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OCEAN CURRENTS.

Causes of the Oceanic Currents.—The Equatorial Stream.—The Gulf Stream.—Its Influence on the Climate of the West European Coasts.—The Cold Peruvian Stream.—The Japanese Stream.

Perpetual motion and change is the grand law, to which the whole of the created universe is subject, and immutable stability is nowhere to be found, but in the Eternal mind that rules and governs all things. The stars, which were supposed to be fixed to the canopy of heaven, are restless wanderers through the illimitable regions of space. The hardest rocks melt away under the corroding influence of time, for the elements never cease gnawing at their surface, and dislocating the atoms of which they are composed. Our body appears to us unchanged since yesterday, and yet how many of the particles which formed its substance, have within these few short hours, been cast off and replaced by others. We fancy ourselves at rest, and yet a torrent of blood, propelled by an indefatigable heart, is constantly flowing through all our arteries and veins.

A similar external appearance of tranquillity might deceive the superficial observer, when sailing over the vast expanse of ocean, at a time when the winds are asleep, and its surface is unruffled by a wave. But how great would be his error! For every atom of the boundless sea is constantly moving and changing its place; from the depth to the surface, or from the surface to the depth; from the frozen pole to the burning equator, or from the torrid zone to the arctic ocean; now rising in the air in the form of invisible vapours, and then again descending upon our fields in fertilising showers.

The waters are, in fact, the greatest travellers on earth; they know all the secrets of the submarine world; climb the peaks of inaccessible mountains, shame the flight of the condor as he towers over the summit of the Andes, and penetrate deeper into the bowels of the earth than the miner has ever sunk his shaft.

Leaving their wanderings through the regions of air to the next chapter, I shall now describe the principal ocean currents, the simple, but powerful agencies by which they are set in motion, their importance in the economy of nature, and their influence on the climate of different countries.

Even in the torrid zone, the waters of the ocean, like a false friend, are warm merely on the surface, and of an almost icy coldness at a considerable depth. This low temperature cannot be owing to any refrigerating influence at the bottom of the sea, as the internal warmth of the earth increases in proportion to its depth, and the waters of profound lakes, in a southern climate, never show the same degree of cold as those of the vast ocean.

The phenomenon can thus only arise from a constant submarine current of cold water from the poles to the line, and strange as it may seem, its primary cause is to be sought for in the warming rays of the sun, which, as we all know, distributes heat in a very unequal manner over the surface of the globe.

Heat expands all liquid bodies, and renders them lighter; cold increases their weight by condensation. In consequence of this physical law, the waters of the tropical seas, rendered buoyant by the heat of a vertical sun, must necessarily rise and spread over the surface of the ocean to the north and south, whilst colder and heavier streams from the higher latitudes flow towards the equator along the bottom of the ocean, to replace them as they ascend.

In this manner, the unequal action of the sun calls forth a general and constant movement of the waters from the poles to the equator, and from the equator to the poles; and this perpetual migration is one of the chief causes by which their purity is maintained. These opposite currents would necessarily flow direct to the north or south, were they not deflected from their course by the rotation of the earth, which gradually gives them a westerly or easterly direction.

The unequal influence of the sun in different parts of the globe, and the rotation of the earth, are, however, not the only causes by which the course of ocean-currents is determined.

Violent storms move the waters to a considerable depth, and retard the flow of rivers, and thus it is to be expected that continuous winds, even of moderate strength, must have a tendency to impel the waters in the same direction.

The steady trade-winds of the tropical zone, and the prevailing westerly winds in higher latitudes, consequently unite their influence with that of the above mentioned causes, in driving the waters of the tropical seas to the west, and those of the temperate zones to the east.

The tides also, which on the high seas generally move from east to west, promote the flow of the ocean in the same direction, and thus contribute to the westerly current of the tropical seas.

Nor must we forget that the obstacles which the ocean-currents meet on their way; such as intervening lines of coast, sand banks, submarine ridges, or mountain chains, have a great influence upon their course, and may even give them a diametrically opposite direction to that which they would otherwise have followed.

Having thus briefly mentioned the origin and causes of the currents, which intersect the seas like huge rivers, I shall now describe such of them as are most important and interesting in a geographical point of view.

