310 Cambridge Philosophical Society : — 



viscous mass would move along a trough inclined at a small angle 

 to the horizon ; and, moreover, it was obvious that the general mass 

 of a glacier did so change its form as to accommodate itself to the 

 changing dimensions of the valley down which it moved. 



On the other hand, it was contended that a substance so hard and 

 brittle as glacial ice could not be said to have the property of visco- 

 sity, and that the different velocities of the central and lateral por- 

 tions of a glacier, and the changes of form which the general mass 

 might undergo, were more attributable to the formation of crevasses 

 and to discontinuous ruptures of the mass, than to any continuous 

 change of form in each infinitesimal portion of it, like that which 

 takes place in a mass which can be properly termed ^dscous. That 

 this view was partly true was obvious, since ruptures and crevasses 

 were actually formed by the unequal motions of different portions of 

 the mass. Those who maintained this latter view held that the 

 glacier moved by actually sliding over its bed ; while those who sup- 

 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 obsen-ed 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 enable 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 iu 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 doubts the fact of the whole mo- 

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

 the property which has been hitherto usually designated as the vis- 

 cosity or plasticity of its mass, and that which consists of a sliding 



