GENERAL FEATURES OF THE EARTH. 39 



extratropical waters (see Fig. 30) outward toward the polar region, 

 to return thence with the polar waters mainly along the western side 

 of the ocean (though partly by the eastern). 



(3.) A flow of the colder current under the warmer when the two 

 meet, since cold water is heavier than warm. 



(4.) A lifting of the deep-seated cold currents to the surface along 

 the sides of a continent or island, or over a submerged bank, as on 

 the west coast of South America. 



(5.) A movement of the circuit, as a whole, some degrees to the 

 north or south with the change of the seasons, or as the sun passes to 

 the north or south of the equator. 



(6.) On the west side of an ocean (see Fig. 30), the cold northerly 

 current is mainly from the polar latitudes ; on the east side, it is mainly 

 from the high temperate latitudes, being the cooled extratropical flow 

 on its return. 



(7.) The tropical current has great depth, being a profound move- 

 ment of the ocean, and it is bent northward in its onward course by 

 the deep, submerged sides of the continents. The Gulf Stream has 

 consequently its main limit 80 to 100 miles from the American coast, 

 where the ocean commences its abrupt depths (p. 11). Hence, a sub- 

 mergence of a portion of a continent sufficient to give the body of the 

 current a free discharge over it would have to be of great depth, — 

 probably two thousand feet at least. 



The usual explanation of the courses is as follows : As the earth 

 rotates to the eastward, the westward tropical flow is due simply to a 

 slight lagging of the waters in those latitudes. But transfer these 

 waters toward the pole, where the earth's surface moves less rapidly 

 (the rate of motion varies as the cosine of latitude), and then they 

 may move faster than the earth's surface, and so have a movement to 

 the eastward. The earth's rotation is not supposed to be a cause of mo- 

 tion in the waters ; but, there being a movement, for other reasons 

 (which it is not necessary here to consider), from the equator toward 

 the poles, and from the higher latitudes toward the equator, it gives 

 easting to the flow in the former direction, and westing to the flow in 

 the latter. 



On the same principle, any waters flowing from the polar regions 

 (where the earth's motion at surface is slow) toward the equator would 

 be thrown mainly against the west side of the oceans (as the Labrador 

 current in the North Atlantic) ; for they have no power to keep up 

 with the earth's motion. But the waters flowing toward the pole, that 

 have not lost much of their previous eastward moving force, may 

 descend to lower latitudes along the east side of the ocean. 



Put the above figure in either the Atlantic or Pacific, and the sys- 

 tem for the ocean will be apparent at a glance. 



