122 Mr. H. H. Howorth on 



If the body be only semi-fluid, this will no longer be the 

 case ; at least the pressure communicated from one portion 

 (say of a sloping canal) to the other, will not be the whole 

 pressure of a vertical column of the material, equal in height 

 to the difference of level of the parts of the fluid considered ;. 

 the consistency or mutual supports of the parts opposes a 

 certain resistance to the pressure, and prevents its indefinite 

 transmission. It must be recollected that in the case of 

 glaciers, the pressing columns are enormous, the origin and 

 termination of many of the largest having not less than 

 4,cx>D feet of difference of level; were they, therefore,, 

 perfectly fluid, or suddenly converted into water, the lower 

 end would begin to move with the enormous velocity of 

 506 feet a second, or would move over 44 millions of feet in 

 24 hours. Now the velocity of the Mer de Glace is only 

 about 2 feet in that time, a difference so enormous that the 

 fluidity of a glacier compared to water will not appear so 

 preposterous as it might at first sight do, considering the 

 small dfegree of transmitted pressure required to be effectual. 



" Again, it has been attempted to be shewn that a glacier 

 is not coherent ice, but is a granular compound of ice and 

 water, possessing under certain circumstances, especially 

 when much saturated with moisture, a rude flexibility 

 sensible even to the hand. Farther, it has been shown that 

 the glacier does fall together and choke its own crevasses- 

 with its plastic substance." 



Forbes then proceeds to argue that— 



(i) From the proved result that the centre flows faster 

 than the sides and bottom there follows a suggestive 

 corollary. " I have no doubt," he says, " that glaciers 

 slide over their beds, as well as that the particles of ice rub 

 over one another, and change their mutual positions: butj. 

 J maintain that the former motion is caused by the latter, 

 and that the motion impressed by gravity upon the super- 

 ficial and central parts of a glacier (especially near its lower 



