142 VISCOUS THEORY OF GLACIER MOTION. [1846. 



cart, or any other comparison with an aggregate of incoherent 

 fragments or individual masses, must be wrong if mine be 

 right. And I feel confident, not only that such an incoherent 

 mass could not move after the manner of a glacier,, but also 

 that attentive inspection of a glacier at once contradicts such 

 an idea. 



On the first point, I maintain that a rugged channel, like 

 that of a glacier, with a moderate slope, being packed with 

 angular solid fragments, would speedily be choked, and that 

 farther pressure from behind (for such a mass can only convey 

 thrusts, not strains) would tend to wedge the fragments more 

 tightly. Some grains of dry sand will slide easily down a 

 plate of glass; but try to thrust it forcibly through a narrowing 

 tube, or even a uniform one, the lower end of which rests on a 

 surface over which the sand has poured, and your effort is vain, 

 the tube will sooner burst ; and even rocks may be blasted 

 rather than the power of the wedge yield.* If the figure of 

 the bed or channel be in any degree irregular, that is, have 

 expansions and contractions, however smooth its surface, how- 

 ever small the sliding angle of ice upon that surface, the 

 choking of a strait or contraction by the piling of the fragments 

 will be as complete and effectual as if the lateral friction were 

 excessive. Now in point of fact we have such cases as this ; 

 a glacier 2000 yards wide (the Mer de Glace at the Tacul) 

 issues by an orifice or strait 900 yards wide ; the glacier of 

 Talefre, a nearly oval basin, pours out its annual overcharge by 

 an orifice the breadth of which is but oae-third of its lesser, 

 one-sixth of its greater diameter.f On the supposition of 

 jostling fragments, the facility of motion is increased, as the 

 comminution is greater. The impossibility of the discharge of 

 a fragmentary solid through a gorge by long stripes fractured 

 parallel to its length, and constituting parallelepiped ons of a 

 certain breadth, is evident. 



* See Huber-Burnand's conclusive experiments on this subject, Ann. de Chimie 

 et de Physique, xli. 166, and Fechner's Repertorium, i. 65. 



f See the Map of the Mer de Glace and its tributaries in my Travels in the 

 Alps of Savoy. 



