﻿86 Mr. J. Ball on the Cause of the Motion of Glaciers. 



term is limited to disruption of a sensible or measurable amount*). 

 In the larger glaciers, and especially those which lie in a deep 

 trough and whose bed is very irregular, a new phenomenon 

 comes into play. Glacier-ice, though imperfectly solidified, is 

 yet rigid enough to transmit very considerable pressures; but 

 there is a limit at which pressure upon ice at 32° F. has the 

 effect of reducing it to the liquid state. At any given moment 

 of the progress of a great glacier, especially in summer, certain 

 points are subjected to enormous pressure. A given square 

 inch in the section of the glacier, instead of sustaining, as 

 Mr. Moseley assumes, merely the pressure of a parallelopiped 

 of equal section lying behind it, may at a given moment have 

 to bear the pressure due to a mass a hundred or a thousand times 

 as great in section. The effect may either be that fracture en- 

 sues at that point, whereupon the pressure is transmitted to an- 

 other adjoining point, and so further ; or else the pressure lique- 

 fies a portion of the ice : the water, even if it cannot escape, oc- 

 cupies less space than it did before ; so that the effect of trans- 

 ferring the maximum pressure from one point to another is 

 accomplished. It is this process that I ventured to compare to 

 the progress of a huge snake, whose movements are effected not 

 by simultaneous effort at every point, but by the transmission of 

 muscular energy from one point to another ; and many who 

 have happened to traverse a great glacier when its motion was at 

 the fastest, will admit that the words which I added, " straining 

 and groaning audibly v — will allow that these are scarcely figu- 

 rative terms. 



Having failed to explain with sufficient clearness what I be- 

 lieve to be the nature of the process, I am not surprised that 

 Mr. Croll fails to see that it is quite consistent with my views 

 that a glacier should advance more rapidly in a sinuous and ir- 

 regular channel than in one of uniform width and slope — because 

 in the former case the irregular distribution of pressure more 

 surely determines the yielding of the mass at one point, and 

 thus leads to the transference of the maximum pressure through- 

 out many successive points. 



The fact that a rise in external temperature causes a notable 

 increase in the rate of advance of a glacier has been familiar to 

 glacier-theorists since the earliest observations of the late Pro- 

 fessor Forbes ; but I fail to see that this militates in favour of 



* I have admitted that, in ray opinion, molecular science is too little ad- 

 vanced to allow us to give a peremptory negative to Mr. Croll's hypothesis 

 as to the passage of heat through ice. If the rein be given to conjecture, 

 I think it more reasonable to suppose that, in all bodies that change their 

 form slowly under pressure, the actual process" by which the particles 

 change their relative positions is nearly akin to, if not identical with, that 

 now well known as fracture and revelation. 



