[BARNES] ANCHOR-ICE FORMATION IDS 



M. Arago's illustrations also furnish us with a satisfactory explan- 

 ation of the curious facts, that the ground-gru makes its first appearance 

 in the more rapid and agitated parts of the stream, and begins to 

 show itself on the bottoms of the stiller parts, and to accumulate there 

 ill quantity, only after a longer continuance of the clear, frosty 

 weather. In the rapids the hydrostatic order is overturned, and the 

 colder, which is also the lighter, water not only mixed with the warmer 

 below, but, at the whirls of the greatest rapids, brouglit suddenly, 

 without much mixing, into direct contact with the bottom, cooled still 

 lower than itself by radiation. If the water is at the temperature of 

 32° Fahr. it can give out no heat to the colder bottom without part 

 of it being converted into ice, the spiculas and crystals of which find 

 a solid body for their attachment at the very point where the heat is 

 given out.^ 



But while in this manner we can explain some of the incidents, 

 may it not be held, as above demonstrated, that the chief cause of the 

 ground-gru is the radiation of heat from the bottoms of the rivers? 

 Every branch of the phenomenon is of easy explanation when we admit 

 the radiation; and among the rest a circumstance to which I have 

 yet made no reference, and that is, the disappearance at the bottom of 

 the water of the immense quantity of heat, 140° of Fahr., which con- 

 stitutes the caloric of fluidity disengaged, when water at 32° Fahr. 

 is converted into ice at the same temperature. 



The answer to our original question then is, That ice is formed 

 sometimes on the surface of running water, and sometimes at the bot- 

 tom, because frost sometimes takes place with a clouded sky, which is 

 incompatible with radiation of heat from the bottom of the stream, 

 and sometimes with a clear sky, when that radiation takes place through 

 tlie water, in the same manner as the experiments of Dr. Wells prove 

 it goes on, under a like sky, through the atmosphere. The bottom 

 is bv this cooled down below the freezing point of water, before the 

 water itself; ice is formed on it, and its detachment by transmitted 

 heat from below prevented as long as the radiation continues. 



' We may observe also, that there is a local source of greater cold of 

 the water in the rapids, in its being brought into more active and extensive 

 contact with the air by a sharp ripple and spray. 



