Professor Forbes's Sixth Letter on Glaciers. 241 



position abcdefg, after other equal intervals at a'Uc'd'e'fg', and 

 at a!'h"d'd"e"f"g'\ by which time it will be seen that the neigh- 

 bour particles have entirely changed their relative positions, 

 and that the mass can have no pretension to be called rigid, 

 but moulds itself after the manner that a fluid or semifluid 

 body does in like circumstances, the centre advancing fastest, 

 and, for some space in the centre, nearly uniformly, whilst the 

 retardation produced by the friction of the banks is most in- 

 tense in their neighbourhood ; which is conformable to what 

 we know of the movement of viscous fluids. It is, therefore, 

 no hypothesis, but a simple statement of a demonstrated fact, 

 that the manner of movement of the surface of a glacier is not 

 such as is consistent with the continuity of a rigid body, hut 

 that it coincides with the manner of motion of a viscous or semi- 

 fluid body, "Whatever may be the difiiculty of conceiving the 

 glacier to be a body thus constituted, the fact admits of no 

 doubt ; — the effects of forces applied on a great scale to bodies, 

 are the best and only conclusive proofs of their real constitu- 

 tion, and worth all molecular theories and minute experiments 

 put together. 



If a body be really of a ]pasty consistence, ductile and plas- 

 tic like lava or tar, such transpositions taking place in the 

 interior of the mass are effected without any injury to the 

 texture or continuity of the substance. With a degree less 

 of plasticity, a violent separation of the parts may take place, 

 but they will, by juxtaposition, soon reunite and take a new 

 set. With a degree more of rigidity, there must be a per- 

 manent bruising and rending of the parts, in order that a 

 semi-rigid body may assimilate all in its movements to a 

 fluid. It must, therefore, be considered as entirely confirma- 

 tory and explanatory of the preceding statements of the seem- 

 ing plasticity of a body so fragile in its elements as pure ice, 

 that the ice of glaciers is found rent in many parts by the 

 forces tending to dislocation, and that, besides, it contains 

 within itself a testimony to the internal partial movements by 

 which its total motion is effected, in the veined structure al- 

 ready alluded to, occasioned by the varying velocity of the 

 Adjacent icy strips k. a d d\ B 6 U b\ &c. This structure 

 is jQot exactly parallel to the direction of motion of the ice, 



