III. Observations of the Mer de Glace.—Part I
In this paper the author communicates the first part of a series of observations upon the Mer de Glace, made during a residence of six weeks at the Montanvert last summer. He corroborates the laws regarding the swifter flow of the central portions of the ice-stream, first established by Prof. Forbes, and shows how the velocity changes as the width of the glacier varies. The Mer de Glace moves through a valley which twice turns a convex curvature to the east, and once to the west. The points of swiftest motion at these curves are found to be not central, but thrown to that side of the valley towards which the glacier turns its convex curvature. It has hitherto been believed that the portion of the Mer de Glace derived from the Glacier du Géant moved swiftest. The author shows that the tributaries which form the Mer de Glace lose their individuality in the trunk stream, the latter flowing as if it proceeded from a single source. The point of maximum motion is sometimes on the eastern, sometimes on the western side of a line drawn along the centre of the glacier, the change from side to side depending upon the curvature of the valley. The locus of the point of swiftest motion in a glacier which moves through a sinuous valley, is exactly similar to that of a river moving through a sinuous channel; it forms a curve more deeply sinuous than the valley itself, and crosses the centre of the valley at each point of contrary flexure. A rare opportunity of determining the comparative velocities of a glacier at its surface and close to its bed, was furnished by a precipice of ice 140 feet in height, which was exposed near the Tacul. Three stakes were fixed in this, precipice, one at the top, the other near the bottom, and a third in the face of the precipice at a height of nearly 40 feet above the bottom; the velocities of the three stakes were found to be 6 inches, 4.59 inches, and 2.56 inches per day; thus furnishing additional proof of the correctness of the law first predicted by Prof. Forbes, and confirmed subsequently by his own observations and those of M. Martins.