122 Temperature of the Sea an:i the Air 



If we consider the film of water immediately at the surface 

 of the sea, having the atmosphere on the one side of it and 

 the bulk of the water on the other, it is strictly comparable 

 with the film of water surrounding the bulb of the thermometer, 

 when exposed to the atmosphere in the way described above; 

 and the air playing upon it must produce exactly the same 

 effect in the one case as in the other. The evaporation lowers 

 the temperature of the aqueous film, which proceeds to extract 

 heat from the neighbouring bodies namely, in the one case 

 the air and the bulb of the thermometer, and in the other 

 case the air and the layer of water immediately below the 

 surface film. If we imagine for a moment the surface film 

 separated from the bulk of the water below it by a diaphragm 

 impervious to heat and exposed to the atmosphere so as 

 to suffer evaporation and lowering of temperature, then on 

 the removal of the diaphragm it would immediately sink 

 away from the surface and its place would be taken by warmer, 

 and therefore less dense, water from below. In the case of 

 sea-water this effect would be slightly intensified by the con- 

 centration produced by evaporation. But while the water 

 below supplies some of the heat rendered latent by the 

 evaporation of the water, the air above it supplies its share, 

 and is cooled. In both cases the heat thus lost is made good 

 by the direct radiation from the sun. Through a moderately 

 dry atmosphere the rays pass with comparatively little heating 

 effect, but are largely absorbed on entering the water. 

 Consequently the loss of heat which the water suffers by 

 evaporation at the surface of separation is made good more 

 abundantly than that sustained by the air; and the difference 

 in power of absorption of radiant heat exhibited by these 

 two substances is thus sufficient to keep up a permanent 

 difference of temperature between the water and the air 

 immediately above it. 



Starting with air and water at the same temperature, we 

 may imagine the process as taking place in three acts. First, 

 the water at the surface evaporates, and the air on the one 

 side, and the water on the other, are cooled ; second, in order 

 to make up for the heat thus rendered latent and lost, the 



