210. Are we to understand by this that the ice of the Léchaud is squeezed to one-tenth of its former volume? By no means. It is mainly a change of form, not of volume, that occurs at Trélaporte. Previous to its compression, the glacier resembles a plate of ice lying flat upon its bed. After its compression, it resembles a plate fixed upon its edge. The squeezing, doubtless, has deepened the ice.
[§ 30.] Longitudinal Compression of a Glacier.
211. The ice is forced through the gorge at Trélaporte by a pressure from behind; in fact the Glacier du Géant, immediately above Trélaporte, represents a piston or a plug which drives the ice through the gorge. What effect must this pressure have upon the plug itself? Reasoning alone renders it probable that the pressure will shorten the plug; that the lower part of the Glacier du Géant will to some extent yield to the pressure from behind.
212. Let us test this notion. About three-quarters of a mile above the Tacul, and on the mountain slope to the left as we ascend, we observe a patch of verdure. Thither we climb; there we plant our theodolite, and set out across the Glacier du Géant, a line, which we will call line No. 1 (F F' upon sketch, p. 68).
213. About a quarter of a mile lower down we find a practicable couloir on the mountain side; we ascend it, reach a suitable platform, plant our instrument, and set out a second line, No. 2 (G G' upon sketch). We must hasten our work here, for along this couloir stones are discharged from a small glacier which rests upon the slope of Mont Tacul.
214. Still lower down by another quarter of a mile, which brings us near the Tacul, we set out a third line, No. 3 (H H' upon sketch), across the glacier.
215. The daily motion of the centres of these three lines is as follows:—
| Inches | Distances asunder | ||
| No. 1 No. 2 No. 3 | 50·55 15·43 12·75 | } } | 545 yards. 487 " |
216. The first line here moves five inches a day more than the second; and the second nearly three inches a day more than the third. The reasoning is therefore confirmed. The ice-plug, which is in round numbers one thousand yards long, is shortened by the pressure exerted on its front at the rate of about eight inches a day.