THE WORK OF THE OCEAN. 337 



strong, they determine a general movement of the surface-waters in 

 their own direction, the surface-water being dragged along at a rate 

 somewhat less than that of the wind itself. The constant trades appear 

 to be the chief generators of the equatorial ocean-currents. Once 

 generated, these currents may be concentrated and their courses modi- 

 fied. The currents generated by trades are turned north and south 

 when directed against a continent ; they are modified by the configura- 

 tion of the bottom if the water be shallow, and always and everywhere, 

 except, at the equator, they are deflected by the rotation of the earth, 

 in the northern hemisphere to the right, and in the southern to the left. 

 The pole-ward currents generated in the equatorial region by the trades, 

 and directed by the winds, the lands, the configuration of the bottom, 

 and the rotation of the earth, determine compensating currents from 

 high latitudes to low, and the same influences which control the course 

 of the former direct the latter as well. 



Since the atmospheric movements are so far constant that there is 

 a prevailing direction of winds in all latitudes, the winds, as well as 

 differences of density and differences of level, insure a general and con- 

 tinual circulation of sea-water. The geological effects of this circula- 

 tion are direct and indirect; direct, by gradation of the bottom over 

 which they flow, and indirect, by the modifications of chmate they pro- 

 duce. Since rotation deflects the pole-ward currents to the east sides 

 of the oceans (west sides of the continents) and the equator-ward move- 

 ments to the west sides of the oceans (east sides of the continents), the 

 east shores of the oceans are warmer than the west in corresponding 

 latitudes, and the west sides of the continents are both warmer and 

 moister ^ than the east sides. 



The most obvious disturbance of sea-water resulting from the winds 

 is the generation of waves. Waves are not primarily parts of the general 

 oceanic circulation. Since they are generated in other ways than by 

 winds, and since the gradational effects of waves are independent of 

 their origin, the effects of wind-waves will not be considered separately. 



Movements generated by attraction. — One of the movements of the 

 sea-water which is not primarily circulatory results from the attraction 

 of the moon and sun. The tide is really the result of the inequalities 

 of the attraction of these bodies on different parts of the earth. The 



^ This does not hold for tropical latitudes. 



