﻿Mr. J. Ball on the Cause of the Motion of Glaciers, 85 



time, so long as the phenomena exhibited by ice are strictly 

 analogous to those offered by other substances of the same class, 

 I shall not think that any special hypothesis is required for their 

 explanation. 



In conclusion I venture to state the explanation of the cause 

 of glacier-motion which appears to me to be best consistent with 

 the facts, and which I am content to submit to those who are 

 familiar with the phenomena, without entering into detailed ar- 

 guments, already well-known to those who have attended to the 

 subject, and which could not be fully set forth without infrin- 

 ging unduly on the space accorded to me. 



In the first place, I must dwell for a moment on the fact, 

 familiar yet constantly forgotten, that glacier-ice is a very differ- 

 ent substance from the ice that is formed in winter on the sur- 

 face of lakes and rivers. However slight the external indica- 

 tions of the fact may be, it is a congeries of separate fragments 

 more or less perfectly welded together, but not possessing a 

 uniform or common crystalline structure, and showing by the 

 frequent presence of air-bubbles, and by its behaviour when ex- 

 posed to radiant heat, an inferior degree of solidity. On this 

 account I objected to Mr. Moseley's experiments on shearing- 

 force, because I held them to be inapplicable to glacier-ice ; and 

 I further hold that, if the most careful experimenter were to 

 determine the shearing-force of a given square inch of glacier- 

 ice, it would be unsafe to conclude that the resistance offered by 

 an adjoining square inch, or a number of other equal sections of 

 the glacier, would be found nearly equal. 



I further note, as an essential characteristic of the great 

 glaciers whose onward motion is rapid, that they are traversed 

 by fissures often of great width, and of depth great even rela- 

 tively to the dimensions of the glacier. 



Finally, I consider it to be an established fact that all consider- 

 able glaciers, even those of the second order when they lie below 

 what is called the snow-line, have a fixed internal temperature 

 of 32° Fahr., from which they never vary by more than a mi- 

 nute fraction, and that the influence of the seasons, still more 

 that of day and night, penetrates but a very moderate distance 

 from their exterior surfaces. 



Glacier-ice is a substance which at the temperature of freezing 

 is capable of yielding, very slowly, to moderate pressure. A 

 portion of the motion of all glaciers, and the whole, or nearly 

 the whole, of the motion of glaciers of the second order (that is 

 to say, of those which lie on the slopes of mountains, but do not 

 fill a definite trough or valley-channel) is due to this cause, and 

 is effected independently of fracture and regelation (if the former 



