THE OCEAN . 337 



eastern coasts of the continents, where in some places they form 

 distinct currents. 



Cause of ocean currents. It is now generally believed that 

 the equatorial drifts are produced by the trade-winds. Outside 

 the tropics the winds do not blow in one direction all the time, 

 and so do not produce persistent currents. In regions of strong 

 monsoon winds, as about India, the drift of the surface waters 

 changes with the shifting winds (Figs. 233 and 235), thus showing 

 that steady winds are able to produce movements of the water. 



If the ocean covered all the earth, the westward drift of the 

 equatorial waters, caused by the trade-winds, would go round and 

 round the earth. But the continents prevent this, and where the 

 waters of the equatorial drift reach their shores, they are turned 

 from their westerly course to the north or south. 



After the moving waters pass out of the zone of the trade-winds, 

 their course is directed chiefly (1) by the shores, (2) by the pre- 

 vailing winds, and (3) by the rotation of the earth. Their courses 

 are therefore given them partly by the causes which make them, 

 and partly by other causes which direct them. 



Climatic effects of ocean currents. The ocean current itself does 

 not warm or cool the land, but the air over a warm ocean current is 

 heated by the water, and may then be carried over to the land. 

 In middle latitudes, for example, the westerly winds carry the air 

 warmed by the warm currents over to the coasts of the continents 

 lying east of them. This makes the coasts on the east sides of the 

 oceans, in the intermediate zones, warmer in winter than they 

 would be otherwise, and gives them, at the same time, plenty 

 of moisture. The winter temperature of the west coast of north- 

 ern Europe (Fig. 217) is less severe than it would be but for the Gulf 

 Stream. The warm current in the North Pacific lessens the cold of 

 winter along the northern part of the west coast of North America. 

 Similar results would be found in the southern hemisphere, if there 

 were land so situated as to feel the effects of the* warm currents in 

 the southern oceans. 



Warm currents often give rise to fogs both at sea and on land. 

 When the wind blows over a warm current, such as the Gulf Stream, 

 it takes up a goodly supply of moisture. If it then blows over 



