98 ROYAL SOCIETY OF CANADA. 



M. Arag-o, in his paper^ refers to three circumstances, as partly, 

 at least, explanatory of the formation of gromid-grii in rnnning water. 



1st. The inversion, by the- motion of the current, of the hydros- 

 tatic order, by which the water at the surface, cooled by the cold air, 

 and which at all points of the temperature of water under 39° Fahr. 

 would, in still water, continue to float on the surface, is mixed with 

 (he 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°, sim- 

 ilar 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 crys- 

 tals 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 reflection 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 tlie 

 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 formation of ice either at the bottom or at the sur- 

 face. 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 anothfier solution of the problem. 



M. Arago, in his conclusion, does not present these three circum- 

 stances as a complete 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 tem- 

 perature 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, float- 

 ing 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. 



