Page:The American Cyclopædia (1879) Volume II.djvu/85

 ATLANTIC OCEAN 73 The circulation of the water in the Atlantic ocean can be stated in very general terms to consist of two gigantic eddies orVrevolving streams, the one in the northern Atlantic, the other in the southern or Ethiopia basin ; the former revolving from left to right, the other from right to left ; both giving out offshoots of greater or less importance on their outer cir- cumference. Both originate in the equatorial current, which consists of two parallel parts, the northern and southern, separated by a narrower return current, called the Guinea current. The southern equatorial current, starting from the coast of Africa and striking the coast of South America at Cape San Roque, divides itself into two branches. The southern one follows the coast of Brazil under the name of the Brazil- ian current, dividing about the latitude of the tropic of Capricorn into two branches, the smaller one following the coast, but gradually growing narrower and weaker, nearly as far as the extremity of South America. The larger and wider portion strikes toward the southeast in the direction of the Cape of Good Hope, under the name of the southern connecting current ; a short distance west of this cape the current turns north and follows the coast of Africa, under the name of the South Atlantic current, toward the equator, where the cir- cuit is completed. This current is accom- panied in its northern course, and between it and the coast, by a branch of the cold An- tarctic current, the waters of which can be traced for a long distance by their temperature. The northern branch of the south equatorial current follows the coast of South America from Cape San Roque to the Antilles, where it penetrates into the Caribbean sea, jointly with the larger north equatorial current. Thus a por- tion of the waters of the South Atlantic is carried into the North Atlantic, for which apparently no return is made as far as surface currents are concerned. After entering the Caribbean sea, the current is driven through the straits of Yucatan into the gulf of Mexico. The prin- cipal mass of the water then turns to the east- ward along the northern coast of Cuba, while a smaller and less known branch is said to fol- low the western and northern coasts of the gulf, ultimately falling in again with the for- mer. After passing the southern extremity of Florida the current receives the name of the Gulf stream, and passes north through the nar- rows of Bernini between Florida and the Ba- hama banks into the Atlantic ocean. It now follows the coast of the United States at a somewhat variable distance to about the lati- tude of Chesapeake bay, when it turns east. On the S. side of the banks of Newfoundland it is pressed in by the polar current, and ac- cording to some authors ceases to exist as a .special current. It is most probable that a por- tion of its waters continues its course eastward across the ocean, bending south between the Azores and the coast of Portugal, and finally returning along the coast of Africa to the equa- torial current, and thus completing the circuit. A small offset enters the Mediterranean through the strait of Gibraltar. Another small branch separates at Cape Finisterre, sweeps around the bay of Biscay in a northerly direction, and dies out finally on -the coast of Ireland. This is Rennel's current, named so after its dis- I coverer. From the region east of the banks "of Newfoundland, the waters of the Gulf stream or of the general ocean drift (the ques- tion being disputed) move northward toward the coasts of northern Europe, to which they carry their heat, passing the North Cape, and j reaching nearly to Nova Zembla. Interweav- ing with the polar current, a branch passes up the N. coast of Spitzbergen, another around the west to the N. coast of Iceland, another along the W. coast of Greenland into Davis strait. A polar current, carrying large quan- tities of ice at certain seasons, descends along the W. shore of Davis strait and the coasts of Labrador and Newfoundland, and passes, part of it under the Gulf stream, and part be- tween that stream and the coast of the United States. -Cause of currents. The various theo- ries propounded to explain the circulation of the water in the ocean have been based 1, on the effect of permanent winds; 2, on differ- ences of density due to evaporation ; 3, on dif- ferences of density due to temperature ; 4, on the rotation of the earth ; 5, on difference of barometric pressure ; and finally, on combina- tions of these causes. The first author to leave a theory of currents was Kepler, who attributed them to the rotation of the earth, remarking that as the water is only in loose contact with the earth, it cannot follow the rotation east- ward as fast, and remains behind. He was followed and sustained by Varenius in 1650. Vossius and Fournier a little later adopted the heat and evaporation theories, but in a rather extravagant form, the former supposing the heat of the sun to expand and attract the water of the ocean into a kind of long mountain ridge, which, following the sun, broke on the coast of America, producing the currents running along the shore ; a curious glimpse of the usual tidal theory. Fournier supposed, on the con- trary, a hollow or valley formed by evapora- tion in the ocean in the tropics, causing a con- stant rush of the polar waters to fill it up. Coming down to Franklin, we find him an ad- vocate of the trade-wind theory for the Gulf stream, while, later, Humboldt explained the phenomenon by the rotation of the earth. Major Rennel, in his work on ocean currents, divides the currents into two classes. Drift currents, according to him, are the effect of the permanent winds on the surface of the water, by which the superficial layers are set in mo- tion ; when a drift current meets with an ob- stacle, the general surface is raised by accumu- lation, and the water in trying to return to its level produces a deeper and generally more rapid fiow called a stream current. The equa- torial current is an example of the former, the