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viously known to be the quality in glaciers in virtue of which their 

 motion down their valleys is produced by gravitation. Designating 

 Mr. Faraday's fact under the term " regelation," Dr. Tyndall de- 

 scribed the capability of glacier ice to undergo changes of form, as 

 being not true viscosity, but as being the result of vast numbers of 

 successively occurring minute fractures, changes of position of the 

 fractured parts, and regelations of those parts in their new positions. 

 The terms fracture and regelation then came to be the brief expres- 

 sion of his idea of the plasticity of ice. He appears to have been led 

 to deny the applicability of the term viscosity through the idea that 

 the motion occurs by starts due to the sudden fractures of parts in 

 themselves not viscous or plastic. The crackling, he pointed out 

 might, according to circumstances, be made up of separate starts 

 distinctly sensible to the ear and to the touch, or might be so slight 

 and so rapidly repeated as to melt almost into a musical tone. He 

 referred to slight irregular variations in the bending motion of the 

 line marked by a row of pins on a glacier by Prof. Forbes, as being 

 an indication of the absence of any quality that could properly be 

 called viscosity, and of the occurrence of successive fractures and 

 sudden motions in a material not truly viscous or plastic. I can only 

 understand his statements on this subject by supposing that he con- 

 ceived the material between the cracks to be rigid, or permanent in 

 form, when existing under strains within the limit of its strength, or 

 when strained less than to the point of fracture. 



This theory appeared to me to be wrong* ; and I then published, 



* While the offering of my own theory as a substitute for Professor Tyndall's 

 views seems the best argument I can adduce against them, still I would point to 

 one special objection to his theory. No matter how fragile, and no matter how much 

 fractured a material may be, yet if its separate fractured parts be not possessed of 

 some property of internal mobility, I cannot see how a succession of fractures is 

 to be perpetuated. A heap of sand or broken glass will either continue standing, 

 or will go down with sudden falls or slips, after which a position of repose will be 

 attained ; and I cannot see how the addition of a principle of reunion could tend 

 to reiterate the fractures after such position of repose has been attained. When 

 these ideas are considered in connexion with the fact that while ice is capable of 

 standing, without immediate fall, as the side of a precipitous crevasse, or of lying 

 without instantaneous slipping on a steeply sloping part of a valley, it can also 

 glide along, with its surface nearly level, or very slightly inclined, I think the 

 improbability of the motion arising from a succession of fractures of a substance 

 having its separate parts devoid of internal mobility will become very apparent. 

 If, on the other hand, any quality of internal mobility be allowed in the fragments 

 between the cracks, a certain degree at least of plasticity or viscosity is assumed, 



