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an inclined plane and retarded by friction as a constant force ; it 

 was due to the fact of the cohesion of the constituent particles of the 

 mass at its lower surface being insufficient to resist the tendency of 

 such an enormous weight of ice to descend down a plane even of 

 very small inclination. A continuous disintegration is thus pro- 

 duced, promoted probably, in a greater or less degree, by a constant 

 but very gradual thawing of the ice at the lower surface. In this 

 manner it is easy to see that the motion must depend on the rate of 

 disintegration, and therefore must be nearly a uniform, and not an 

 accelerated motion. 



Professor Forbes had an undoubted claim to the credit of being 

 the first to suggest and insist upon the capability of the general mass 

 of a glacier to change its form under existing conditions, as a cause 

 of glacial motion. The above explanation of the sliding of the mass 

 Mr. Hopkins claimed for himself. 



Still it was felt that further investigation was required respecting 

 the property of glacial ice which had been designated as its viscosity. 

 There was no conclusive evidence that glacial ice would bear any 

 considerable extension without breaking, for numberless crevasses 

 were formed wherever the ice appeared to be subjected to any great 

 extending force. Again, it was equally certain that the contiguous 

 portions of a dislocated glacial mass, though retaining their perfect 

 solidity, did become reunited into one continuous and unbroken 

 mass. These facts were not sufficiently explained by the assertion 

 that glacial ice was viscous. The true explanation appeared to 

 have been afforded by an observation made some time ago by Dr. 

 Faraday, and the more recent experiments of Dr. Tyndall. The 

 former observed that two pieces of ice in perfect contact would 

 freeze together so as to become one perfectly continuous mass, 

 though the surrounding temperature should be much higher than 

 32° ; and the latter gentleman had shown, by a striking form of the 

 experiment, the extreme facility and rapidity with which a piece of 

 common ice, after being crushed and broken into numberless frag- 

 ments, will reunite into one continuous mass of transparent ice. 

 This process had been designated by the term " r eg elation " and 

 manifestly some corresponding term was required to designate the 

 property which, in ice, rendered that process so complete. Such 

 terms as viscous and plastic failed to express adequately the property 

 in question. At the same time it should be remarked that, so far as 

 glacial motion depended on the facility with which the glacial mass 

 might change its form, the manner in which that change was effected 

 was of secondary importance, and did not diminish whatever value 

 attached to Professor Forbes's first recognition of this change as an 

 important cause of glacial motion. In one respect, however, the 

 mechanism of the motion would be in some measure affected. Dr. 

 Tyndall contends, and in a paper recently presented to the Royal 

 Society has collected a considerable amount of evidence to show, 

 that glacial ice would bear no more linear extension, independently 

 of lateral compression, than ordinary specimens of ice would lead us 

 to suppose, and consequently, when acted on by extending forces, it 



