one of the five great hydrographical divisions of the globe, lying between America on the W., and Europe and Africa on the E., and extending from the Arctic circle on the N., to the Antarctic circle on the S. Meridian lines from Cape Horn in South America, and from Cape Agulhas in Africa, prolonged to the Antarctic circle, complete its boundaries on the E. and W. It extends 8600 miles from N. to S., and its breadth varies from 1800 to 5400 miles, the estimated area being 25,000,000 square miles. It contains Iceland, the British Isles, the Azores, Madeira, the Canary Islands, the islands in the Gulf of Guinea, and those of the Gulf of St Lawrence, the Antilles, and Newfoundland. The principal gulfs and bays are the Bay of Biscay, and the Gulfs of Guinea, Mexico, and St Lawrence.

This ocean, according to Humboldt, presents all the indications of a valley, as if a flow of eddying waters had been directed first towards the N.E., then towards the N.W., and back again to the N.E. The parallelism of the coast north of 10° S. latitude, the projecting and receding angles, the convexity of Brazil opposite to the Gulf of Guinea, that of Africa under the same parallel with the Gulf of the Antilles, all favour this apparently speculative view. In this Atlantic valley, as is almost everywhere the case in the configuration of large continental masses, coasts deeply indented and rich in islands are situated opposite to those possessing a different character. The depth of this ocean is extremely various; and is enormous, both to the N. and S. of the equator. Thus, Captain Sir Edward Belcher, R.N., in Lat. 0.4. N. Long. 10.6. W., sounded to the depth of 3065 fathoms = 18,390 feet; Captain Barnett, R.N., in Lat. 41.2. N. Long. 44.3. W., sounded to the depth of 3700 fathoms, or 22,200 feet; Captain Sir James Ross, R.N., in 1840, to 2677 fathoms, or 16,062 feet; and afterwards in Lat. 33.3. S. Long. 9.1. E. to 4600 fathoms, or 27,600 feet; and Captain H. M. Denham, H.M.S. Herald, has obtained soundings at the vast depth of 7706 fathoms = 46,236 feet, or about 8¾ English miles. This, which is the deepest sounding ever yet taken, occurred on 30th October 1852, in Lat. 36.49. S. Long. 37.6. W. Two American captains have also obtained soundings at the depth of 3100 and 3500 fathoms.

The intertropical part of the Atlantic is under the influence of the trade-winds, which frequently extend to the 32d degree of N. or S. latitude. The winds blow almost invariably in the same direction, from E. to W., occasionally varying a few points to the N. or S., on the N. and S. sides of the equator. A region of calms, varying from 3 to 10 degrees of latitude, according to the season of the year, separates the two trade-winds. It is, however, daily agitated by a squall, which begins about two o'clock in the afternoon, and continues about an hour. This region does not always occupy the same part, but its central line may be placed about the 5th degree of N. latitude. In some tracts the influence of the trade-winds extends to the shores; but in general it does not come within 200 miles of the coasts.

The current of this ocean first known to mariners, is called the equinoctial, and is within the tropics, especially flowing from the coast of Senegal to the Caribbean Sea. Its direction is constantly from E. to W., with a mean rapidity of 9 or 10 miles in 24 hours. This current is attributed to the impulse which is given to the surface of the sea by the trade-winds. "In the channel," says Humboldt, "which the Atlantic has dug between Guiana and Guinea on the meridian of 20 or 23 degrees, and from the 8th or 9th to the 2d or 3d degrees of N. latitude, where the trade-winds are often interrupted by winds blowing from the S. or S.S.W., the equinoctial current is more inconstant in its direction. The equinoctial current is felt, though feebly, even beyond the tropic of Cancer, in the 26th and 28th degrees of latitude. In the vast basin of the Atlantic, at 600 or 700 leagues from the coasts of Africa, vessels from Europe bound to the West Indies find their sailing accelerated before they reach the torrid zone." In the Caribbean Sea, the motion of this current is much accelerated by the action of another current. The Mozambique current, flowing from N. to S. between Madagascar and the eastern coast of Africa, bends to the north of the Lagallas bank, round the southern point of Africa, and advances with much violence along its western coast beyond the equator to the island of St Thomas. It gives a north-westerly direction to a portion of the waters of the South Atlantic, causing them to strike Cape St Augustin, and to follow the shores of Guiana beyond the mouth of the Orinoco, the Boca del Drago, and the coast of Paria. The coast of America presenting a barrier to the equinoctial current; its waters are driven with velocity through the strait formed by Cape Catuche and Cape St Antonio, into the Gulf of Mexico; and after following the bendings of the coast, force their way back into the open sea N. of the Straits of Bahama, where it forms a warm rapid current, known as the Gulf Stream. At first its rapidity is sometimes as great as five miles an hour, but it decreases as it proceeds towards the north, at the same time that its breadth increases and its waters become cool. Between Cayo Bicacano and the bank of Bahama the breadth is only 15 leagues, while in the latitude of 29½ degrees it is 17; and in the parallel of Charlestown, opposite Cape Henlopen, from 40 to 50 leagues. "The waters of the Mexican Gulf forcibly drawn to N.E. preserve their warm temperature to such a point, that in 40 and 41 degrees of latitude I found them at 22.5° cent. (= 72.4° Fahr.) when out of the current; the heat of the ocean at its surface was scarcely 17.5° (= 63.5° Fahr.). In the parallel of New York and Oporto, the temperature of the Gulf Stream is consequently equal to that of the seas of the tropics in the 18th degree of latitude, as, for instance, in the parallel of Porto Rico and the islands of Cape Verde."—(Humboldt.) In Lat. 41.25. and Long. 67., where it is nearly 80 leagues broad, it turns suddenly to the E., and almost touches the southern edge of the Great Newfoundland Bank, from whence it continues its course E. and E.S.E. to the Azores. On the meridian of Corvo and Flores, the most westerly islands of the Azores group, the breadth of the current is 180 leagues. From the Azores it turns towards the Straits of Gibraltar, the island of Madeira, and the Canary group. There are several other minor currents in this vast ocean, as the branch... Atlantis sent off by the Gulf Stream before reaching the Western Azores, which at certain seasons of the year flows towards Ireland and Norway. The experiments of Sir James Ross and Captain Denham show that, after the depth of 200 or 300 fathoms, the temperature of the Atlantic waters is stationary at 40° Fahr., whatever be the temperature at the surface. This uniformity of temperature in the waters of great oceans renders them the means of mitigating the extremes of tropical and polar regions.—See Rennell's Investigation of the Currents of the Atlantic Ocean; Humboldt's Cosmos, Personal Narrative, &c.