These oceanic streams exceed all the rivers in the world in breadth and depth as well as length. The equatorial current in the Atlantic is 160 miles broad off the coast of Africa, and towards its mid-course across the Atlantic its width becomes nearly equal to the length of Great Britain: but as it then sends off a branch to the N.W., it is diminished to 200 miles before reaching the coast of Brazil. The depth of this great stream is unknown; but the Brazilian branch must be very profound, since it is not deflected by the river La Plata, which crosses it with so strong a current that its fresh muddy waters are perceptible 500 miles from its mouth. When currents pass over banks and shoals, the colder water rises to the surface and gives warning of the danger. In summer, the great north polar current coming along the coasts of Greenland and Labrador, together with the current from Davis’s Straits, brings icebergs to the margin of the Gulf-Stream. The difference between the temperatures of these two oceanic streams brought into contact is the cause of the dense fogs that brood over the banks of Newfoundland. The north polar current runs inside of the Gulf-Stream, along the coast of North America to Florida, and beyond it—since it sends an under-current into the Caribbean Sea. Counter-currents on the surface are of such frequent occurrence that there is scarcely a strait joining two seas that does not furnish an example—a current running in along one shore, and a counter-current running out along the other. One of the most remarkable occurs in the Atlantic: it begins off the coast of France, and, after sending a mass of water into the Mediterranean, it holds a southerly direction at some distance from the continent of Africa; till, after passing Cape Mesurada, it flows rapidly for 1000 miles due east to the Bight of Biafra in immediate contact with the equatorial current, running with great velocity in the opposite direction, and seems to merge in it at last. Periodical currents are frequent in the eastern seas: one flows into the Red Sea from October to May, and out of it from May to October. In the Persian Gulf this order is reversed; in the Indian Ocean and China Sea the waters are driven alternately backwards and forwards by the monsoons. It is the southwesterly monsoon that causes inundations in the Ganges, and a tremendous surf on the coast of Coromandel. The tides also produce periodical currents on the coasts and in straits, the water running in one direction during the flood, and the contrary way in the ebb. The Roost of Sumburgh, at the southern promontory of Shetland, runs at the rate of 15 miles an hour; indeed, the strongest tidal currents known are among the Orkney and Shetland islands; their great velocity arises from local circumstances. Currents in the wide ocean move at the rate of from one to three miles an hour, but the velocity is less at the margin and bottom of the stream from friction.