CLOUDS AND RIVERS, ICE AND GLACIERS. 121 



the expansion by cold culminates. Up to the point of 

 solidification the increase of volume is slow and gradual ; 

 while in the acb of solidification it is sudden, and of 

 overwhelming strength. 



312. By this force of expansion the Florentine Acade- 

 micians long ago burst a sphere of copper nearly three 

 quarters of an inch in thickness. By the same force 

 the celebrated astronomer Huyghens burst in 1667 iron 

 cannons a finger breadth thick. Such experiments 

 have been frequently made since. Major Williams 

 during a severe Quebec winter filled a mortar with 

 water, and closed it by driving into its muzzle a plug 

 of wood. Exposed to a temperature 50° Fahr. below 

 the freezing point of water, the metal resisted the 

 strain, but the plug gave way, being projected to a 

 distance of 400 feet. At Warsaw howitzer shells 

 have been thus exploded ; and you and I have shi- 

 vered thick bomb-shells to fragments, by placing them 

 for half an hour in a freezing mixture. 



313. The theory of the shafts and pits referred to 

 at the beginning of this section is this : — The water 

 at the surface of the shaft is warmed by the sun, say to 

 a. temperature of 39° Fahr. The water at the bottom, 

 in contact with the ice, must be at 32° or near it. The 

 heavier water is therefore at the top ; it will descend 

 to the bottom, melt the ice there, and thus deepen the 

 shaft. 



314. The circulation here referred to undoubtedly 



