248 



HELMHOLTZ 



taken from the frozen surface of the river, with its two 

 plane terminal surfaces between the plates of the press. If 

 I begin to work, the block is broken by pressure; every 



FIG. 114 



FIG. 115 



crack which forms extends through the entire mass of the 

 block; this splits into a heap of larger fragments, which 

 again crack and are broken the more the press is worked. 

 If the pressure is relaxed, all these fragments are, indeed, 

 reunited by freezing, but the aspect of the whole indicates 

 that the shape of the block has resulted less from pliability 

 than from fracture, and that the individual fragments have 

 completely altered their mutual positions. 



The case is quite different when one of the cylinders 

 which we have formed from snow or ice is placed between 

 the plates of the press. As the press is worked the creak- 

 ing and cracking is heard, but it does not break ; it gradually 

 changes its shape, becomes lower and at the same time 

 thicker; and only when it has been changed into a tolerably 

 flat circular disc does it begin to give way at the edges 

 and form cracks, like crevasses on a small scale. FIG. 114 

 shows the height and diameter of such a cylinder in its 

 original condition; FIG. 115 represents its appe-arance after 

 the action of the press. 



A still stronger proof of the pliability of ice is afforded 

 when one of our cylinders is forced through a narrow 



