It was thus placed beyond doubt that the point of maximum motion of the Mer de Glace, at the place referred to, is not the centre of the glacier. But, to make assurance doubly sure, I examined the comparative motion along three other lines, and found in all the same undeviating result.
This result is not only unexpected, but is quite at variance with the opinions hitherto held regarding the motion of the Mer de Glace. The reader knows that the trunk-stream is composed of three great tributaries from the Géant, the Léchaud, and the Talèfre. The Glacier du Géant fills more than half of the trunk-valley, and the junction between it and its neighbours is plainly marked by the dirt upon the surface of the latter. In fact four medial moraines are crowded together on the eastern side of the glacier, and before reaching the Montanvert they have strewn their débris quite over the adjacent ice. A distinct limit is thus formed between the clean Glacier du Géant and the other dirty tributaries of the trunk-stream.
Now the eastern side of the Mer de Glace is observed on the whole to be much more fiercely torn than the western side, and this excessive crevassing has been referred to the swifter motion of the Glacier du Géant. It has been thought that, like a powerful river, this glacier drags its more sluggish neighbours after it, and thus tears them in the manner observed. But the measurement of the foregoing three lines shows that this cannot be the true cause of the crevassing. In each case the stakes which moved quickest lay upon the dirty portion of the trunk-stream, far to the east of the line of junction of the Glacier du Géant, which in fact moved slowest of all.
LAW OF MOTION SOUGHT.
The general view of the glacier, and of the shape of the valley which it filled, suggested to me that the analogy with a river might perhaps make itself good beyond the limits hitherto contemplated. The valley was not straight, but sinuous. At the Montanvert the convex side of the glacier was turned eastward; at some distance higher up, near the passages called Les Ponts, it was turned westward; and higher up again it was turned once more, for a long stretch, eastward. Thus between Trélaporte and the Ponts we had what is called a point of contrary flexure, and between the Ponts and the Montanvert a second point of the same kind.
CONJECTURE REGARDING CHANGE OF FLEXURE.
Supposing a river, instead of the glacier, to sweep through this valley; its point of maximum motion would not always remain central, but would deviate towards that side of the valley to which the river turned its convex boundary. Indeed the positions of towns along the banks of a navigable river are mainly determined by this circumstance. They are, in most cases, situate on the convex sides of the bends, where the rush of the water prevents silting up. Can it be then that the ice exhibits a similar deportment? that the same principle which regulates the distribution of people along the banks of the Thames is also acting with silent energy amid the glaciers of the Alps? If this be the case, the position of the point of maximum motion ought, of course, to shift with the bending of the glacier. Opposite the Ponts, for example, the point ought to be on the Glacier du Géant, and westward of the centre of the trunk-stream; while, higher up, we ought to have another change to the eastern side, in accordance with the change of flexure.
On the 25th of July a line was set out across the glacier, one of its fixed termini being a mark upon the first of the three Ponts. The motion of this line, measured on a subsequent day, and reduced to its daily rate, was found to be as follows:—
Fourth Line.—Daily Motion.
| No. of stake. | Inches. | |
| East 1 | moved | 61/2 |
| 2 | " | 8 |
| 3 | " | 121/2 |
| 4 | " | 151/4 |
| 5 | " | 151/2 |
| 6 | " | 183/4 |
| 7 | " | 181/4 |
| 8 | " | 183/4 |
| 9 | " | 191/2 |
| 10 | moved | 21 |
| 11 | " | 201/2 |
| 12 | " | 231/4 |
| 13 | " | 231/4 |
| 14 | " | 21 |
| 15 | " | 221/4 |
| 16 | " | 171/4 |
| 17 | " | 15 West. |