This is a subject on which it is needless to enlarge. Every one knows that the tides have the power of transporting solid matter; though most of us, perhaps, do not fully appreciate the magnitude of their accumulated effects, working as they do with untiring energies upon the coasts of islands and continents all over the world. It is not, however, so generally known that the ocean is traversed in all directions by powerful currents, which, from their regularity, their permanence, and their extent, have been aptly called the rivers of the ocean. We do not mean here to inquire into the causes of these currents, upon which the progress of physical science has thrown considerable light: neither can we hope to describe even the principal currents that prevail over the vast tracts of water which constitute about three-fourths of the entire surface of our globe. We shall content ourselves with tracing the course of one great system, which may serve to give some idea of their general character and enormous power. This system would seem to have its origin with a stream that flows from the Indian Ocean toward the southwest, and then doubling the Cape of Good Hope, turns northward along the African coast. It is here called the South Atlantic Current. When it encounters the shores of Guinea, it is diverted to the west, and stretches across the Atlantic, traversing forty degrees of longitude until it reaches the projecting promontory of Brazil in South America. In this part of its course it is known as the Equatorial Current, because it follows pretty nearly the line of the Equator: it varies in breadth from two hundred to five hundred miles, and it travels at the mean rate of thirty miles a day, though sometimes its velocity is increased to seventy or eighty. Next, under the name of the Guyana Current, it pursues a northwesterly direction, following the line of the coast; and passing close to the island of Trinidad, becomes diffused, and almost seems to be lost, in the Caribbean Sea. Nevertheless, it again issues with renewed energy from the Gulf of Mexico, and rushing through the Straits of Florida at the rate of four and five miles an hour, it issues once more into the broad waters of the Atlantic. From this out it is called the Gulf Stream, and is well known to all who are concerned in Transatlantic navigation; for it sensibly accelerates the speed of vessels which are bound from America to Europe, and sensibly retards those sailing from Europe to America.