44 PHYSIOGRAPHIC GEOLOGY. 



The rate of flow of the tropical current is increased somewhat after 

 striking the borders of a continent, because of the diminished deptli. As it 

 passes on beyond the parallel of 30° and 35°, the flow becomes more and more 

 easterly in course, in consequence of loss of motion by friction. In the 

 tropical region, the movement ivesticard indicates a less rapid rate of move- 

 ment than the earth's surface, in its daily eastward rotation. But beyond 

 30° the rate of flow is faster than the rotation there ; and hence the result is 

 an eastward movement. As the waters continue on to the Arctic, friction 

 further diminishes the flow, and while part goes on northeastward north of 

 Asia, the rest lags and goes northward and northwestward. From the full 

 polar seas the waters must of necessity escape southward ; the lagging part 

 takes a course along the Greenland border and down Baffin Bay, making 

 the Labrador current ; and also a submarine course along the western half 

 of the ocean's bottom, while the rest returns along the ocean's bottom, — 

 especially along its eastern half, — and thus the Atlantic circuit is completed. 



In the accompanying sketch, WE is the equator with 30° and 60° par- 

 allels of latitude north and south of it. North, the ellipse represents the 



general movement in the north Atlantic ; the branch 

 26 



at P, the flow poleward, and the current at L, the 



returning Labrador current. 



The trends of the continental coasts, and their larger 

 bays, gulfs, or seas, and bordering island groups, have 

 much influence on the course and character of the 

 current. Owing to the position of the north coast of 

 South America with reference to the opposite coast of 

 Africa, the circuit-stream of the south Atlantic as it 

 flows westward contributes a considerable branch to 

 the north Atlantic circuit ; and because of the outlet 

 among the East India Islands, the circuits of the north 

 and south Pacific lose part of their waters by their 

 passing off into the Indian Ocean ; and still they are plainly distinguishable 

 off Japan, and off Australia, in the currents and their temperature. 



The most remarkable example of the effect of gulfs or seas and islands 

 is that afforded by the West India seas and islands. The West India sea 

 faces part of the slowly advancing ocean-stream. It has an area of nearly 

 2,000,000 square miles. Though rudely fenced in by the Windward Islands, 

 there are spaces over 3000 feet deep between the most of them, and less 

 than twice this at the chief entrance. Prom the Caribbean Sea the waters, 

 after a circuit, escape partly between the islands northwestward ; but part 

 pass the narrow Yucatan channel with an hourly movement of one fourth of 

 a mile, and raise the level of the Gulf of Mexico three feet above its nat- 

 ural level, and, at the same time, act as a hydrostatic reservoir to make of 

 the escaping waters the Gulf Stream, which flows through the Florida straits 

 (according to Commander Bartlett) at a mean rate of three miles an hour, 

 and at a maximum, for 15 miles in the axis of the stream, as high as b\ 





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