WHAT IS A G LACIER f 



3 



glaciers, was thirty-seven inches per day. Kear the sides of 



the glacier, however, the movement was reduced to two or 



three inches. The rate of motion during 



the winter was only about one half that 



during the summer. 



A further resemblance of the motion of 



a glacier to that of a river appears in the 



fact that the ice near the top moves faster 



than that near the bottom. At a point in 



the Mer de Glace where the side of the gla- 

 cier is exposed, presenting a wall of ice 



about one hundred and fifty feet in height, 



Professor Tyndall drove three stakes ; one 



at the summit of the ice, another thirty-five 



feet from the bottom, and another four feet 



from the bottom. Upon examination of 



them, at the end of twenty-four hours, it 



appeared that, while the top stake had 



moved forward six inches, the middle one 



had moved but four and a half inches, and 



the bottom stake but two and two thirds of 



an inch. 



In all these experiments the influence of fiiction is clearly 



visible. The ice of the glacier is retarded by the friction of 



the sides and bottom of 

 the channel through which 

 it moves, so that the most 

 rapid motion is upon the 

 surface, near the middle, 

 the part farthest removed 

 from this retarding influ- 

 ence. 

 A little attention to this last principle will prepare the 



mind for crediting the observations which more recently 



have been reported from the large glaciers in Greenland and 



Alaska, showing a motion tifteen or twenty times that of 



the Alpine glaciers. As the cross-section of a glacier is in 



Fig. 2.— The continu- 

 ous lines define the 

 valley occupied hy 

 the glacier. The 

 dotted line with the 

 arrow - heads indi- 

 cates the line of 

 most rapid motion 

 in the ice. showing 

 its more sinuous 

 course. 



PlO. S. — a, b. c, are stakes driven in the vertical 

 wall of the side of a glacier ; a', b'. c'. are 

 the points occupied at a subsequent date. 



