Mr. Hopkins on Glacial Theories. 311 



over its bed. Dr. Tyndall has recently observed proof of this latter 

 motion in various parts of glaciers as well as near their lovrer extre- 

 mities ; and all the phsenomena of polished and striated rocks indi- 

 cate most clearly that such motion must have existed in the ancient 

 glaciers to which such phenomena are referred. But how was it 

 conceivable that a glacier should thus slide over a surface on which 

 there must be many and considerable inequalities, and at inclinations 

 sometimes not exceeding 2° or 3° ? And if it did thus slide, how 

 was it that it did not move, as bodies ordinarily move down inclined 

 planes, with an uccelei-ated moWon ? These questions were frequently 

 dwelt upon formerly. They were completely answered by the expe- 

 riments made by Mr. Ho])kins, and described by him in the ' Trans- 

 actions ' of this Society in 1844 (vol. viii. part 1), and in his first 

 letter "On the Motion of Glaciers," dated November 19 of that 

 year, and inserted in the Philosophical Magazine. The motion in 

 question was not at all analogous to that of a body descending down 

 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 sUding of the mass 

 Mr. Hopkins claimed for himself. 



Still it was felt that further investigation was requii-ed 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 tlic surrounding temperature should be much higher than 

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

 experiment, the extreme facility and rajjidity with whicli a \ncce. of 

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

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



