OCEANIC AND ATMOSPHERIC CURRENTS. 475 



over almost half of the distance from pole to pole. Another great current originates 

 in the Indian Ocean, flows into the China Sea through the Straits of Malacca, thence 

 into the North Pacific between the coast of Asia and the Philippine Islands; thence 

 crosses the ocean to the north-westward, modifying the climate of Oregon and Alaska. 



All the heated water thus poured from the tropical ocean, and all raised from it by 

 evaporation and transported through aerial channels to feed the rivers of the temperate 

 and polar regions, must tind its way back by counter currents. Heat, according to the 

 dictum of modern science, may be reduced to force. The force of the sun's rays 

 poured upon the tropical oceans is sufficient to raise thousands of yards into the air 

 five hundred cubic miles of water every day, and to put and keep in motion the 

 mighty currents which sweep back and forth from the equator to the poles. The 

 study of the course, direction, and elevation of these currents has as yet only begun. 

 We know that sometimes, as on the eastern coast of America, the currents of warm 

 and cold water run side by side in opposite directions ; sometimes a warm current is 

 on the surface, and sometimes below it. In the Gulf Stream the warm current is 

 above, the cold below ; while on the coast of Japan a cold current from the Sea of 

 Okhotsk runs on the surface, giving rise to a fishery not inferior in magnitude to that 

 caused on the banks of Newfoundland by the cold current from Baffin's Bay. Enough, 

 however, is now known of oceanic currents to warrant the assumption that they are 

 mainly governed by the great law of gravitation. The lighter water flows upon the 

 surface, the heavier flows underneath. But the specific gravity of ocean water depends 

 upon two things, the temperature and the amount of salts contained. The heated 

 water from the tropics is rendered lighter than that which surrounds it of the same 

 saltness, and so floats on the surface ; but the cold currents from the poles are less 

 saline, and consequently lighter than the tropical waters of the same temperature. 

 When these two opposing currents meet there is a struggle ; but at length the one 

 which is really specifically heavier sinks, while the lighter rises. So facile is the 

 movement of fluids among each other, that a diflference in gravity which we can hardly 

 detect with our nicest instruments may be abundantly sufficient to decide which of two 

 opposing currents shall run above and which below. 



The air has currents as well as the ocean, and these have much to do in modifying 

 the climate of the Tropical World. Rarified by the intense heat of a vertical sun, the 

 air within the tropics rises in perpendicular columns high above the surface of the 

 earth, and thence flows oif toward the poles ; while, to fill up the void, cold air cur- 

 rents come rushino; in from the Arctic and Antarctic regions ; but the rotation of the 

 earth gradually diverts the direction of these cold currents, and changes them into the 

 trade winds which regularly blow over the greater part of the tropical ocean from east 

 to west, and materially contribute to the health and comfort of the navigator whom 

 they waft over the equatorial waters. 



These atmospheric currents in another way still more powerfully influence the 

 climate and productions of the Tropical World ; for upon them, in conjunction with 

 the character and direction of the great mountain ranges, depends the supply of water 

 upon the surface of the globe, and thus determine the fertility or barrenness of the 

 soil. It may be laid down as a rule that wherever within the tropics there is an 

 abundant supply of moisture, vegetation grows in rank luxuriance ; and wherever this 

 is wanting, the land is a desert; and wherever, as in the llanos of Southern America, 



