LETTEE XI. 



ON THE TEMPERATURE OE THE WATERS, AND ON 

 THEIR INFLUENCE ON CLIMATE. 



The temperature of the waters, of the rivers, the 

 lakes, the seas, is, while the heat is increasing, 

 usually somewhat lower than that of the solid 

 ground, if exposed to the same amount of heating 

 influence. 



The capacity of water to take heat into its sub- 

 stance, is about four times as great, on the average, 

 as that of the ground; in other words, water 

 requires four times as much heat as does the 

 ground, to raise the temperature of an equal 

 weight of it a given number of degrees. Moreover 

 the rays of the sun penetrate through the water, 

 although with less ease than through the air, to 

 a great depth, before they are wholly absorbed or 

 converted into sensible heat. Their warming 

 power is therefore extended through a greater 

 mass, — a mode of action which is very much 

 favoured by the mobility of water, by the waves 

 and the currents. 



Besides, the open surface of the water is sub- 



