Illustrations of the viscous theory of glaciers

Part III. The author inquires, in this part of his paper, into the motion of those comparatively small isolated glacial masses, reposing in the cavities of high mountains or oncols , and called by De Saussureglaciers of the second order . A glacier of this description in the neighbourhood of the Hospice du Simplon, lodged in a niche on the northern face of the Schœnhorn, immediately behind the Hospice, and at an elevation of about 8000 feet above the sea, was selected for observation. The average velocity of its descent was found to be about one inch and a half in twenty-four hours : those parts in which the slope was 20° moving with a velocity about one-third greater than those in which the slope was 10°. The author next enters into general views on the annual motion of glaciers, and on the influence of seasons ; and gives tabular details of the observations made with reference to these questions at two stations ; the one on the Glacier des Bossons, and the other at the Glacier des Bois, which is the outlet of the Mer de Glace towards the valley of Chamouni. In both these glaciers, the motion in summer exceeds that in winter in a greater proportion as the station is lower, and consequently exposed to more violent alternations of heat and cold. He also found that the variations of velocity due to season are greatest where the variations in the temperature of the air are greatest, as in the lower valleys ; excepting that variations of temperature below the freezing-point produce scarcely any appreciable change in the rate of motion of the ice. He concludes with some general illustrations of the plastic or viscous theory of glacier motion. A glacier, he contends, is not a mass of fragments or parallelopipedons ; neither is it a rigidly solid body ; and although it may be extensively intersected by crevices, these “ crevasses” are comparatively superficial, and do not disturb the general continuity of the mass in which they occur. The water contained in these crevices is only the principal vehicle of the force which acts upon it : and the irresistible energy with which the whole icy mass descends from hour to hour with a slow but continuous motion bespeaks of itself the operation of a fluid pressure acting on a ductile or plastic material.