Professor Forbes on thv Leading Phenomena of Glaciers. 24f 



the resistance in front is enormous^ the tendency of the separa- 

 tion planes^ which are those of apparent cleavage, will be very 

 highly inclined. As the lower end of the glacier is approached, 

 the resistance continually diminishes, the line of least resist- 

 ance becomes more and more nearly horizontal ; and finally,* 

 when the lower end of the glacier is reached, the planes fall 

 away altogether, and the upper layers roll over the lower ones, 

 now wholly unsupported. Such we have seen to be the actual 

 phenomena of the Mer de Glace. 



Imagine a long narrow trough or canal stopped at both 

 ends, and filled to a considerable depth with treacle, honey, 

 tar, or any such viscid fluid. Imagine one end of the trough 

 to give way, the bottom still remaining horizontal, if the fric- 

 tion of the fluid against the bottom be greater than the friction 

 against its own particles, the upper strata will roll over the 

 lower ones, and protrude in a convex slope, which will be pro- 

 pagated backwards towards the other or closed end of the 

 trough. Had the matter been quite fluid, the whole would 

 have run out and spread itself on a level ; as it is, it assumes 

 precisely the conditions which we suppose to exist in a glacier. 

 The greatest disturbance or maximum separation of the parts 

 takes place at the lower end, and there (the retardation at 

 the sides being proportional to the absolute velocity) the sepa- 

 ration will be most violent, and 

 the loops on the surface will be 

 most elongated. Near the origin 

 the declivity is less, and the loops 

 are more transverse. This is~true 

 of the glacier. 



Now, let the trough be a little inclined, so as to aid the 

 gravitating force derived from the mere depth of the fluid. 

 Each particle will be urged on by a force due to the slope, 

 diminished by the resistances opposed to it. The particles 

 near the lower termination of the stream have no resistances, 

 except their attachment to those behind them, they therefore 

 roll straight on ; but those in the middle of the glacier will 

 easier raise the weight of a certain superincumbent stratum 

 of ice, than push the entire glacier before them ; they may do 



