236 



HELMHOLTZ 



great. At the edges and in the lower layers of the glacier, 

 as in a flow of water, it is considerably smaller than in the 

 centre of the surface. 



The upper sources of the Mer de Glace also have a slower 

 motion, the Glacier du Geant thirteen inches a day, and 



FIG. no 



the Glacier du Lechaud nine inches and a half. In different 

 glaciers the velocity is in general very various, according 

 to the size, the inclination, the amount of snow-fall, and 

 other circumstances. 



Such an enormous mass of ice thus gradually and gently 

 moves on, imperceptibly to the casual observer, about an 

 inch an hour the ice of the Col du Geant will take 120 

 years before it reaches the lower end of the Mer de Glace 

 but it moves forward with uncontrollable force, before 

 which any obstacles that man could oppose to it yield like 

 straws, and the traces of which are distinctly seen even on 

 the granite walls of the valley. If, after a series of wet 

 seasons, and an abundant fall of snow on the heights, the 

 base of a glacier advances, not merely does it crush dwell* 



