Our Salt'JVater World 325 



with that of the normal temperature of the latitude. 

 However, this is true only of the surface water. It has 

 been shown that the temperature diminishes very rap- 

 idly with the depth, in tropical and temperate latitudes 

 particularly, till at great depths the ice-cold tempera- 

 ture of 35° Fahr. everywhere prevails. At the equator, 

 where the surface temperature is about 80°, the de- 

 crease with the depth is so rapid that at 360 feet from 

 the top the temperature is less than 61°; at 190 feet 

 it falls to 50°; at 4,200 feet it again falls to 40°; and 

 at a little more than one and three-fourth miles it is 

 36°. Below this depth it falls at a much slower rate, 

 till in some regions it approaches very close to freezing 

 temperature, which, for sea water, is about 28.6°. In 

 the equatorial belt the solar heat affects the water of 

 the upper four hundred feet; but it is a remarkable 

 fact that immediately beneath this sun-heated stratum 

 the water in the North Atlantic as far as the fortieth 

 latitude is warmer than that at the same depth at the 

 equator. 



In a peculiar sense the ocean has its rivers and 

 lakes as well as the land, its mighty cu'^rents and 

 areas of distinct specialization. The rivers cross 

 and recross, they diverge and come together, and in 

 many cases they do so without mingling their waters 

 to any great extent. The lakes, or little s'^as, are 

 no less distinctive in their characteristics. They are 

 not distinguished by the character of their currents 

 or margins, however, but by the floating vegetation 

 they contain. 



The nearest of these tracts to our shores is the Sar- 

 gasso Sea in the Atlantic, between Bermuda and the 



