548 Professor Tyndall, [Jane 4, 



with his anticipations and reasonings will prove alike their sagacity 

 and their truth. 



The most commanding view of the Mer-de-Glace and its tribu- 

 taries is obtained from a point above the remarkable cleft in the 

 mountain range underneath the Aiguille de Charmoz, which is 

 sure to attract the attention of an observer standing at the Mont- 

 envert. This point, which is marked G on the map of l^orbes, the 

 speaker succeeded in attaining. A Tubingen Professor once visited 

 the glaciers of Switzerland, and seeing these apparently rigid masses 

 enclosed in sinuous valleys, went home and wrote a book, flatly 

 denying the possibility of their motion. An inspection from the 

 point now referred to would have doubtless confirmed him in his 

 opinion ; and indeed nothing can be more calculated to impress the 

 mind with the magnitude of the forces brought into play than the 

 squeezing of the three tributaries of the Mer-de-Glace through the 

 neck of the valley at Trelaporte. But let us state numerical results. 

 Previous to its junction with its fellows, the Glacier du Geant mea- 

 sures 1134 yards across. Before it is influenced by the thrust of 

 the Tal^fre, the Glacier de Lt^chaud has a width of 825 yards ; while 

 the width of the Talefre branch across the base of the cascade, 

 before it joins the Lechaud, is approximately 638 yards. The 

 sum of these widths is 2597 yards. At Trelaporte those three 

 branches are forced through a gorge 893 yards wide, with a central 

 velocity of 20 inches a day ! The result is still more astonish- 

 ing, if we confine our attention to one of the tributaries, — that of the 

 Lechaud. Before its junction with the Talefre, the glacier has a 

 width of 37i English chains. At Trelaporte this broad ice river 

 is squeezed to a driblet of less than 4 chains in width, that is to say, 

 to about one-tenth of its previous horizontal transverse dimension. 



Whence is the force derived which drives the glacier through 

 the gorge? The speaker believed that it must be a pressure from 

 behind. Other facts also suggest that the Glacier du Geant is 

 throughout its length in a state of forcible longitudinal compression. 

 Now, taking a series of points along the axis of this glacier, — if 

 these points, during the descent of the glacier, preserved their dis- 

 tances asunder perfectly constant, there could be no longitudinal 

 compression. The mechanical meaning of this term, as applied to 

 a sulastance capable of yieldirig like ice, must be that the hinder 

 points are incessantly advancing upon the forward ones. The 

 speaker was particularly anxious to test this view, which first oc- 

 curred to him from a priori considerations. Three points, ABC, 

 were therefore fixed upon the axis of the Glacier du Geant, A 

 being the highest up the glacier. The distance between A and B 

 was 545 yards ; and that between B and C was 487 yards. The 

 daily velocities of these three points, determined by the theodolite, 

 were as follows : — 



A . . 20 • 55 inches. 

 B . . 15-43 „ 

 C . . 12-75 „ 



