180 The Realm of Nature CHAP. 



wind. Ocean currents are very different from surface drift. 

 They are usually narrow tracts of the sea surface, the water 

 of which flows steadily and strongly in a definite direc- 

 tion, passing through the rest of the sea without appreciable 

 mixing, as a river runs through a meadow. Some of these 

 ocean rivers flow steadily in a constant direction at the rate 

 of nearly 4 miles an hour ; thus it is matter of importance 

 to sailors to map out the ocean so that they may avoid 

 or take advantage of the currents in making a passage. 

 Solar energy in one form or another is undoubtedly the 

 power that keeps the whole system of oceanic circulation in 

 motion, and the rotation of the Earth ( 89) together with 

 the form of the coast-lines of the continents direct the flow 

 of currents. Sun-power acts on the hydrosphere (a) by 

 "raising the temperature in the tropical regions far above 

 that in the polar zones, thus causing expansion and altering 

 the level ; () by causing great evaporation in the tropical 

 regions, great rainfall in equatorial regions and moderate 

 rainfall in the temperate zone, thus altering both level and 

 density; (<r) by setting up the whole system of winds. Some 

 difference of opinion exists as to the chief cause of oceanic 

 movement, but it is usually allowed that the most powerful 

 is the wind. All three, however, act together and reinforce 

 each other. If the student compares the map of ocean 

 currents (Plate XVIII.) with those of temperature, of salinity, 

 and of prevailing winds (Plates XV. VIII. V. VI.), he will 

 see that the currents circulate in the same way as the winds 

 and around nearly the same centres, which lie close to the 

 regions of maximum sea - temperature and salinity. All 

 ocean currents are more or less irregular in form and speed ; 

 they usually flow as parallel streams separated by spaces of 

 still water, and vary in position and strength, as the winds 

 do, with the time of year. Plate XVIII. should be specially 

 referred to in reading the following paragraphs. 



243. Equatorial Currents of the Atlantic. The trade 

 winds blowing from the west coast of Africa drive the 

 surface water before them in rapid currents. The North 

 Equatorial Current, sweeping along the north-west coast 

 of Africa past the Canary Islands, turns toward the west 



