ICE AND GLACIERS. 



Ill 



Such observations have in recent times been made by 

 several observers, especially by Forbes and by Tyndall. They 

 show that in summer the middle of the Mer de Glace moves 

 through twenty inches a day, while towards the lower terminal 

 cascade the motion amounts to as much as thirty-five inches in 

 a day. In winter the velocity is only about half as great. At 



FIG. 19. 



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 sur- 

 face. 



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

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

 Glacier du Lechaud nine inches and a half. In different 

 glaciers the velocity is in general very various, according to the 



