Cqap. XVII.] THE WINDS AND EVArORATlON. 187 



with the natural action of vis-inertice as described in 

 Book 11. , according to which the water flows west in 

 the equatorial regions ; because, the westward pressure 

 in those regions is the greatest of all current- creating 

 forces : also, the water carried west in the zone of 

 greatest force must return east through the zones 

 of lesser force in higher latitudes ; and, therefore, 

 flows from the equator on the west side of each 

 ocean, and returns to the equator on the east : and, 

 the under- currents which convey cold water to 

 the equator, tend to the eastern side of the ocean ; 

 because, their natural tendency westwards must yield 

 to the greater force of the westward tendency of the 

 upper strata wherever the latter can draw no supply 

 directly from the east. The current which flows 

 west in the equatorial regions draws its supply from 

 higher latitudes, because in those latitudes the force 

 of westward pressure is less ; and it draws this 

 supply chiefly through under-currents, because the 

 westward pressure at the bottom of the ocean is less 

 than at the surface. 



Doubtless, if no more powerful agencies were in 

 play, then, the disturbed equilibrium resulting from 

 difl'erences of temperature and other conditions in 

 polar and equatorial regions, would cause an inter- 

 mixing of the waters of the ocean, by means of a 

 system of currents causing a constant mterchange of 

 equatorial and polar waters. But, we have to con- 

 sider how the system of oceanic circulation which 