In the northern part of the Atlantic, between Europe, North Africa, and the New World, the waters are constantly performing a vast circular or rotatory movement. Under the tropics they proceed like the trade-winds from east to west, assisting the progress of the ships that sail from the Canaries to South America, and rendering navigation in a straight line from Carthagena de Indias to Cumana (stream upwards) next to impossible. This westerly current receives a considerable addition from the Mozambique stream, which, flowing from north to south between Madagascar and the coast of Caffraria, proceeds round the southern extremity of Africa, and after rapidly advancing to the north, along the western coast of that continent, as far as the island of St. Thomas, unites its waters with those of the equatorial current, and continues its course right across the Atlantic. In this manner the combined tropical streams reach the eastern extremity of South America (Cape Roque), where they divide into two arms. The one flowing to the south follows the south-eastern coast, and gradually takes a south-easterly direction, between the tropic of Capricorn and the mouth of the La Plata river, beyond the limits of the trade-winds. Its traces show themselves to the south-east of the Cape of Good Hope, and are finally lost far in the Indian Ocean.

The northern arm of the equatorial stream flows along the north-eastern coast of South America; constantly raising its temperature under the influence of a tropical sun, and progressing with a rapidity of a hundred miles in twenty-four hours (six feet and a half in a second), after having been joined by the waters of the Amazon river. Thus it continues to flow to the east, until the continent of Central America opposes an invincible barrier to its farther progress in this direction, and compels it to follow the windings of the coast of Costa Rica, Mosquitos, Campeche, and Tabasco. It then performs a vast circuit along the shores of the Mexican Gulf, and finally emerges through the Straits of Bahama into the open ocean.

Here it assumes a new name, and forms what navigators call the Gulf-stream, a rapid current of tepid water, which, flowing in a diagonal direction, recedes farther and farther from the coast of North America as it advances to the north-east. Under the forty-first degree of latitude it suddenly bends to the east, gradually diminishing in swiftness, and at the same time increasing in width.

Thus it flows across the Atlantic, to the south of the great bank of Newfoundland, where Humboldt found the temperature of its stream several degrees higher than that of the neighbouring and tranquil waters, which form, as it were, the banks of the warm oceanic current. Ere it reaches the western Azores, it divides into two arms, one of which is driven, partly by the natural impulse of its stream, but principally by the prevailing westerly and north-westerly winds, towards the coasts of Europe; while the other, flowing towards the Canary Islands and the western coast of Africa, finally returns into the equatorial current.

In this manner the waters are brought back to the point from which they came, after having performed a vast circuit of 20,000 miles, which it took them nearly three years to accomplish. According to Humboldt's calculations, a boat left to the current, and moving along without any other assistance, would require about thirteen months to float from the Canary Islands to the Caribbean Sea as far as Caraccas. From Caraccas to the Straits of Florida, it would remain another ten months on the way, for though the direct distance is but short, the current has to perform an enormous circuit of 2500 miles, and flows but slowly in those confined seas. But the accumulated waters having now to force their passage through the narrow channel between Cuba and the Bahama Islands on one side, and Florida on the other, attain so considerable a velocity, that the whole distance from the Havannah to the Bank of Newfoundland, is traversed in forty days. During this passage the Gulf-stream particularly deserves its name, and is easily distinguished from the surrounding waters by its higher temperature and its vivid dark blue colour. Numerous marine animals of the tropical seas,—the flying fish, the neat velella, the purple ianthina, the crosier nautilus, accompany it to latitudes which otherwise would prove fatal to their existence; and, trusting its tepid stream, float or swim along to the north or the north-east.

At the extremity of the Bank of Newfoundland, it becomes broader, wavers more or less in its course, according to the prevailing winds, and at the same time decreases in rapidity, so that the boat would most likely still require from ten to eleven months for this last station of its journey, ere it once more reached the Canary Islands.

The direction of the Gulf-stream explains to us how the productions of tropical America are so frequently found on the shores of the Eastern Atlantic. Humboldt relates that the main-mast of the "Tilbury," a ship of the line, wrecked during the seven years' war on the coast of San Domingo, was carried by the Gulf-stream to the North of Scotland; and cites the still more remarkable fact, that casks of palm oil belonging to the cargo of an English vessel, which foundered on a rock near Cape Lopez, likewise found their way to Scotland, having thus twice traversed the wide Atlantic; first borne from east to west by the equatorial current, and then carried from west to east, between 45° and 55° N. latitude, by means of the Gulf-stream.

Major Rennell ("Investigation of Currents") relates the peregrinations of a bottle, thrown overboard from the "Newcastle," on the 20th of January, 1819, in lat. 38° 52″, and long. 66° 20″, and ultimately found on the 2nd of June, 1820, on the shore of the Island of Arran.

On the 16th of April, 1853, another bottle cast into the waters in the vicinity of the Bank of Newfoundland, on the 15th of March, 1852, was found near Bayonne, not far from the mouth of the Adour.

On the coasts of Orcadia, a sort of fruit, commonly known by the name of Molucca, or Orkney beans, are found in large quantities, particularly after storms of westerly wind.

These beans are the produce of West Indian trees (Anacardium occidentale), and find their way from the woods of Cuba and Jamaica, to the Ultima Thule of the ancients, by means of the Gulf-stream.

