MOTION. If a glacier is in equilibrium—that is, neither growing larger nor smaller—the ice annually flowing through any cross-section of the glacier must exactly equal the total an nual accumulation above and the waste below that section. As the accumulation above and the waste below a section through the neve-line is greater than for any other section, the flow, under uniform conditions, is the greatest there; and it is less and less through sections more and more distant from the neve-line, whether they are higher in the reservoir or lower in the dissipator.
That the ice of glaciers is in motion down the valley has long been known, both from the observation that large stones are carried down on the surface of the ice and from the general reasoning given above. It was not, however, until Agassiz and Forbes began their classical researches that any quantitative value of the mo tion of the ice was obtained. Since then many measurements have been made on various gla ciers, and we now have a fair knowledge of this matter. It has been found that at any section the velocity of the ice is greatest at the centre and diminishes as we approach the sides. When, however, a glacier has a sinuous course, the greatest velocity is not in the exact centre, but is displaced toward the convex side, so that the line of maximum velocity is more sinuous than the glacier itself. The velocity diminishes also from the surface of the ice toward the bed of the glacier. The observations on which this statement rests are neither numerous nor satis fying; nevertheless, they are sufficient to estab lish the fact. It will appear that, as Forbes said, the flow of a glacier is very much like that of a river; if we consider a river which is flow ing into a sandy region, where the water is gradually lost by seepage, the analogy is still more striking. In valley glaciers of fairly uni
form slope the velocity is greatest at the neve line and diminishes as we ascend or descend from there. This law is subject to many excep tions; if the valley contracts in descending, there must be an increase in velocity; if the slope of the valley increases, this will also increase the velocity; in glaciers which reach the sea and break off in bergs the velocity increases as we approach the end, as a result of the lack of support in front. There is also a slight move ment into the glacier in the reservoir, which is greatest where the accumulation is greatest, and one toward the surface in the dissipator, which is greatest where the waste is greatest. As to actual values in velocities, we find that the Mer de Glace has the greatest velocity of any glacier in the Alps, its maximum amounting to inches a day. The greatest velocity of the Aletsch, the largest glacier of the Alps, is 20 inches a day. For other Alpine glaciers we find various Velocities down to an inch or two a day, or even less for the smaller ones. Of larger glaciers, the Muir in Alaska has a velocity of about seven feet, near where it reaches tide water; and one of the larger ice-streams of Green land, the Upernivik, was found to have a velocity of 99 feet a day at one point near its end.