The ordinary velocity of the principal currents of the ocean is from one to three miles per hour; but when the boundary lands converge, large bodies of water are driven gradually into a narrow space, and then, wanting lateral room, are compelled to raise their level. Whenever this occurs, their velocity is much increased. The current which runs through the Race of Alderney, between the island of that name and the main land, has a velocity of about eight English miles an hour. The late Captain Hewett found that in the Pentland Firth the stream, in ordinary spring tides, runs ten miles and a half an hour, and about thirteen miles during violent storms. The greatest velocity of the tidal current through the “Shoots or New Passage,” in the Bristol Channel, is fourteen miles an hour; and Captain King observed, in his recent survey of the Straits of Magellan, that the tide ran at the same rate through the “First Narrows,” and about eight geographical miles an hour in other parts of those straits. The course of currents on the British shores is ascertained to be as winding as that of ordinary rivers. Sometimes they run between banks of sand, which consist of matter thrown down at certain points where the velocity of the stream has been retarded, but it very frequently occurs, that as in a river one bank is made of low alluvial gravel, while the other is composed of some hardy and lofty rocks constantly undermined, so the current in its bends strikes here and there upon a coast which then forms one bank, whilst a shoal under water forms the other. If the coast be formed of solid materials, it yields slowly; so also if it be of great height, for in that case a large quantity of matter must be removed before the sea can penetrate to any distance. Currents depend, like tides, on no temporary or accidental circumstances, but on the laws which preside over the motions of the heavenly bodies. The height to which tides rise, and the violence and velocity of the currents, depend in a great measure on the actual configuration of the land, the contour of a long line of continental or insular coast, the depth and breadth of channels, the peculiar form at the bottom of the seas—in a word, on a combination of circumstances which are made to vary continually by many igneous and aqueous causes, and among the rest, by the tides and currents themselves. Although these agents of decay and reproduction are local in reference to periods of short duration, such as those which history embraces, they are nevertheless universal, if we extend our views to a sufficient lapse of ages. {28}