﻿496 M. A. Heim on Glaciers. 



probable. The phenomenon observed by MM. Bertin, Grad, and 

 Dupre, that in lamellae cut horizontally out of the lower part of 

 the glacier coloured rings with a black cross are seen when they 

 are viewed in the polarizing microscope, does not necessarily in- 

 dicate actual crystalline structure ; amorphous glass can yield 

 the same phenomenon through strains forced upon it by external 

 pressure. It appears to me that the effect in the glass also would 

 necessarily be permanent, if the violent pressure had operated 

 for many years. Experiments on a small scale, for the purpose 

 of producing by pressure the crystallographic orientation in 

 pieces of ice, gave no result. This is not surprising ; for I could 

 not, as many a glacier does, operate with a pressure of 5 cwt. 

 per square inch, or factors of similar magnitude. Already in the 

 27th volume of the Philosophical Magazine Sir John Herschel 

 conjectured a parallel arrangement of the optic axes, but not on 

 grounds corresponding with M. Grades explanation. 



I must not, however, omit to mention that, according to the 

 calculations of an Englishman (Canon Moseley) in the Philoso- 

 phical Magazine for May, 1869, the resistance of the ice to the 

 shearing-forces of the glacier-motion would be too great for the 

 ice to be broken by gravity alone and thus the glacier to move 

 by its own weight. If I rightly understand the experiments of 

 Tyndall and my own on the remoulding of plates of ice, the ice 

 therein and in the glacier is not compelled to shear, but to break 

 by the bending. The shearing, tangential displacements take 

 place along the fissures previously produced by fracture (bend- 

 ing) . The very peculiar mechanical conditions of the ice seem 

 to me not to have been sufficiently considered in the calculation. 

 One of these is its extraordinary brittleness even at 0° C. ; the 

 same pressure which a mass of ice will sustain in a state of rest 

 for along time without breaking, breaks it immediately a shock 

 is added. Another is the occurrence of crowds of minute air- 

 bubbles, which must very much diminish the compactness of the 

 ice. In the lower part of a glacier, where air-bubbles are almost 

 absent, we have the old, only half regelated secondary capillary 

 fissures, along which fresh fracture is easier : while in this region 

 the primary fissures are somewhat less numerous, the secondary 

 (which are longer preserved) exhibit a peculiar abundance of 

 plaits. So many factors, unmeasured in their effect, and scarcely 

 known in their mode of operation, are involved in the mechanics 

 of glacier- motion, that the result of a calculation based on the little 

 that is known cannot possibly induce me antecedently to reject the 

 explanation by pressure, with which all the facts of which I have 

 any knowledge, and all that I have seen, agree so perfectly. Ac- 

 cording to the explanation of glacier-motion given by Mr. Moseley 

 (Phil. Mag. Jan. 1863 and Aug. 1869), the total motion must 



