552 Professor Tj/ndall, [June 4, 



and of a river in a sinuous channel has been already pointed out. 

 But the analogy fails in one important particular : the river, and 

 much more so a mass of flowing treacle, honey, tar, or melted 

 caoutchouc, sweeps round its curves without rupture of continuity. 

 The viscous mass stretches, but the icy mass breaks, and the 

 " excessive crevassing " pointed out by Prof. Forbes himself, is the 

 consequence. Secondly, the inclinations of the Mer-de-Glace and 

 its three tributaries were taken, and the association of transverse 

 crevasses with the changes of inclination was accurately noted. 

 Every traveller knows the utter dislocation and confusion pro- 

 duced by the descent of the Mer-de-Glace from the Chapeau down- 

 wards. A similar state of things exists in the ice-cascade of the 

 Talefre. Descending from the Jardin, as the ice approaches the 

 fall, great transverse chasms are formed, which at length follow 

 each other so speedily as to reduce the ice masses between them to 

 mere plates and wedges, along which the explorer has to creep 

 cautiously. These plates and wedges are in some cases bent and 

 crumpled by the lateral pressure, and on some masses vortical forces 

 appeared to have acted turning large pyramids 90° round, so as to 

 set their structure at right angles to its normal position. The ice 

 afterwards descends the fall, the portions exposed to view being a 

 fantastic assemblage of frozen boulders, pinnacles, and towers, some 

 erect, some leaning, falling at intervals with a sound like thunder, 

 and crushing the ice crags on which they fall to powder. The 

 descent of the ice through this fall has been referred to as a proof 

 of its viscosity ; but the description just given does not, it was 

 believed, harmonize with our ideas of a viscous substance. 



But the proof of the non-viscosity of the substance must be 

 sought at places where the change of inclination is very small. 

 Nearly opposite I'Angie there is a change from 4 to 9 degrees, and 

 the consequence is a system of transverse fissures which renders the 

 glacier here perfectly impassable. Further up the glacier, trans- 

 verse crevasses are produced by a change of inclination from 3 

 to 5 degrees. This change of inclination is accurately protracted 

 in Fig. 5 ; the bend occurs at the point B ; it is scarcely percepti- 



Fig. 5. 



ble, and still the glacier is unable to pass over it without breaking 

 across. Thirdly, The crevasses are due to a state of strain from 

 which the ice relieves itself by breaking : the rate at which they 

 widen may be taken as a measure of the amount of relief demanded 

 by the ice. Both the suddenness of their formation, and the slow- 

 ness with which they widen are demonstrative of the non-viscosity 

 of the ice. For were the substance capable of stretching even 

 at the small rate at which they widen there would be no necessity 

 for their formation. 



