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ported the viscous theory contended that there was no such sliding 

 motion, or if it existed at all, it constituted but a small part of the 

 whole observed progressive motion of the surface of the glacier. 



In the warmth of discussion these theories came to be considered 

 as more antagonistic than they really were. It was manifestly 

 possible that the lower surface of the glacier might slide, and thus 

 cause a part of the observed motion of the upper surface ; which 

 might also have an additional motion, due to the more rapid pro- 

 gression of the upper portions of the mass as compared with that of 

 its lower portions retarded by friction, as in the case of a semifluid 

 mass. On this point Mr. Hopkins quoted the following passage 

 from one of his letters " On the Mechanism of Glacial Motion," ad- 

 dressed to the editors of the Philosophical Magazine in 1844-45. If 

 observations "should concur in showing an approximate equality in 

 the motions of the upper and lower surfaces of a glacier, every candid 

 and impartial mind must admit, I conceive, the sliding in preference 

 to the viscous theory ; but if, on the contrary, it should be proved 

 that the velocity of the upper bears a large ratio to that of the lower 

 surface, the claims of the latter theory must be at once admitted." 

 Since this was written, several observations had been made by dif- 

 ferent persons, which agreed m showing that the upper surface of a 

 glacier does move faster than the lower surface ; but the only obser- 

 vations Mr. Hopkins had met with which enabled us to compare the 

 actual amounts of those motions, had been made by Prof. Forbes 

 himself near the extremity, Mr. Hopkins believed, of one of the gla- 

 ciers at Chamouni. The result was that the upper surface moved 

 about twice as fast as the lower one, thus proving that in this in- 

 stance the motion of the upper surface was due in nearly equal de- 

 grees to the two causes above mentioned, and that both theories 

 had so far equal claims to be admitted. 



But at present no one probably doubted the fact of the whole 

 motion of a glacier being made up of that motion which it derives 

 from the property hitherto usually designated as the viscosity or 

 plasticity of its mass, and that which consists of a sliding over its 

 bed. Dr. Tyndall had recently observed proofs of this latter motion 

 in various parts of glaciers as well as near their lower extre- 

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

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

 glaciers to which such phsenomena 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 accelerated motion ? These questions were frequently 

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

 riments made by Mr. Hopkins, 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 



