Supposing the points of maximum motion to be determined for a great number of lines across the glacier, the line uniting all these points is what mathematicians would call the locus of the point of maximum motion. At Trélaporte this line would lie east of the centre; at the Ponts it would lie west of the centre; hence, in passing from Trélaporte to the Ponts, it must cross the axis of the glacier. Again, at the Montanvert, it would lie east of the centre, and between the Ponts and the Montanvert the axis of the glacier would be crossed a second time. Supposing the dotted line in [Fig. 21] to represent the middle line of the glacier, then the defined line would represent the locus of the point of maximum motion. It is a curve more deeply sinuous than the valley itself, and it crosses the axis of the glacier at each point of contrary flexure.
LOCUS OF POINT OF SWIFTEST MOTION.
To complete our knowledge of the motion of the Mer de Glace, we afterwards determined the velocity of its two accessible tributaries—the Glacier du Géant, and the Glacier de Léchaud. On the 29th of July, a line of stakes was set out across the former, a little above the Tacul, and their motion was subsequently found to be as follows:
Sixth Line.—Daily Motion.
| No. of stake. | Inches. | |
| 1 | moved | 11 |
| 2 | " | 10 |
| 3 | " | 12 |
| 4 | " | 13 |
| 5 | " | 12 |
| 6 | moved | 123/4 |
| 7 | " | 101/2 |
| 8 | " | 10 |
| 9 | " | 9 |
| 10 | " | 5 |
The width of the glacier at this place we found to be 1134 yards, and its maximum velocity, as shown by the foregoing table, 13 inches a day.
On the 1st of August a line was set out across the Glacier de Léchaud, above its junction with the Talèfre: it commenced beneath the block of stone known as the Pierre de Béranger. The displacements of the stakes, measured on the 3rd of August, gave the following results:—
Seventh Line.—Daily Motion.