especi- 

 ally through the winds (which in the centre arrive with their 

 temperature already approximated to that of the glacier). The 

 margins have in some measure a more continental, the centre a 

 more oceanic climate. Hence one would think that the edges 

 of the glacier would move faster than, or at least as fast as the 

 centre, if variations of temperature effected the motion. 



I have not the remotest intention to summarily reject Mr. 

 Moseley's views ; but I thought it admissible to state what at 

 present appears to me opposed to them, in order to justify my 

 continuing to hold Professor TyndalFs explanation. Perhaps 

 Mr. Moseley will be able to remove these difficulties and, especi- 

 ally by measurements of the interior temperature of glaciers, 

 give his views a better foundation. 



In the Philosophical Magazine for March 1869, a " new 

 theory of glacier-motion " is advanced by Mr. Croll, which, as 

 regards the shearing-forces, is free from the difficulties of Pro- 

 fessor TyndalPs theory. I must omit a discussion of this 

 theory, because I do not yet clearly understand it. According 

 to it an explanation of the vertical arrangement of the optic axes 

 would be very easy. 



2. Experiments with "killed" Gypsum. 



The theory of plasticity, as given by Forbes, led immediately 

 to experiments with semiliquid masses. At that time the dif- 

 ference between viscous and non-viscous semiliquids was neg- 

 lected. Both yield to pressure; but while the former extend 

 on being drawn, and contract transversely, so as even to form 

 thin threads, the latter resist change of form by traction or are 

 torn across. In the former the parts are rearranged by traction 

 and by pressure ; in the latter, pressure produces a rearrange- 

 ment, traction either no effect or division. Ice is solid and 

 very brittle ; neither by pressure nor by traction are its particles 



