Rivers during' Winter, 133 



of radiation : when the atmospherical temperature is very low, 

 they will, of course, freeze the water which touches them. 



It is unnecessary to examine here, whether heat radiates 

 through a thick layer of water, as Mr M'Keever supposes, as 

 the most simple observation is sufficient to overthrow it. 



Where is the person who has not observed, that the strong 

 radiation which the Irish philosopher admits, would be more 

 plainly manifested, or as completely, in still water than in run- 

 ning water ; but no one has seen a piece of still water frozen at 

 the bottom ? 



Let us throw aside all these absurd explanations, and, for 

 want of better, analyze perspicuously the physical condition of 

 the question. 



If liquids of different densities are thrown into a vessel, the 

 heavy will sink to the bottom, the light keep at the top. 



This principle in hydrostatics is general. It applies as well 

 to liquids possessing different chemical properties, as to portions 

 of one and the same liquid whose densities are dissimilar, in 

 consequence of inequalities in the temperature. 



Liquids, like all other bodies, solid or gaseous, increase in 

 density as their temperature diminishes. 



Water alone, in a certain small extent of' the thermometric 

 scale, presents a singular exception to this rule. Suppose wa- 

 ter is taken at -|- 10° centigr. and gradually cooled, at 9° we 

 shall find it denser than at 10°, at 8° more than 9°, at T more 

 than 8°, and so on till 4° ; at this point condensation will cease. 

 In going from 4° to 3° for example, there is a manifest diminu- 

 tion of density. This diminution will go on till the temperature 

 falls from 3 to 2, from 2 to 1, and from 1 to zero. To con- 

 clude, water has a maximum of density, which does not coin- 

 cide with its term of congelation. At 4° above zero is the 

 maximum of density. 



There is nothing so simple as to point out in what manner 

 the congelation of stagnant water takes place. 



Let us suppose, as is always the case, that at the moment 

 when the wind blowing from the north produces ice, the water 

 throughout to be at -f- 10°. The cooling of the hquid, by 

 coming in contact with the glacial air, will be affected from the 

 exterior to the interior. The surface which, hypothetical ly 



