FORMED AT THE BOTTOM OF RUNNING WATER. 337 



perature of water under 39° Fahr. would, in still water, continue to float on the sur- 

 face, is mixed with the warmer water below, and thus the whole body of water to the 

 bottom is cooled alike by a mechanical action of the stream : 



2nd. The aptitude to the formation of crystals of ice on the stones and asperities of 

 the bottom, in the water wholly cooled to 32°, similar to the readiness with which 

 crystals form on pointed and rough bodies in a saturated saline solution : 



3rd. The existence of a less impediment to the formation of crystals in the slower 

 motion of the water at the bottom, than in the more rapid one near, or at the surface. 



There is no denying the justness of these three positions, and yet the slightest re- 

 flection teaches us that neither singly nor combined do they aid us in answering the 

 main question before us, " Why is ice formed sometimes at the surface of running 

 water, and sometimes at the bottom ?" All the circumstances, or conditions, referred 

 to by M. Arago, are present when ice, as most frequently takes place, is in the course 

 of being formed only on the surface, as well as when the formation is going on at the 

 bottom. Were we to admit them as an answer to our question, then running water 

 ought always to freeze first at the bottom. But a most extensive experience teaches 

 us that this is not the case. The illustrations of M. Arago, indeed, just and true in 

 themselves, are not to be overlooked when we would investigate and explain the for- 

 mation of ice either at the bottom or at the surface. They will serve to enlighten us 

 greatly in both these events, but they have no exclusive relevancy to either, and we 

 must therefore look out for another solution of the problem. 



M. Arago, in his conclusion, does not present these three circumstances as a com- 

 plete explanation ; but he says, the reader may ask why he has not done so, and he 

 answers to this, " that we have no observations which prove that this kind of ice is 

 seen, until the temperature of the whole of the water is at zero " (centigr.) ; and that 

 it is not certain that the little icy particles, seen by Mr. Knight, floating on a milldam, 

 at the time ground-ice was forming in the stream, and which may have acquired in 

 contact with the air a temperature below zero (centigr.), do not play an important 

 part in the phenomenon which he has overlooked. 



In regard to the former of these points, I cannot say what M. Arago would have 

 deduced from it, had it been established in one way or the other. The observations 

 made on the Don on the 5th of January show that the temperature of the whole 

 water was not quite down to 32° Fahr. when the ground-gru was forming in large 

 quantity. In regard to the latter, the little icy particles seen by Mr. Knight, the 

 same condition belongs to them that belongs to the circumstances professedly adduced 

 by M. Arago as explanations ; that is, they occur as well when the ice is forming on 

 the surface only as when it is forming on the bottom. They account well, however, 

 for the collections of frozen matter seen by him at the sides of the stones opposed to 

 the stream, in parts where its velocity had a certain modification. 



And here I may advert to the explanation offered by the Rev. Mr. Eisdale, in his 

 paper already referred to. From the information he received, he was led to believe 



2x2 



