In order to comprehend the origin of these pelagic rivers, it is necessary to consider the laws which govern the atmospheric currents, in particular the trade-winds. "Hence," says Maury, "in studying the system of oceanic circulation, we set out with the very simple assumption, that from whatever part of the ocean a current is found to run, to that same part a current of equal volume is bound to return; for on this principle is based the whole system of currents and counter currents." The differences of temperature between equinoctial and polar countries generate two opposing currents, the upper one proceeding from the Equator to the Poles, the lower one directed from the Poles towards the Equator. On reaching the Equator, the cold current of air from the Poles is warmed and rarefied, and ascends to the upper beds of the atmosphere, whence it is again led to its point of departure; there it is again cooled, and returns with the lower current towards the tropical regions. But the rotatory movement of the earth modifies the direction of these atmospheric currents. The movement by which it is carried from west to east being almost nothing at the Poles, but inconceivably rapid under the Equator, it follows that the cold air, in proportion as it advances towards the Tropics, ought to incline a little towards the west. This is just what takes place with these counter currents. The north-east trade-winds, which prevail in the northern hemisphere, move in a sort of spiral curve, turning to the west as they rush from the Poles to the Equator, and in the opposite direction as they move from the Equator towards the Poles; the immediate cause of this motion being the rotation of the earth on its axis. "The earth," says Dr. Maury, "moves from west to east. Now, if we imagine a particle of atmosphere at the North Pole, where it is at rest, to be put in motion in a straight line towards the Equator, we can easily see how this particle of air, coming from the very axis of diurnal rotation, where it did not partake of the diurnal motion, would, in consequence of its own vis inertiæ, find as it travelled south that the earth was slipping from under it, as it were, and it would appear to be coming from the north-east and going towards the south-west; in other words, it would be a north-east wind."
In the same manner, the upper currents of air, which proceed towards the Poles with equatorial rapidity, ought to outstrip the atmospheric beds, which are gifted with much smaller rapidity of motion towards the Poles, and turn them towards the east in consequence. These are the south-west and north-west counter trade-winds, which, passing above the north and south-east trades, often sweep the surface of the sea in the latitudes of the Temperate zone. The two trades are separated by a belt more or less broad, where the friction experienced at the surface of the sea neutralizes their impulse towards the west; in general, the current of air there is an ascending current. This belt, which does not exactly correspond with the Equator, is called the Zone of Calms, where atmospheric tempests frequently occur, and the winds make the entire tour of the compass, which has acquired for them the name of tornadoes.
The trade-winds, whose movement towards the west is retarded by the friction which the waves of the ocean oppose to them, communicate to these waves, by a sort of reaction, a tendency towards the west, or, to speak more exactly, towards the south-west in the northern hemisphere, and towards the north-west in the opposite hemisphere. The currents on the surface of the water which result from this reaction, reunite under the Equator, and form the grand equinoctial current which impels the waters of the east towards the west. This movement is stronger at the edges than in the middle of the current, because the force which produces it acts there with more energy: it results from this, that the currents bifurcate more readily when any obstacle presents itself to its movement. In the Atlantic Ocean, bifurcation takes place a little to the south of the Equator; the southern branch descends along the coast of Brazil, and probably returns by reascending along the west coast of Africa. The northern branch follows the coast of Brazil and Guiana, enters the Sea of the Antilles, and directs its course, reinforced by the current which reaches it from the north-east, into the Bay of Honduras, traverses the Yucatan Channel, and enters the Gulf of Mexico, whence it debouches by the Florida Channel, under the name of the Gulf Stream. Of this oceanic marvel Dr. Maury observes that "there is a river in the bosom of the ocean; in the several droughts it never fails, and in the mightiest floods it never overflows; its banks and its bottom are of cold water, while its current is of warm; it takes its rise in the Gulf of Mexico, and empties itself into the Arctic Seas. This mighty river is the Gulf Stream. In no other part of the world is there such a majestic flow of water; its current is more rapid than the Amazon, more impetuous than the Mississippi, and its volume is more than a thousand times greater. Its waters, as far as the Carolina coast, are of indigo blue; they are so distinctly indicated that their line of junction can be marked by the eye." Such is Dr. Maury's description of this powerful current of warm water, which traverses the Atlantic Ocean, and influences in no slight manner the climate of Northern Europe, and especially our own shores.
The Gulf Stream thus described by the American savant issues from the Florida Channel, with a breadth of thirty-four miles, and a depth of two thousand two hundred feet, moving at the rate of four and a half miles per hour. The temperature of the water in the vicinity is about thirty degrees Cent. From the American coast the current takes a north-east direction towards Spitzbergen, its velocity and volume diminishing as it expands in breadth. Towards the forty-third degree of latitude it forms two branches, one of which strikes the coast of Ireland and of Norway, whither it frequently transports seeds of tropical origin: it also warms the frozen waters of the glacial sea. The other branch, inclining towards the south, not far from the Azores, visits the coast of Africa, whence it returns to the Antilles. Throughout this vast circuit may be seen all sorts of plants and driftwood, with waifs and strays of every description borne on the bosom of the ocean. "Midway the Atlantic, in the triangular space between the Azores, Canaries, and Cape de Verd Islands, is the great Sargasso Sea, covering an area equal in extent to the Mississippi Valley: it is so thickly matted over with the Gulf Weed (Sargassum bacciferum), that the speed of vessels passing through it is actually retarded, and to the companions of Columbus it seemed to mark the limits of navigation; they became alarmed. To the eye at a little distance it seemed sufficiently substantial to walk upon." These moving vegetable masses, always green, which tail off to a steady breeze, serving as an anemometer to the mariner, afford an asylum to multitudes of mollusks and crustaceans.
