NOTES ON ANCHOR ICE. 177 



would seem impossible that it could have been deposited unless formed 

 where found. It has been found upon smooth rock in rapid water 

 ten feet in depth ; and it has been seen to burst up from the flat rock 

 bed of the St. Lawrence, at the head of the Longue Sault rapids, 

 where there is a depth of twelve feet. I have seen it rising to a sur- 

 face already nearly covered with it, and at the same time, have felt it 

 with a pole upon the stony bottom in upwards of twelve feet of 

 water. 



A remarkable instance of the formation of ice under a considerable 

 depth of water occurred in the winter of '.56-'5r, at New York. 

 The gutta-percha pipe — about 2|- inches diameter — which supplies 

 " Black well's Island" with Croton water, was frozen solid while rest- 

 ing on the bottom in a tide way at a depth of at least twenty-five feet 

 below the surface. The flow in the pipe had been arrested by an 

 obstruction, and the East River was at the time covered with floating 

 ice. As the water within the pipe was fresh and very pure it could 

 become solid while the surrounding salt water remained liquid, but it 

 may have been over-grown with anchor ice. 



The temperature of running water falls considerably below 32* 

 without congealing, and therefore anchor ice is not melted. It is pro- 

 bably often formed in water at a temperature as low as 28° or 29*^, 

 which is not impossible if the air is 30*'' below zero ; and although 

 no anchor ice is formed under the solid surface ice, that which has 

 been carried there does not melt, — shewing that the water in motion 

 under the surface ice below rapids does not soon recover the warmth it 

 has lost in traversing the shallow open reaches above. 



The appearance of the open water above the Lachine Rapids, after 

 a cold period, seems to prove both the place of origin and the fact of 

 rising of anchor ice ; but how it is formed, and why it rises, are 

 questions of interest which I have never seen explained, and to obtain 

 answers to which is the object of these notes. 



"With respect to the mode of formation, it is analagous to that by 

 which dew or hoar frost is formed on the surface of the earth, and 

 is probably due to radiation from the warmer bed of the stream to 

 the colder surface current, and still colder atmosphere. When the 

 temperature of the air rises above 40", the surface of the snow covered 

 ground remains colder than the atmosphere, and radiation ceases. 

 When it ceases the power which kept anchor ice at the bottom is 

 suspended ; and from this circumstance, and the regular rising of the 



