GLACIAL MOTION.
That glacial ice actually moves after the analogy of a semi-fluid has been abundantly demonstrated by observation. In the year 1827 Professor Hugi, of Soleure, built a hut far up upon the Aar Glacier in Switzerland, in order to determine the rate of its motion. After three years he found that it had moved 330 feet; after nine years, 2,354 feet; and after fourteen years Louis Agassiz found that its motion had been 4,712 feet. In 1841 Agassiz began a more accurate series of observation upon the same glacier. Boring holes in the ice, he set across it a row of stakes which, on visiting in 1842, he found to be no longer in a straight line. All had moved downwards with varying velocity, those near the centre having moved farther than the others. The displacements of the stakes were in order, from side to side, as follows: 160 feet, 225 feet, 269 feet, 245 feet, 210 feet, and 125 feet. Agassiz followed up his observations for six years, and in 1847 published the results in his celebrated work System Glacière.
Fig. 16.
But in August, 1841, the distinguished Swiss investigator had invited Professor J. D. Forbes, of Edinburgh, to interest himself in solving the problem of glacial motion. In response to this request, Professor Forbes spent three weeks with Agassiz upon the Aar Glacier. Stimulated by the interest of this visit, Forbes returned to Switzerland in 1842 and began a series of independent investigations upon the Mer de Glace. After a week’s observations with accurate instruments, Forbes wrote to Professor Jameson, editor of the Edinburgh New Philosophical Journal, that he had already made it certain that “the central part of the glacier moves faster than the edges in a very considerable proportion, quite contrary to the opinion generally maintained.” This letter was dated July 4, 1842, but was not published until the October following, Agassiz’s results, so far as then determined, were, however, published in Comptes Rendus of the 29th of August, 1842, two months before the publication of Forbes’s letter. But Agassiz’s letter was dated twenty-seven days later than that of Forbes. It becomes certain, therefore, that both Agassiz and Forbes, independently and about the same time, discovered the fact that the central portion of a glacier moves more rapidly than the sides.
In 1857 Professor Tyndall began his systematic and fruitful observations upon the Mer de Glace and other Alpine glaciers. Professor Forbes had already demonstrated that, with an accurate instrument of observation, the motion of a line of stakes might be observed after the lapse of a single clay, or even of a few hours. As a result of Tyndall’s observations, it was found that the most rapid daily motion in the Mer de Glace in 1857 was about thirty-seven inches. This amount of motion was near the lower end of the glacier On ascending the glacier, the rate was found in general to be diminished; but the diminution was not uniform throughout the whole distance, being affected both by the size and by the contour of the valley. The motion in the tributary glaciers was also much less than that of the main glacier.
This diminution of movement in the tributary glaciers was somewhat proportionate to their increase in width. For example, the combined width of the three tributaries uniting to form the Mer de Glace is 2,597 yards; but a short distance below the junction of these tributaries the total width of the Mer de Glace itself is only 893 yards, or one-third that of the tributaries combined. Yet, though the depth of the ice is probably here much greater than in the tributaries, the rapidity of movement is between two and three times as great as that of any one of the branches.[AM]
[AM] See Tyndall’s Forms of Water, pp. 78-82.