326 Of the Propagation of Heat 



But if, for the purposes of life and vegetation, it be 

 necessary that the ground, the rivers, the lakes, and the 

 trees be defended from the cold winds from the poles, it 

 may be asked how this inundation of cold air is to be 

 warmed ? I answer by the waters of the ocean, which 

 there is the greatest reason to think were not only de- 

 signed principally for that use, but particularly prepared 

 for it. 



Sea water contains a large proportion of salt in solu- 

 tion ; and we have seen that the condensation of a sa- 

 line solution, on its being cooled, follows a law which is 

 extremely different from that observed in regard to pure 

 water ; and which (as may easily be shown) renders it 

 peculiarly well adapted for communicating Heat to the 

 cold winds which blow over its surface. 



As sea water continues to be condensed as it goes on 

 to cool, even after it has passed the point at which fresh 

 water freezes, the particles at the surface, instead of re- 

 maining there after the mass of the water had been 

 cooled to about 40°, and preventing the other warmer 

 particles below from coming in their turns and giving off 

 their Heat to the cold air (as we have seen always hap- 

 pens when fresh or pure water is so cooled), these cooled 

 particles of salt water descend as soon as they have 

 parted with their Heat, and in moving downward force 

 other warmer particles to move upwards ; and in con- 

 sequence of this continual succession of warm .particles 

 which come to the surface of the sea, a vast deal of Heat 

 is communicated to the air, — incomparably more than 

 could possibly be communicated to it by an equal quan- 

 tity of fresh water at the same temperature, as will ap- 

 pear by the following computation. 



Without taking into the account that very great ad- 



