Mr, Hopkins on the Mechanism of Glacial Motion. 245 



It would seem difficult to reconcile the notion of a glacier's 

 being " dragged down and pulled," with the hypothesis of its 

 being " urged by a Ibrce from above." 



In the second place, I maintain that the mechanical reason- 

 ing by which it is attempted to be proved that there would be 

 an indefinite number of such surfaces, is untenable, as is shown 

 by the reasoning of art. 4*. 



Again, allowing for the moment the formation of separation 

 planes, 1 do not allow that they could be " very highly in- 

 clined" to the surface of the glacier, assuming the pushing 

 force to act as it would do in the case of a semifluid mass, in 

 which case the direct tendency of the force would be to form 

 separation planes approximately parallel to the bed of the 

 glacier. The direction, however, in which there would be 

 the greatest tendency to form these planes, would be that in 

 which the pushing force resolved in that direction should bear 

 the greatest ratio to the resistance in that direction, arising 

 from the cohesion of the mass. I do not believe that the 

 plane of fracture could be " highly inclined." That a nearly 

 horizontal force should produce nearly vertical fractures, is, 

 prima facie^ a mechanical contradiction ; if it be not really so, 

 it must at least require some proof beyond assertion. 



The continued verticality of transverse fissures presents an 

 obvious objection to the general theory of Professor Forbes. 

 He has attempted to answer it by saying that the portion of 

 a glacier near its surface moves with the same velocity to a 

 considerable depth. I am at a loss to reconcile this, as a 

 general statement, with the assertion that, near the lower end 

 of the glacier, " the upper layers roll over the lower ones." 

 If this assertion be correct, how can the verticality of trans- 

 verse fissures be preserved, as I believe it is, near the lower 

 extremity of a glacier, as well as in every other part of it ? 

 It is very possible that the motion may be such as here de- 

 scribed at the extremity of a glacier like that of the Rhone, 

 where the inclination of the glacial valley suddenly diminishes 

 at the termination of the glacier. But is this the motion in 

 the glaciers of Chamouni, Grindelwald and the Aar? I believe 

 the assertion rests entirely on theoretical considerations, but 

 it is on the evidence of observation alone that the fact can be 

 admitted. 



6. It has been remarked, at the conclusion of my previous 



* In Mr. Hodgkinson's experiments on the breaking of cast iron by a 

 crushing force,the fracture always took place along one surface of separation, 

 and not along many, and it manifestly must do so whenever the body frac- 

 tured possesses the cohesion of a solid body. The case is very analogous 

 to that of a glacier, if we suppose, as I am now doing, for the sake of the 

 argument, that the glacier is urged forward by a force « tergo. 