Large quantities of American drift-wood are transported by the same current to the dreary shores of Iceland,—a welcome gift to the inhabitants of a region where the highest tree is but a dwarfish shrub, and cabbages of the size of an apple are raised, as a great rarity, in the governor's garden.

A short time before Humboldt visited the island of Teneriffe, the sea had thrown out the trunk of a North American cedar-tree (Cedrela odorata), covered with the mosses and lichens that had grown upon it in the virgin forest.

The Gulf-stream has even contributed to the discovery of America, for it is well known that Columbus was strengthened in his belief in the existence of a western continent, by the stranding on the Azores of bamboos of an enormous size, of artificially carved pieces of wood, of trunks of a species of Mexican pine, and of the dead bodies of two men, whose features, resembling neither those of the inhabitants of Europe nor of Africa, indicated a hitherto unknown race. But not only lifeless and inanimate objects find their way across the wide Atlantic by means of the Gulf-stream and its spreading waters; the living aborigines of the distant regions of America have also sometimes been driven towards the coasts of Europe by the combined action of the currents and the winds. Thus, James Wallace tells us that, in the year 1682, a Greenlander in his boat was seen by many people near the south point of the island of Eda, but escaped pursuit. In 1684 another Greenland fisherman appeared near the island of Wistram. An Esquimaux canoe, which the current and the storm had cast ashore, is still to be seen in the church of Burra. In Cardinal Bembo's "History of Venice," it is related that, in the year 1508, a small boat with seven strange-featured men, was captured by a French vessel in the North Sea. The description given of them corresponds exactly with the appearance of the Esquimaux; they were of a middle-size, of a dark colour, and had a broad face with spreading features, marked with a violet scar. No one understood their language. They were clothed in seal-skins. They ate raw flesh, and drank blood as we do wine. Six of these men died on the journey; the seventh, a youth, was presented to the King of France, who at that time was residing at Orleans.

The appearance of so-called Indians on the coast of the German Sea, under the Othos and Frederic Barbarossa, or even, as Cornelius Nepos, Pomponius Melas, and Pliny relate, at the time when Quintus Metellus Celer was proconsul in Gaul, may be explained by similar effects of the current and continuous north-easterly winds. A king of the Boians made a present of the stranded dark-coloured men to Metellus Celer. Gomara, in his "General History of the Indies," expresses a belief that these Indians were natives of Labrador, which would be doubly interesting as the first instance recorded in history of the natives of the Old and the New World having been brought into contact with each other. We can easily account for the appearance of Esquimaux on the North European coasts in former times; as during the eleventh and twelve centuries, their race was much more numerous than at present, and extended, as we know, from the researches of Rask and Finn Magnussen, from Labrador to the good Winland, or the shores of the present State of Massachusetts and Connecticut.

If we compare the climates on the opposite coasts of the Northern Atlantic, we find a remarkable difference in favour of the Old World. The frozen regions of Labrador, lie under the same degree of latitude as Plymouth, where the myrtle and laurel remain perpetually verdant in the open air. In New York, which has a more southern situation than Rome, the winter is colder than at Bergen in Norway, which lies 20° farther to the north. While on the northern coasts of the old continent, the waters remain open a great part of the year, even beyond the latitude of 80°, the ice never completely thaws on the opposite shores of Greenland. What a contrast between the Feroë islands, where the harbours are never frozen, where fertile meadows afford pasturage to numerous flocks of sheep, and even crops of barley reward the labours of the husbandman, and the frightful wildernesses on the shores of Hudson's Straits!—and yet both are situated under the same latitude of 62°.

The milder winter and earlier spring which characterise the north-west coast of Europe, are due, in some measure, to the prevailing westerly winds; but there can be no doubt that they are mainly owing to the influence of the Gulf-stream, which, as we have seen, conveys the heated waters of the Mexican Gulf far to the north-east, and thus imparts warmth to the climate of our native isle. In both seas, on the contrary, which bound the peninsula or island of Greenland, icy currents descend, and continue their course to the south, along the coasts of North America. Near Newfoundland their temperature, in May, is found to be 14° lower than that of the air, and even in spring and the early summer they carry along with them immense ice-blocks, which are frequently drifted as far south as the latitude of New York, and finally disappear in the Gulf-stream.

It is evident that the cold of winter must be increased, and the spring retarded along the North American coasts by these cold streams, just as the coasts of Europe are favoured by streams of a contrary nature; and thus the ocean-currents go a great way to explain the remarkable differences of climate between the opposite shores of the Northern Atlantic.

