206 WRITINGS OF JOSEPH HENRY. [1855- 



principles already stated that the line of separation would 

 in preference pass through a house, as this is the direction of 

 least resistance, for the cellar may be considered as a line of 

 fissure between the two masses of earth, or a crack already 

 commenced. 



During a very cold night when the temperature is rapidly 

 diminishing, and the ground covered with snow slightly 

 encrusted on the surface by previous thawing and freezing, 

 a continued series of minute explosions may be heard de- 

 pending in frequency and loudness upon the thickness or 

 thinness of the crust. In some cases it resembles a crack- 

 ling, and at others a series of distant though not loud or 

 sharp explosions. 



There is a phenomenon connected with ice in rivers 

 which has given rise to much discussion as to its cause. I 

 allude to the freezing which takes place at the bottom of run- 

 ning streams, where in some cases the ice remains until it is 

 separated by its buoyancy and rises to the surface. It pre- 

 sents a peculiar angular appearance, and is sometimes known 

 by the name of anchor ice. Its formation appears to be an 

 exception to the general rule of the freezing of water, which 

 on account of the decreasing density usually takes place at 

 the surface. It was at first supposed that it was due to the 

 radiation of heat through the clear water above ; but Arago 

 has shown that this explanation cannot be the true one, 

 since rays of low temperature cannot pass through water, 

 and hence no such radiation can take place. A more prob- 

 able explanation has been given, I think, by the same 

 author, in referring it to the fact that still water can be re- 

 duced below the freezing point without congealing, and that 

 it will immediately be converted into ice if a bit of solid 

 matter be thrown into the vessel in which the experiment is 

 made, which may serve as a nucleus for the crystallization. 

 When water in this state is passing through a rapid channel 

 it is mixed together and the coldest as well as the warmest 

 part is brought into contact with the bed of the stream, the 

 materials of which acting as a point of rest serve as a basis 

 of crystallization. 



