186 THE OCEAN. [Book VIH. 



Besides the objections urged by Sir Jobn Herschel 

 against the theory which makes differences in specific 

 gravity the prime cause of the currents of the ocean, 

 it must be observed that, if differences in specific 

 gravity, resulting from the difference of temperature 

 and other conditions in polar and equatorial regions, 

 were the principal cause of ocean currents — in conse- 

 quence of the tendency of the heated and cold water 

 to exchange positions in order to re-establish their 

 equilibrium in specific gravity — then, the heated 

 water flowing from the equator would be under that 

 influence of change of latitude which tends to carry 

 it eastwards, and the cold water from the polar regions 

 would be under that influence of change of latitude 

 which tends to carry it westwards ; so that, therefore, 

 the warm water would naturallv flow from the 

 equator on the east side of the ocean, and the cold 

 water as naturally flow to the equator on the west 

 side of the ocean ; whereas in fact, with the existing 

 currents of the ocean, the very reverse is the case ; 

 the cold water is brought to the equator partly by 

 currents running towards the equator on the eastern 

 side of each ocean, and partly by under-currents 

 rising to the surface chiefly in the eastern parts of 

 the ocean ; then, it flows westwards through the equa- 

 torial regions, and flows from the equator on the west 

 of the ocean, as warm currents ; having been heated 

 during its course through the equatorial regions. 

 This actual course of the currents is in accordance 



