From Tyndall’s observations it appears also that the line of most rapid motion is not exactly in the middle of the channel, but is pushed by its own momentum from one side to the other of the middle, so as always to be nearer the concave side; in this respect conforming, as far as its nature will permit, to the motion of water in a tortuous channel.
Fig. 17.
It is easy to account for this differential motion upon the surface of a glacier, since it is clear that the friction of the sides of the channel must retard the motion of ice as it does that of water. It is clear also that the friction of the bottom must retard the motion of ice even more than it is known to do in the case of water. In the formation of breakers, when the waves roll in upon a shallowing beach, every one is familiar with the effect of the bottom upon the moving mass. Here friction retards the lower strata of water, and the upper strata slide over the lower, and, where the water is of sufficient depth and the motion is sufficiently great, the crest breaks down in foam before the ever-advancing tide. A similar phenomenon occurs when dams give way and reservoirs suddenly pour their contents into the restricted channels below. At such times the advancing water rolls onwards like the surf with a perpendicular front, varying in height according to the extent of the flood.
Seasoning from these phenomena connected with moving water, it was naturally suggested to Professor Tyndall that an analogous movement must take place in a glacier. Choosing, therefore, a favourable place for observation on the Mer de Glace where the ice emerged from a gorge, he found a perpendicular side about one hundred and fifty feet in height from bottom to top. In this face he drove stakes in a perpendicular line from top to bottom. Upon subsequently observing them, Tyndall found, as he expected, that there was a differential motion among them as in the stakes upon the surface. The retarding effect of friction upon the bottom was evident. The stake near the top moved forwards about three times as fast as the one which was only four feet from the bottom.
Fig. 18.
The most rapid motion (thirty-seven inches per day) observed by Professor Tyndall upon the Alpine glaciers occurred in midsummer. In winter the rate was only about one-half as great; but in the year 1875 the Norwegian geologist, Helland, reported a movement of twenty metres (about sixty-five feet) per day in the Jakobshavn Glacier which enters Disco Bay, Greenland, about latitude 70°. For some time there was a disposition on the part of many scientific men to doubt the correctness of Holland’s calculations. Subsequent observations have shown, however, that from the comparatively insignificant glaciers of the Alps they were not justified in drawing inferences with respect to the motion of the vastly larger masses which come down to the sea through the fiords of Greenland. The Jakobshavn Glacier was about two and a half miles in width and its depth very likely more than a thousand feet, making a cross-section of more than 1,400,000 square yards, whereas the cross-section of the Mer de Glace at Montanvert is estimated to be but 190,000 square yards or only about one-seventh the above estimate for the Greenland glacier. As the friction of the sides would be no greater upon a large stream than upon a small one, while upon the bottom it would be only in proportion to the area, it is evident that we cannot tell beforehand how rapidly an increase in the volume of the ice might augment the velocity of the glacier.