Now, such a change of flexure actually occurs opposite the passages called Les Ponts, and at this place the view just enunciated was tested. It was immediately ascertained that the point of swiftest motion here lay at a different side of the axis from that observed lower down. But to confer strict numerical accuracy upon the result, stakes were fixed at certain distances from the western side of the glacier, and others at equal distances from the eastern side. The velocities of these stakes were compared with each other, two by two, a stake on the western side being always compared with a second one which stood at the same distance from the eastern side. The results of this measurement are given in the following table, the numbers denoting inches:

1st pair 2nd pair 3rd pair 4th pair 5th pair
West1517¼22¼23¾23¾
East12½15¼15½18¼19½

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:

1st pair 2nd pair 3rd pair
West12¾1517¼
East14¾17½19

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.

Fig. 1.