[BARNES] ANCHOR-ICE FORMATION 109 



is 45° Fahr., and that of the earth, of course, the same; and on the 

 remission of its cause, the gronnd-gru is here speedily detached from 

 the bottom, by the transmission of heat from below. Not so in the 

 Neva. There, according to Kiipffer, the mean temperature is only 

 38°. 75; and under the action of a frost so severe as to form three 

 feet depth of ice, although the condition of the clear sky might not 

 continue, previously formed gru vt'ould, at that mean temperature, be 

 of great permanence. This applies also to the Siberian rivers. 



