1st pair 2nd pair 3rd pair 4th pair 5th pair
West 15 17-1/4 22-1/4 23-3/4 23-3/4 East 12-1/2 15-1/4 15-1/2 18-1/4 19-1/2
It is here seen that in each case the _western_ stake moved more swiftly than its eastern fellow stake; thus proving, beyond a doubt, that opposite the Ponts the western side of the Mer de Glace moves swiftest--a result precisely the reverse of that observed where the curvature of the valley was different.
But an additional test of the explanation is possible. Between the Ponts and the promontory of Trélaporte the glacier passes another point of contrary flexure, its convex curvature opposite to Trélaporte being turned towards the base of the Aiguille du Moine, on the eastern side. A series of stakes was placed across the glacier here; and the velocities of those placed at certain distances from the western side were compared, as before, with those of stakes placed at the same distances from the eastern side. The following table shows the result of these measurements; the numbers, as before, denote inches:
West 12-3/4 15 17-1/4 East 14-3/4 17-1/2 19
Here we find that in each case the _eastern_ stake moved faster than its fellow. The point of maximum motion has therefore once more crossed the axis of the glacier.
Determining the point of maximum motion for a great number of transverse sections of the Mer de Glace, and uniting these points, we have what is called the _locus_ of the point. The dotted line in the annexed figure represents the centre of the Mer de Glace; the hard line which crosses the axis of the glacier at the points A A is then the locus of the point of swiftest motion. It is a curve more deeply sinuous than the valley itself, and it crosses the central line of the valley at each point of contrary flexure. The position of towns upon the banks of rivers is usually on the convex side of the stream, where the rush of the water renders silting-up impossible; and the same law which regulated the flow of the Thames, and determined the position of the towns upon its banks, is at this moment operating with silent energy among the Alpine glaciers.
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: