Another peculiarity of glacier motion is now to be noticed.
Before any observations had been made upon the subject, it was surmised by Prof. Forbes that the portions of a glacier near its bed were retarded by friction against the latter. This view was afterwards confirmed by his own observations, and by those of M. Martins. Nevertheless the state of our knowledge upon the subject rendered further confirmation of the fact highly desirable. A rare opportunity for testing the question was furnished in 1857 by an almost vertical precipice of ice, constituting the side of the Glacier du Géant, exposed near the Tacul. The precipice was about 140 feet in height. At the top and near the bottom stakes were fixed, and by hewing steps in the ice I succeeded in fixing a stake in the face of the precipice at a point about forty feet above the base.[31] After the lapse of a sufficient number of days, the progress of the three stakes was measured; reduced to the diurnal rate, the motion was as follows:
| Top stake | 6.00 inches. |
| Middle stake | 4.59 „ |
| Bottom stake | 2.56 „ |
We thus see that the top stake moved with more than twice the velocity of the bottom one, while the velocity of the middle stake lies between the two. But it also appears that the augmentation of velocity upwards is not proportional to the distance from the bottom, but increases in a quicker ratio. At a height of 100 feet from the bottom, the velocity would undoubtedly be practically the same as at the surface. Measurements made upon an adjacent ice-cliff proved this. We thus see the perfect validity of the reason assigned by Forbes for the continued verticality of the walls of transverse crevasses. Indeed a comparison of the result with his anticipations and reasonings will prove alike their sagacity and their truth.
The most commanding view of the Mer de Glace and its tributaries 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 Montanvert. This point, marked G on the map of Forbes, I succeeded in attaining. A Tübingen 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 Trélaporte.
But let me state numerical results. Previous to its junction with its fellows, the Glacier du Géant measures 1,134 yards across. Before it is influenced by the thrust of the Talèfre, the Glacier de Léchaud has a width of 825 yards; while the width of the Talèfre branch across the base of the cascade, before it joins the Léchaud, is approximately 638 yards. The sum of these widths is 2,597 yards. At Trélaporte 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 astonishing if we confine our attention to one of the tributaries—that of the Léchaud. This broad ice-river, which before its junction with the Talèfre has a width of 825 yards, at Trélaporte is squeezed to a driblet of less than 88 yards 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? No doubt pressure from behind. Other facts also suggest that the Glacier du Géant is throughout its length in a state of forcible longitudinal compression. Taking a series of points along the axis of this glacier—if these points, during the descent of the glacier, preserved their distances asunder perfectly constant, there could be no longitudinal compression. The mechanical meaning of this term, as applied to a substance capable of yielding like ice, must be that the hinder points are incessantly advancing upon the forward ones. I was particularly anxious to test this view, which first occurred to me on à priori grounds. Three points, A, B, C, were therefore fixed upon the axis of the Glacier du Géant, 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 „ |
The result completely corroborates the foregoing anticipation. The hinder points are incessantly advancing upon those in front, and that to an extent sufficient to shorten a segment of this glacier, measuring 1,000 yards in length, at the rate of 8 inches a day. Were this rate uniform at all seasons, the shortening would amount to 240 feet in a year. When we consider the compactness of this glacier, and the uniformity in the width of the valley which it fills, this result cannot fail to excite surprise; and the exhibition of force thus rendered manifest must be mainly instrumental in driving the glacier through the jaws of the granite vice at Trélaporte.