On this occasion I cannot omit directing the reader's attention to the influence which the far-distant barrier of Central America has upon the climate of Great Britain. Supposing yon narrow belt of land to be suddenly whelmed under the ocean, then instead of circuitously winding round the Gulf of Mexico, the heated waters of the equatorial current would naturally flow into the Pacific, and the Gulf-stream no longer exist. We should not only lose the benefit of its warm current, but cold polar streams, descending farther to the south would take its place, and be ultimately driven by the westerly winds against our coasts. Our climate would then resemble that of Newfoundland, and our ports be blocked up during many months, by enormous masses of ice. Under these altered circumstances, England would no longer be the grand emporium of trade and industry, and would finally dwindle down from her imperial station to an insignificant dependency of some other country more favoured by Nature.

On examining other coast-lands, in different parts of the globe, we shall everywhere find the influence of the reigning currents producing analogous effects to those I have already mentioned.

The Southern Atlantic is not warmed like the European seas by tepid streams, it is exposed on all sides to the free afflux of the cold waters of the Antarctic Ocean, and during the summer months to the influence of drift ice. Thus, the southern extremity of America, Terra del Fuego, the Falkland Islands, South Georgia, Sandwich Land, and other isles of the southern ocean, have a much colder climate than the European coasts and islands situated under the same latitude.

Let us for instance compare the temperature of the Falkland Islands and of Port Famine in the Straits of Magellan, with that of Dublin, which is situated at an equal distance from the line.

		Mean Temperature.
Latitude.	Winter.	Summer.	Annual.
Dublin	53° 21′ N.	+4·	0° R.	15·	3°	9·	6°
Port Famine	53° 38′ S.	+0·	6	10·	0	5·	3
Falkland Islands	52° 0′ S.	4·	36	11·	8	8·	24
Feroë Islands	62° 2′ N.	3·	9	11·	6	7·	1

Thus the climate of the Falkland Islands is, as we see, not very different from that of the Feroë Islands, although the latter lie ten degrees farther from the equator.

In the Pacific Ocean, as well as in the Atlantic, we find a westerly current filling the whole breadth of the tropical zone, from the coast of America to that of Australia and the Indian Archipelago. The best known of its affluxes is the cold Peruvian stream, which, emerging from the Polar Sea, flows with great rapidity along the shores of Chili and Peru, and does not take a westerly direction, before reaching the neighbourhood of the line. It has everywhere a remarkably low temperature, comparatively to the latitude, and this sufficiently accounts for the equal and temperate climate on the coasts of Chili and Peru. Thus, the mean temperature of Callao (12° S. lat.) is only 20° R. while in Rio Janeiro (23° S. lat.), though so much farther from the line, the annual warmth rises to 23·2° R.

In the beginning of November, Humboldt found at Callao the temperature of the sea within the current not higher than 15·5°, while outside the stream it rose to 26° or even 28·5° R.

Even in the vicinity of the equator, after the current has already assumed a westerly direction, its mean temperature does not exceed 20·5°. But as it advances towards the west, its temperature gradually rises to 27° or 28°.

On the western banks of the Pacific the equatorial stream divides into several branches. Part of its waters flow to the south, a greater quantity penetrates through the channels of the south Asiatic Archipelago into the Indian Ocean, the remainder turns to the north-east, on the confines of the Chinese Sea, leaves the eastern coast of the Japanese Islands, and then spreads its warm waters under the influence of north-westerly winds over the northern part of the Pacific. Thus the Japanese stream plays here the same part as the Gulf-stream in the Atlantic, and exerts a similar, though less mighty influence over the climate of the west coast of America, as it is neither so large nor so warm, and, having to traverse a wider ocean, in higher latitudes, naturally loses more of its heat during the passage.

Japan Junks.

It is owing to this stream that Sitcha enjoys a mean annual temperature of +7° R., while Nain in Labrador, situated under the same latitude, is indebted to the Greenland current for a summer of +7·8°, a winter of -18·5°, and a miserable annual temperature of -3·6°. On the west coast of North America the analogous trees grow 3° or 4° nearer to the pole, and the aboriginal tribes go naked as far to the north as 52°, a simplicity of toilet that would but ill suit the Esquimaux of Labrador.

Besides their beneficial influence on different climates the ocean-currents tend to equalise, or to maintain the equilibrium of the saline composition of sea-water, and thus secure the existence of numberless marine animals. Their movements also contribute to the formation of sand-banks, where at certain seasons legions of fishes deposit their spawn and invite the persecutions of man.

The rapidity of currents is very different, but always important enough to be taken into account by navigators. The well-informed seaman makes use of them to traverse wide spaces with greater rapidity, and, after an apparently circuitous course, arrives sooner and more safely at his journey's end than the ignorant steersman, who vainly endeavours to strive against their power.