OCEAN CIRCULATION. 475 



of its tropical warmth to countries between 

 which and the source of heat in the current, 

 lies the broad bosom of the great Atlantic. 

 The polar currents, on the contrary, pour their 

 cold waters upon the heated shores of the burn- 

 ing tropics, thus mitigating the intensity of 

 their temperature, and communicating a grate- 

 ful coolness to regions otherwise comparatively 

 intolerable. 



It has been before mentioned that there is a 

 notable difference with regard to the amount 

 per cent, of their saline ingredients in the waters 

 of the tropical seas, and of those lying more 

 to the north, in consequence of the greater 

 amount of evaporation suffered by the former 

 compared with the latter. A current, there- 

 fore, setting out from tropical regions, and ex- 

 tending to the Polar seas, will convey its high 

 charge of saline matter with it, and throughout 

 its track. On the other hand, a current setting 

 out from the Poles will carry with it water less 

 charged with saline contents. By this means 

 a perpetual circulation of these ingredients is 

 maintained, and the uniform composition of 

 ocean-water is secured. The importance of 

 these two classes of duties fulfilled by marine 

 currents to the preservation of an uniformity 

 of temperature and composition in the whole, 

 can scarcely be exaggerated ; and there can be 



