232 Professor Forbes on the Leading Phenomena of Glaciers. 



case of glaciers, the pressing columns are enormous, the origin 

 and termination of many of the largest having not less than 

 4000 feet of diflference of level ; were they, therefore, perfect- 

 ly 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 com- 

 pared to water will not appear so preposterous as it might at 

 first sight do, considering the small degree of transmitted pres- 

 sure required to be eff'ectual. 



Again, it has been attempted to shew 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 shewn that the glacier does fall to- 

 gether and choke its own crevasses with its plastic substance. 



When a glacier passes from a narrow gorge into a wide val- 

 ley, it spreads itself, in accommodation to its new circumstan- 

 ces, as a viscous substance would do, and when embayed be- 

 tween rocks, it finds its outlet through a narrower channel than 

 that by which it entered. This remarkable feature of Glacier 

 Motion, already several times adverted to, had not been brought 

 prominently forward, until stated by M. Rendu, now Bishop 

 of Annecy, who has described it very clearly in these words, 

 — " 11 y a une foule de faits qui sembleraient faire croire que 

 la substance des glaciers jouit d'une espece de ductilite qui lui 

 permet de se modeler sur la localite qu**elle occupe, de s'amincir, 

 de se retrecir, et de s'etendre, comme le ferait une pate moUe. 

 Cependant, quand on agit sur un morceau de glace, qu'on le 

 frappe, on lui trouve une rigidite qui est en opposition directe 

 avec les apparences dont nous venons de parler. Peut-etre 

 que les experiences faites sur de plus grandes masses donne- 

 raient d'autres resultats."* 



* Theorie des Glaciers de la Savoie, p. 84. Whilst T am anxious to shew how 

 far the sagacious views of M. Rendu coincide with, as they also preceded my own, 

 it is fair to mention, that all my experiments were made, and indeed by far the 



