With regard to the diminution of the glaciers réservoirs, or, in other words, to the manner in which the ice disappears, notwithstanding the continual additions made to it, we have the following remarkable passage:—"In seeking the cause of the diminution of glaciers, it has occurred to my mind that the ice, notwithstanding its hardness and its rigidity, can only support a given pressure without breaking or being squeezed out. According to this supposition, whenever the pressure exceeds that force, there will be rupture of the ice, and a flow in consequence. Let us take, at the summit of Mont Blanc, a column of ice reposing on a horizontal base. The ice which forms the first layer of that column is compressed by the weight of all the layers above it; but if the solidity of the said first layer can only support a weight equal to 100, when the weight exceeds this amount there will be rupture and spreading out of the ice of the base. Now, something very similar occurs in the immense crust of ice which covers the summits of Mont Blanc. This crust appears to augment at the upper surface and to diminish by the sides. To assure oneself that the movement is due to the force of pressure, it would be necessary to make a series of experiments upon the solidity of ice, such as have not yet been attempted."[G] I may remark that such experiments substantially verify M. Rendu's notion.
But it is his observations and reasoning upon the glaciers d'écoulement that chiefly interest us. The passages in his writings where he insists upon the power of the glaciers to mould themselves to their localities, and compares them to a soft paste, to lava at once ductile and liquid, are well known from the frequent and flattering references of Professor Forbes; but there are others of much greater importance, which have hitherto remained unknown in this country. Regarding the motion of the Mer de Glace, Rendu writes as follows:—
MEASUREMENT OF MOTION.
"I sought to appreciate the quantity of its motion; but I could only collect rather vague data. I questioned my guides regarding the position of an enormous rock at the edge of the glacier, but still upon the ice, and consequently partaking of its motion. The guides showed me the place where it stood the preceding year, and where it had stood two, three, four, and five years previously; they showed me the place where it would be found in a year, in two years, &c.; so certain are they of the regularity of the motion. Their reports, however, did not always agree precisely with each other, and their indications of time and distance lack the precision without which we proceed obscurely in the physical sciences. In reducing these different indications to a mean, I found the total advance of the glacier to be about 40 feet a year. During my last journey I obtained more certain data, which I have stated in the preceding chapter. THE SIDES OF THE GLACIER RETARDED. The enormous difference between the two results arises from the fact that the latter observations were made at the centre of the glacier, which moves more rapidly, while the former were made at the side, where the ice is retained by the friction against its rocky walls."[H]
An opinion, founded on a grave misapprehension which Rendu enables us to correct, is now prevalent in this country, not only among the general public, but also among those of the first rank in science. The nature of the mistake will be immediately apparent. At page 128 of the 'Travels in the Alps' its distinguished author gives a sketch of the state of our knowledge of glacier-motion previous to the commencement of his inquiries. He cites Ebel, Hugi, Agassiz, Bakewell, De la Beche, Shirwell, Rendu, and places them in open contradiction to each other. Rendu, he says, gives the motion of the Mer de Glace to be "242 feet per annum; 442 feet per annum; a foot a day; 400 feet per annum, and 40 feet per annum, or one-tenth of the last!" ... and he adds, "I was not therefore wrong in supposing that the actual progress of a glacier was yet a new problem when I commenced my observations on the Mer de Glace in 1842."[I]
In the 'North British Review' for August, 1859, a writer equally celebrated for the brilliancy of his discoveries and the vigour of his pen, collected the data furnished by the above paragraph into a table, which he introduced to his readers in the following words:—"It is to Professor Forbes alone that we owe the first and most correct researches respecting the motion of glaciers; and in proof of this, we have only to give the following list of observations which had been previously made.
| Observers. | Name of glacier. | Annual rate of motion. | |
| Ebel | Chamouni | 14 | feet |
| Ebel | Grindelwald | 25 | " |
| Hugi | Aar | 240 | " |
| Agassiz | Aar | 200 | " |
| Bakewell | Mer de Glace | 540 | " |
| De la Beche | Mer de Glace | 600 | " |
| Shirwell | Mer de Glace | 300 | " |
| M. Rendu | Mer de Glace | 365 | " |
| Saussure's Ladder | Mer de Glace | 375 | " |
... Such was the state of our knowledge when Professor Forbes undertook the investigation of the subject."
I am persuaded that the writer of this article will be the first to applaud any attempt to remove an error which, advanced on his great authority, must necessarily be widely disseminated. The numbers in the above table certainly differ widely, and it is perhaps natural to conclude that such discordant results can be of no value; but the fact really is that every one of them may be perfectly correct. This fact, though overlooked by Professor Forbes, was clearly seen by Rendu, who pointed out with perfect distinctness the sources from which the discrepancies were derived.