Professor Forbes on the Leading Phenomena of Glaciers. 235 



proof to the top and bottom of the ice-stream, for it seems 

 difficult to make this experiment in a satisfactory manner. In 

 the case of a glacier 600 feet deep, the upper hundred feet will 

 move nearly uniformly, on the principles already mentioned ; 

 hence, crevasses, formed from year to year, will not incline 

 sensibly forwards on this account, especially as the action of 

 trickling water is to maintain the verticality of the sides. I 

 conceive that this is a perfectly sufficient answer to an objection 

 which, at one time, I myself urged against the hypothesis of 

 the glacier moving most rapidly. Of the fact, I entertain no 

 doubt, though I see much difficulty in obtaining a satisfactory 

 proof of it. 



I have no doubt that glaciers slide over their beds, as well 

 as that the particles of ice rub over one another, and change 

 their mutual positions ; but I maintain, that the former motion 

 is caused by the latter, and that the motion impressed by 

 gravity upon the superficial and central parts of a glacier 

 (especially near its lower end) pull the lateral and inferior 

 parts along with them. One proof, if I mistake not, of such 

 an action is, that a deep current of water will flow under a 

 smaller declivity than a shallow one of the same fluid.* And 

 this consideration derives no slight confirmation in its appli- 

 cation to glaciers, from a circumstance mentioned by M. Elie 

 de Beaumont, which is so true, that one wonders it has not 

 been more insisted on, — namely, that a glacier, where it de- 

 scends into a valley, is like a body pulled asunder or stretched, 

 and not like a body forced on by superior pressure alone. 



Secondly, We have already seen how enormous would be 

 the velocity of a glacier if suddenly converted into a fluid, 

 and how prodigious a force is absorbed, as it were, by the con- 

 sistency or solidity of the ice. The moderate, though marked 

 diff^erence, found between the lateral and central velocity of 



* It is well known that the mean hydraulic depth, or the ratio of the section 

 of a stream to the perimeter of contact with its bed, is the most important element 

 (together with the declivity) in determining its velocity, or the eflfectual moving 

 force which acts upon it. Now, in the case of common friction, that of a solid 

 body, neither the absolute nor the relative depth of the sliding body can have any 

 influence in determining its motion. 



