1858.] 



on the Mer-de- Glace. 



547 



PIC. I 



point referred to. Fig. 1 represents a sketch of the Mer-de- 

 Glace. The dotted line is drawn along the centre of the glacier ; 

 the defined line which crosses the axis of the glacier at the points 

 A A is then the locus of the point of swiftest 

 motion. It is a curve more deeply sinuous than 

 the valley itself, and crosses the central line of 

 the valley at each point of contrary flexure. 

 The speaker drew attention to the fact that the 

 position of towns upon the banks of rivers is 

 usually on the convex side of the stream, where 

 the rush of the water renders silting-up impos- 

 sible : the Thames was a case in point ; and the 

 same law which regulated its flow and deter- 

 mined the position of the adjacent towns, is at 

 this moment operating with silent energy among 

 the Alpine glaciers. 



Another peculiarity of glacier motion is now 

 to be noticed. 



Before any observations had been made upon 

 the subject, it was surmised by Prof. Forbes that 

 the portions of a glacier near its bed were re- 

 tarded by friction against the latter. This view 

 was afterwards confirmed by his own observa- 

 tions, and by those of M. Martins. Nevertheless the state of our 

 knowledge upon the subject rendered further confirmation of the 

 fact highly desirable. A rare opportunity for testing the question 

 was furnished by an almost vertical precipice of ice, constituting 

 the side of the Glacier du Geant, which was exposed near the 

 Tacul. The precipice was about 140 feet in height. At the top 

 and near the bottom stakes were fixed, and by hewing steps in the 

 ice, the speaker succeeded in fixing a stake in the face of the pre- 

 cipice at a point about 40 feet above the base. After the lapse of a 

 suflicient number of days, the progress of the three stakes was mea- 

 sured ; reduced to the diurnal rate, the motion was as follows : — 



Top stake 

 Middle stake 

 Bottom stake 



6-00 inches. 

 4-59 „ 

 2'5Q „ 



We thus see that the top stake moved with more than twice 

 the velocity of the bottom one; while the velocity of the 

 middle stake lies between the two. But it also appears that the 

 augmentation of velocity upwards is not proportional to the distance 

 from the bottom, but increases in a quicker ratio. At a height of 

 100 feet from the bottom, the velocity would undoubtedly be prac- 

 tically the same as at the surface. Measurements made upon an 

 adjacent ice cliff proved this. We thus see the perfect validity of 

 the reason assigned by Forbes for the continued verticality of the 

 walls of transverse crevasses. Indeed a comparison of the result 



