When a glacier moves through a sinuous valley, the locus of the points of maximum motion does not coincide with the centre of the glacier, but, on the contrary, always lies on the convex side of the central line. The locus is therefore a curved line more deeply sinuous than the valley itself, and crosses the axis of the glacier at each point of contrary flexure.
189. The dotted line on the Outline Plan ([page 68]) represents the locus of the point of maximum motion, the firm line marking the centre of the glacier.
190. Substituting the word river for glacier, this law is also true. The motion of the water is ruled by precisely the same conditions as the motion of the ice.
191. Let us now apply our law to the explanation of a difficulty. Turning to the careful measurements executed by M. Agassiz on the glacier of the Unteraar, we notice in the discussion of these measurements a section of the "Système glaciaire" devoted to the "Migrations of the Centre." It is here shown that the middle of the Unteraar glacier is not always the point of swiftest motion. This fact has hitherto remained without explanation; but a glance at the Unteraar valley, or at the map of the valley, shows the enigma to be an illustration of the law which we have just established on the Mer de Glace.
[§ 26.] Motion of Axis of Mer de Glace.
192. We have now measured the rate of motion of five different lines across the trunk of the Mer de Glace. Do they all move alike? No. Like a river, a glacier at different places moves at different rates. Comparing together the points of maximum motion of all five lines, we have this result:
MOTION OF MER DE GLACE.
| At Trélaporte | 20 | inches | a | day. |
| At les Ponts | 23 | " | " | |
| Above the Montanvert | 26 | " | " | |
| At the Montanvert | 34 | " | " | |
| Below the Montanvert | 33[C] | " | " |
[C] This is probably under the mark. I think it likely that the swiftest motion of this portion of the Mer de Glace in 1857 amounted to a yard in twenty-four hours.
193. There is thus an increase of rapidity as we descend the glacier from Trélaporte to the Montanvert; the maximum, motion at the Montanvert being fourteen inches a day greater than at Trélaporte.