194 PROFESSOR FORBES ON THE VISCOUS THEORY OF GLACIER MOTION. 
On the Jirst 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 narrow- 
ing 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, however 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 com- 
plete 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 one- 
third of its lesser, one-sixth of its greater diameter-^. 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 parallelopipedons of a certain 
breadth, is evident. 
Crevasses . — In the second place, I maintain that actual inspection shows that a glacier 
is not the mass of fragments nor of parallelopipedons which some persons have, naturally 
enough at first sight, supposed it to be. In truth there is not an approach to such a 
condition in those glaciers which move over moderate slopes of considerable extent, 
which have very properly been assumed by all writers as the criterial examples of any 
theory ; for it is not denied that portions of glaciers and glacier tributaries do some- 
times fall piecemeal over precipices, each fragment descending by its separate and 
individual gravity, in the manner of an avalanche, although I am disposed to believe, 
indeed am sure, that the number of such instances is smaller than is usually imagined ; 
and the angle requisite for such a tumultuous mode of descent is far greater than it 
has, perhaps, always hitherto been considered to be. To him who would form a just 
estimate of the mechanical constitution of a glacier — who would consider it as a 
whole — without always distracting his attention from the length and breadth of the 
problem by a minute attention to its lesser features, — I would earnestly recommend 
the frequent and attentive survey of a glacier or glaciers from a considerable eleva- 
tion above their level and under varying effects of light. Had I confined myself to 
studying crevasses on the surface of the glacier, measuring their depths, injecting the 
ice with fluids and taking its temperature ; useful and important as these inquiries 
* 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. 