The Gulf Stream plays a grand part in the Atlantic system. It carries the tepid water of the equinoctial regions into the high latitudes; beyond the fortieth parallel the temperature is sixteen degrees Cent. Urged by the south-west winds which predominate in that zone, its tepid waters mix with those of the Northern Sea, softening the rigour of the climate in these regions. To the south of the great bank of Newfoundland, the warm current, in vast volume rushing from the Florida Straits, meets the cold currents descending from the Arctic Circle through Baffin's Bay and the Sea of Greenland, running with equal velocity towards the south. A portion of these waters reascend towards the Pole along the western coast of Greenland. It is to this conflict of the polar and equatorial waters, that the formation of the banks of Newfoundland is ascribed. Each of these great currents having unceasingly deposited the débris carried in its bosom, the bank has been thus formed bit by bit in the concourse of ages.
The difference of temperature between the Gulf Stream and the waters it traverses gives birth inevitably to tempests and cyclones. In 1780 a terrible storm ravaged the Antilles, in which twenty thousand persons perished. The ocean quitted its bed and inundated whole cities; the trunks of trees, mingled with other débris, were tossed into the air. Numerous catastrophes of this kind have earned for the Gulf Stream the title of the King of the Tempests. In consequence of the numerous nautical documents which have been placed at the command of the National Observatory of Washington, and the admirable use made of them by the late Naval Secretary and his assistants, the directions and range of these cyclones engendered by the Gulf Stream may be foreseen, and their most dangerous ravages turned aside. As an example of the utility of Dr. Maury's labours in settling the direction of storms in the traject of the Gulf Stream, we quote a well-known instance: In the month of December, 1859, the American packet San Francisco was employed as a transport to convey a regiment to California. It was overtaken by one of these sudden storms, which placed the ship and its freight in a most dangerous position. A single wave, which swept the deck, tore out the masts, stopped the engines, and washed overboard a hundred and twenty-nine persons, officers and soldiers. From that moment the unfortunate steamer floated upon the waters, a waif abandoned to the fury of the wind. The day after the disaster the San Francisco was seen in this desperate situation by a ship which reached New York, although unable to assist her. Another ship met her some days after, but, like the other, could render no assistance. When the report reached New York, two steamers were despatched to her assistance; but in what direction were they to go? what part of the ocean were they to explore? The luminaries of Washington Observatory were appealed to! Having consulted his charts as to the direction and limits of the Gulf Stream at that period of the year, Dr. Maury traced on a chart the spot to which the disabled steamer was likely to be driven by the current, and the course to be taken by the vessels sent to her assistance. The crew and passengers of the San Francisco were saved before their arrival. Three ships, which had seen their distressing situation, had been able to reach them, and the steamers sent to their assistance only arrived to witness the safety of the passengers and crew. But the point where the steamer foundered shortly after they were transferred to the rescuing ships was precisely that indicated by Dr. Maury. If the ships sent to their assistance had reached in time, the triumph of Science would have been complete.
The equinoctial currents of the Pacific are very imperfectly known. It is believed, however, that they traverse the Great Ocean in its whole length, and bifurcate opposite the Asiatic coast, where the weakest branch bends northward until it encounters the polar current from Behring's Straits, when it returns along the Mexican coast. The larger branch inclines towards the south, passing round Australia, where it is met by one or many counter currents coming from the Indian Ocean—of the complicated and dangerous nature of which both Cook and La Peyrouse speak.
The cold waters from the Antarctic Pole are carried towards the Equator by three great oceanic rivers. The first bifurcates in forty-five degrees; one portion goes round Cape Horn; the other—Humboldt's current—ascends the Chilian and Peruvian coasts up to the Equator, ameliorating the rainless climate as it goes, and making it delightful. A second great current takes the direction of the African coast, and is divided at the Cape, ascending both the east and west coasts of Africa. On either side of the warm current which escapes from the intertropical parts of the Indian Ocean, but especially along the Australian coast, a polar current wends its way from the Antarctic regions, carrying supplies of cold water to modify the climate and restore the equilibrium in that part of the world. This cold current turns at first towards the west, then towards the south in the direction of Madagascar; more to the south still it is driven back by the polar current from Cape Horn. It is thus that the warm waters from the Bay of Bengal, pressed by the Indian polar current, circulate between Africa and Australia, one lateral branch of the current sweeping along the south coast of this vast continent.
The monsoons which reign in the Indian Ocean tend still more to complicate the currents, already sufficiently intricate and confused. But it is not intended at present to occupy the reader's attention further with these questions of intricate currents.