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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 Geant 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 preci- 

 pice 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. 



Attention is drawn to the immense exertion of force necessary to 

 drive the glacier through the neck of the valley at Trelaporte. The 

 sum of the width of the three tributaries of the Mer de Glace before 

 they mutually act upon each other is 2597 yards. All these are 

 squeezed through a gorge at Trelaporte, measuring 893 yards across. 

 The Glacier de Lechaud, which, before its junction with the Talefre, 

 possesses a width of upwards of 37 chains, is squeezed at Trelaporte 

 to a driblet less than 4 chains wide. 



As a natural consequence of this obstacle in front, the Glacier du 

 Geant is generally in a state of longitudinal compression. But the 

 mechanical meaning of this term must be that the points behind are 

 incessantly advancing upon those in front, the distance between two 

 points upon the axis of the glacier being thus gradually diminished. 

 The daily motion of these points upon the axis of this glacier was 

 determined, which led to the remarkable result, that a section of the 

 glacier 1000 yards in length is shortened by the thrust to which it 



