Mr. Hopkins on the Motion of Glaciers. 15 



down which the mass would descend if its motion were unim- 

 peded by any lateral obstacle, we shall have (art. 3.) to sin a 

 nearly, as the force which this portion alone of the glacier is 

 capable of exerting to overcome any obstacles to its motion. 

 Now it is well known that the flanks of almost every glacier 

 are much crevassed and dislocated, especially where there is 

 any abrupt obstacle to the motion as above represented ; and 

 this being the case, there is no reason to suppose that the ice, 

 of which the motion must be arrested in the corners at C and 

 C, should present a greater impediment to the motion of the 

 central portion of the glacier than the rocky sides of the valley 

 present in other places. B C C B' will no more be able to 

 support itself at rest in the position above represented, than a 

 beam is able to support itself in a horizontal position resting 

 on its extremities, when the parts near the extremities are fis- 

 sured and broken. 



If B C C B' would advance by its weight, it must, dfortiori, 

 advance when acted on by the mass ABB' A', which, if the 

 former portion were impeded, would exert an additional force 

 to urge it onwards. 



If the impediment should offer itself in the form of a hill ri- 

 sing suddenly from the centre of the valley, the motion through 

 the valleys on either side of the hill would admit of the same 

 explanation. Whenever a glacier meets with an obstacle to its 

 motion, its state of dislocation immediately shows itself. 



This explanation was given in my second memoir, and its 

 importance insisted on in opposition to the opinion that the 

 power of a glacier to move through a comparatively narrow 

 gorge is derived from the great plasticity of glacial ice. For 

 the purpose of proving this explanation incorrect, Prof. Forbes 

 recently made some observations on the Mer de Glace, by 

 which he showed that a line originally straight on the surface 

 of the glacier gradually assumed the form of a curve of conti- 

 nuous curvature, proving thereby that the parts of the glacier 

 did not slide past each other as supposed in the preceding ex- 

 planation. He found with some difficulty (as he has stated) 

 a portion of the glacier near its side uninterrupted by fissures. 

 It was doubtless subjected to more than ordinary pressure, by 

 which the diflferent portions into which it must have been pre- 

 viously divided by crevasses, were closely packed together, 

 and constrained to move as a continuous mass. Now, under 

 such circumstances, I should contend that glacial ice has pro- 

 bably a very considerably greater degree ofjtexibility than his 

 observations prove it to have, and therefore these observations 

 were altogether unnecessary for the purpose of refuting my 

 opinions. To render them a test of the truth of the explana- 

 tion above given, they should have been made in those places 



