[BARNES] ANCHOR-ICE FORMATION 69 



back again. On a clear night in winter, with little or no motion in 

 the air, the circumstances are entirely conducive to excessive radiation, 

 "We know from what knowledge we have of the radiation from hot 

 bodies that only a small proportion of the heat can penetrate a la}-'er 

 of water. Hence the heat of the sun is mostly absorl>ed in the first 

 few feet of water. Only a small proportion of the rays ever reach 

 the bottom. On the other hand, the radiation from the bottom is 

 quite different and consists of long rays which we have every reason 

 to believe penetrate the water much more easily. It might at first 

 sight appear that there was a much larger amount of heat radiated 

 into the river by the sun, whose temperature is so high, than is radiated 

 out of the river into space. This is apparently quite true, and but 

 for the absorption by the water of the greater part of the sun's heat, 

 and the consequent lessening of the heat which actually reaches the 

 bottom, it is a question whether anchor-ice would ever form. Very 

 little is known in regard to cold body radiation, and it is not impossible 

 that little or no heat is absorbed by the water, if radiated from the 

 bottom into space at the absolute zero. 



The influence of the sun is everywhere observed in the formation 

 of both frazil and anchor-ice. In the former by warming the water 

 and preventing it from becoming undercooled, and in the latter by 

 loosening the masses of anchor-ice and causing them to rise. Frazil 

 iR never observed to have a bad effect under a strong sun. 



That portion of the heat from the sun, the long waves, that can 

 penetrate the water, is effective in melting off the hold which the anchor- 

 ice has on the bed of a river. A common sight in the early morning, 

 after a cold, clear night, when the sun rises, is the appearance of 

 masses of anchor-ice. These rise and float down with the current in 

 great quantity. Boatmen are very careful when crossing a river never 

 to go when these masses are rising, from the danger of being sur- 

 rounded and caught in a mass of anchor-ice and carried down by the 

 stream helpless into the rapids. 



If we sum up the various facts of common observation in 

 connection with anchor-ice, we see that everything points to radiation 

 as the prime cause. Thus we find that a bridge or cover prevents 

 the ice forming underneath. Such a cover would act as a check to 

 radiation and reflect the heat waves back again to the bottom. Anchor- 

 ice rarely forms under a layer of surface ice. It forms on dark rocks 

 more readily than light ones, which is in accord with what we know 

 in regard to the more copious radiation of heat from dark surfaces. 

 Anchor-ice never forms under a cloudy sky either by day or night, 

 no matter how severe the weather, but it forms very rapidly under a 



