678 [June 18, 



permeated by infiltering water during the summer months ; and if 

 any results were obtained, they were not striking or definite enough 

 to be worth recording. That the lower surface of a glacier in contact 

 with the earth is in a constant state of fusion, even in those cold 

 regions, is proved by the phenomena recorded by Dr. Kane of the 

 Mary Minturn River in lat. 78 54', and the feeder of the Kane 

 Lake, lat. 78 18', which never ceases to flow, summer or winter. 

 Admitting this as a general fact, the sliding of a glacier on its bed is 

 an obvious necessity ; and that it should be unaccelerated is no more 

 a matter of wonder or difficulty of conception than the unaccelerated 

 descent of the weight of a clock which is never abandoned to its own 

 impetus, but brought to rest after every momentary descent by the 

 action of the scapement, or the unaccelerated fall of a body in a 

 resisting medium when the resistance becomes equal to gravity, 

 or of a weight gradually and uniformly lowered by the hand. Per- 

 haps the more general way of conceiving it would be to regard the 

 whole glacier as a mass propped up against a support anyhow inclined, 

 and prevented from tumbling over sideways by lateral stays. Such 

 a mass would rest in its position, if duly supported either by a base- 

 abutment, or by a heap of its own debris ; but if these were slowly 

 abraded, destroyed, or picked away, the whole mass would descend 

 bodily in the exact manner of the withdrawal of support. 



On the disruption of a nearly homogeneous elastic solid in a state 

 of strain, I would add a remark which seems to me of some moment, 

 as explanatory of the greater cohesive strength which is well known 

 to be imparted to cements, especially those of a resinous or gummy 

 nature, by the admixture of extraneous matter in fine powder. If 

 in such a solid there be one portion, however small, weaker than the 

 rest, the strain being uniform, a crack will originate in that place. Now 

 a crack, once produced, has a tendency to run for this plain reason, 

 that at its momentary limit, at the point on which it has just arrived, 

 the divellent force on the molecules there situated is counteracted 

 only by half the cohesive force which acted when there was no crack, 

 viz., the cohesion of the uncracked portion alone. But if a crack 

 anywhere produced be stopped from running by encountering a solid 

 particle of greater cohesive force, fracture will no longer be determined 

 by the accidental deficiency of cohesion at some weak point, but by 

 the average resistance of the whole cementing mass. 





