HOW WATER TAKES UP HEAT. !()!) 



jected, during the day-time, to an almost uninter- 

 rupted evaporation ; that is, to say, a part of the heat 

 tlirown in from without being employed in making 

 vapour, it passes into the state of combined heat, 

 and ceases therefore to contribute to the raising of 

 the temperature. 



When the external temperature is falling, the 

 waters are commonly warmer than the land. It is 

 true that the surface of the water yields up its heat 

 by radiation as readily as does the ground ; but 

 then the water so cooled becomes denser and 

 heavier, and consequently sinks, while some that 

 is warmer rises in its stead, so that nevertheless 

 the cooling can proceed but slowly. Besides, as 

 the temperature falls, the evaporation is diminished, 

 and at last, under certain conditions, of which I 

 shall have to write hereafter, ceases altogether. 



The waters, especially where they are in great 

 abundance, tend therefore to draw the limits of 

 highest and lowest temperature close together. 

 They check the direct action of the sun's heat, 

 and contribute in the same manner to moderate the 

 cool of night and the cold of water. 



Brooks and rivers, whose depth is but small, 

 soon acquire a pretty uniform temperature through 

 their whole mass. When the external temperature 

 falls below 0° C. (32° P.), that of running 

 water cannot be below 0°, because, at the very 

 first formation of ice, as much of the combined 



