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.