The mean depth of the sea is not very exactly ascertained, but certain phenomena observed in the movement of tides are supposed to be incapable of explanation without admitting a mean depth of three thousand five hundred fathoms. It is true that a great number of deep-sea soundings fall short of that limit; but, on the other hand, many others reach seven or eight thousand. Admitting that three thousand fathoms represents the mean depth of the ocean, Sir John Herschel finds that the volume of its waters would exceed three thousand two hundred and seventy-nine million cubic yards. This vast volume of water is divided by geographers into five great oceans: the Arctic, the Atlantic, Indian, Pacific, and Antarctic Oceans. The Arctic Ocean extends from the Pole to the Polar Circle; it is situated between Asia, Europe, and America. The Atlantic Ocean commences at the Polar Circle and reaches Cape Horn. It is situated between America, Europe, and Africa, a length of about nine thousand miles, with a mean breadth of two thousand seven hundred, covering a surface of about twenty-five million square miles, placed between the Old World and the New. Beyond the Cape of Storms, as Cape Horn may be truly called, it is only separated by an imaginary line from the vast seas of the south, in which the waves, which are the principal source of _tides_, have their birth. Here, according to Maury, the young tidal wave, rising in the circumpolar seas of the south, and obedient to the sun and moon, rolls on to the Atlantic, and in twelve hours after passing the parallel of Cape Horn is found pouring its flood into the Bay of Fundy, whence it is projected in great waves across the Atlantic and round the globe, sweeping along its shores and penetrating its gulfs and estuaries, rising and falling in the open sea two or three feet, but along the shore having a range of ten or twelve feet. Sometimes, as at Fundy on the American coast; at Brest on the French coast; and Milford Haven, and the mouth of the Severn in the Bristol Channel, rising and falling thirty or forty feet, "impetuously rushing against the shores, but gently stopping at a given line, and flowing back to its place when the word goes forth, 'Thus far shalt thou go, and no farther.' That which no human power can repel, returns at its appointed time so regularly and surely, that the hour of its approach and the measure of its mass may be predicted with unerring certainty centuries beforehand." The Indian Ocean is bounded on the north by Asia, on the west by Africa, on the east by the peninsula of Molucca, the Sunda Isles, and Australia.