LETTEE XII. . 



THE CURRENTS OF THE SEA. 



I have already mentioned to you that the water 

 of the ocean, at great depths, has a temperature, 

 even under the equator, nearly approaching to the 

 freezing-point. This low temperature cannot 

 depend on any influence of the sea-bottom, since, 

 from all we know of the condition of the mass of 

 the earth, as regards heat, we may be sure that 

 the ground cannot tend to cool the water, though 

 it may have some effect in warming it. Again, 

 we know that the water at the bottom of lakes, 

 and of the southern inland seas, has by no means 

 so low a temperature. The fact however is ex- 

 plained by a continual current of cold water flowing 

 from the polar regions towards the equator. 



The following well-known experiment clearly 

 illustrates the manner of this movement. A glass 

 vessel is to be filled with water with which some 

 powder has been mixed, and is then to be heated at 

 bottom. You will soon see, from the motion of 



