The steady trade-winds of the tropical zone, and the prevailing westerly winds in higher latitudes, consequently unite their influence with that of the above mentioned causes, in driving the waters of the tropical seas to the west, and those of the temperate zones to the east.
The tides also, which on the high seas generally move from east to west, promote the flow of the ocean in the same direction, and thus contribute to the westerly current of the tropical seas.
Nor must we forget that the obstacles which the ocean-currents meet on their way; such as intervening lines of coast, sand banks, submarine ridges, or mountain chains, have a great influence upon their course, and may even give them a diametrically opposite direction to that which they would otherwise have followed.
Having thus briefly mentioned the origin and causes of the currents, which intersect the seas like huge rivers, I shall now describe such of them as are most important and interesting in a geographical point of view.
In the northern part of the Atlantic, between Europe, North Africa, and the New World, the waters are constantly performing a vast circular or rotatory movement. Under the tropics they proceed like the trade-winds from east to west, assisting the progress of the ships that sail from the Canaries to South America, and rendering navigation in a straight line from Carthagena de Indias to Cumana (stream upwards) next to impossible. This westerly current receives a considerable addition from the Mozambique stream, which, flowing from north to south between Madagascar and the coast of Caffraria, proceeds round the southern extremity of Africa, and after rapidly advancing to the north, along the western coast of that continent, as far as the island of St. Thomas, unites its waters with those of the equatorial current, and continues its course right across the Atlantic. In this manner the combined tropical streams reach the eastern extremity of South America (Cape Roque), where they divide into two arms. The one flowing to the south follows the south-eastern coast, and gradually takes a south-easterly direction, between the tropic of Capricorn and the mouth of the La Plata river, beyond the limits of the trade-winds. Its traces show themselves to the south-east of the Cape of Good Hope, and are finally lost far in the Indian Ocean.
The northern arm of the equatorial stream flows along the north-eastern coast of South America; constantly raising its temperature under the influence of a tropical sun, and progressing with a rapidity of a hundred miles in twenty-four hours (six feet and a half in a second), after having been joined by the waters of the Amazon river. Thus it continues to flow to the east, until the continent of Central America opposes an invincible barrier to its farther progress in this direction, and compels it to follow the windings of the coast of Costa Rica, Mosquitos, Campeche, and Tabasco. It then performs a vast circuit along the shores of the Mexican Gulf, and finally emerges through the Straits of Bahama into the open ocean.
Here it assumes a new name, and forms what navigators call the Gulf-stream, a rapid current of tepid water, which, flowing in a diagonal direction, recedes farther and farther from the coast of North America as it advances to the north-east. Under the forty-first degree of latitude it suddenly bends to the east, gradually diminishing in swiftness, and at the same time increasing in width.
Thus it flows across the Atlantic, to the south of the great bank of Newfoundland, where Humboldt found the temperature of its stream several degrees higher than that of the neighbouring and tranquil waters, which form, as it were, the banks of the warm oceanic current. Ere it reaches the western Azores, it divides into two arms, one of which is driven, partly by the natural impulse of its stream, but principally by the prevailing westerly and north-westerly winds, towards the coasts of Europe; while the other, flowing towards the Canary Islands and the western coast of Africa, finally returns into the equatorial current.
In this manner the waters are brought back to the point from which they came, after having performed a vast circuit of 20,000 miles, which it took them nearly three years to accomplish. According to Humboldt's calculations, a boat left to the current, and moving along without any other assistance, would require about thirteen months to float from the Canary Islands to the Caribbean Sea as far as Caraccas. From Caraccas to the Straits of Florida, it would remain another ten months on the way, for though the direct distance is but short, the current has to perform an enormous circuit of 2500 miles, and flows but slowly in those confined seas. But the accumulated waters having now to force their passage through the narrow channel between Cuba and the Bahama Islands on one side, and Florida on the other, attain so considerable a velocity, that the whole distance from the Havannah to the Bank of Newfoundland, is traversed in forty days. During this passage the Gulf-stream particularly deserves its name, and is easily distinguished from the surrounding waters by its higher temperature and its vivid dark blue colour. Numerous marine animals of the tropical seas,—the flying fish, the neat velella, the purple ianthina, the crosier nautilus, accompany it to latitudes which otherwise would prove fatal to their existence; and, trusting its tepid stream, float or swim along to the north or the north-east.
At the extremity of the Bank of Newfoundland, it becomes broader, wavers more or less in its course, according to the prevailing winds, and at the same time decreases in rapidity, so that the boat would most likely still require from ten to eleven months for this last station of its journey, ere it once more reached the Canary Islands.