HELMHOLTfc 



of snow, and then the cylindrical plug, C C, which fits the 

 inner aperture, but moves in it with gentle friction, is 



forced in with the aid of an 

 hydraulic press. The press 

 used was such that the pres- 

 sure to which the snow was 

 exposed could be increased to 

 fifty atmospheres. Of course 

 the looser snow contracts to a 

 very small volume under such 

 a powerful pressure. The 

 pressure is removed, the 

 cylindrical plug taken out, 

 the hollo wr again filled up with 

 snow, and the process re- 

 peated until the entire form 

 is filled with the mass of ice, 

 which no longer gives way 

 to pressure. The compressed 

 snow which I now take out, 

 you will see, has been trans- 

 formed into a hard, angu- 

 lar, and translucent cylinder 

 of ice; and how hard it is 

 appears from the crash which 



FIG. 113 



ensues when I throw it to the ground. Just as the loose 

 snow in the glaciers is pressed together to solid ice, so also 

 in many places ready-formed irregular pieces of ice are 

 joined and form clear and compact ice. This is most re- 

 markable at the base of the glacier cascades. These are 

 glacier falls where the upper part of the glacier ends at a 

 steep rocky wall, and blocks of ice shoot down as avalanches 

 over the edge of this wall. The heap of shattered blocks of 

 ice which accumulate become joined at the foot of the rock- 

 wall to a compact, dense mass, which then continues its way 

 downwards as glacier. More frequent than such cascades, 

 where the glacier-stream is quite dissevered, are places 

 where the base of the valley has a steeper slope, as for in- 

 stance, the places in the Mer de Glace (Fie. 105), at g, of 

 the Cascade of the Glacier du Geant, and at i and h of the 



