248 Professor Forbes's Seventh Letter on Glaciers. 



in the direction in which the effective pressure is greatest ; and 



it is plain, that, owing to the 

 *^* ^* diminishing relation between 



the weight of the superincum- 

 bent particles and the frontal 

 resistance, the direction in 

 which the particles will tend 

 to slide over one another, or 

 to produce rents, will ap- 

 proach verticality at the sur- 

 face, and on the whole will, 

 therefore, tend to produce 

 lines of discontinuity, such as 

 NM. 

 (4.) Considering the glacier at different points of its length. 



Fig. 6. 



it is evident, by similar reasoning, that near the region of 

 the neve a the frontal dip will be all but vertical, because 

 there the horizontal resistance is enormous ; whilst at the 

 lower end b, where it tends to vanish, the shells will tend to 

 parallelism with the bed. It is needless to add, that the 

 relative movements of the particles over one another, produ- 

 cing discontinuity, are not to be confounded with their abso- 

 lute motions in the glacier, exactly as under head (1.) I must 

 however, observe, that as the tendency of any particle due to 

 the hydrostatic pressure will be to describe ultimately the 

 whole curve N m^ M within the glacier, this may account 

 for some of the facts, or supposed facts, which indicate a ten- 

 dency in the ice to expel bodies engaged in it, as well as the 

 convexity of the glacier at all times, and its remarkable rise 

 of surface during winter. 



Lastly, The ablation of the surface of the glacier during its 



