of the tide it receives an accession of near forty feet to its depth of water, which enables the largest West Indiamen and steamers to communicate directly with the city. The change brings not only a supply of water adequate for navigation, but an alternate current every twelve hours, which is just as useful as having a fair wind up and down the river, the regular occurrence of which being certain may immediately be turned to account by previous preparation. The same phenomenon is exhibited on the Solway Firth, the sands of which are so dry at low water that travellers on horseback can cross them, while the tide returns so rapidly as to render this a somewhat hazardous

experiment. Besides the advantage to navigation, to a great metropolis like London, and other cities similarly situated, the tidal flux and reflux is a social convenience of no mean value, and an important sanitary agent. It not only supplies the means of carrying off the drainage, and thus preventing it from endangering health by vitiating the atmosphere, but of having fresh water and air regularly furnished-an arrangement which has been aptly compared to a system of lungs adapted to promote the healthy vital action of maritime populations.

A third movement to which the ocean is subject is known by the name of Currents, which involve not merely the surfacestratum of the sea, but probably extend far below it, where they prevail, and constitute great oceanic highways. The effect of currents was perceived long before any thing was known of their direction and velocity; and Columbus was strengthened in his belief, that land might be reached across the Atlantic westward, by substances which had been drifted from that quarter. A pilot in the service of the King of Portugal, Martin Vicenti, assured him, that after sailing four hundred and fifty leagues to the west of Cape St. Vincent he had taken a piece of carved wood from the sea, evidently not laboured with an iron instrument, which must have floated from some unknown land in a westerly direction. Columbus was also informed by his brother-in-law, Pedro Correo, that he had seen a similar piece of wood off Porto Santo, a small island to the north-east of Madeira, which seemed to have come from the same region; and it was commonly reported that reeds of an immense size had floated to those islands from the west, which the great discoverer fancied were identical with

those described by Ptolemy as growing in India. From the inhabitants of the Azores he learnt that trunks of huge pine-trees had been cast upon the shores, of a species different to any that grew upon the islands; while, towards the close of the fifteenth century, the bodies of two dead men had been drifted to the island of Flores, whose features proclaimed them to belong to an unknown race. These circumstances contributed to confirm Columbus in his theory respecting the existence of a western continent, and strengthened his purpose to venture upon the untracked waste of waters in order to reach it. After the commencement of his great undertaking, when day after day nothing had been seen but a shoreless horizon, and hope had nearly expired in his own breast, while his crew were on the verge of open rebellion, the effect of the oceanic currents restored his confidence, and allayed their clamours. Herbage, fresh and green as if recently plucked, floated by. A branch of thorn, with berries on it, appeared; a reed was picked up, and a staff artificially carved,-significant intimations that an inhabited land lay before the adventurers, which was at length revealed to their gaze, and terminated for ever the mystery which had rested upon the western flood. Upon his second voyage, Columbus found, near one of the islands, to which the Spaniards gave the name of Guadaloupe, the stern-post of a European vessel, the fragment of some wreck which had been borne in a contrary direction across the Atlantic. The preceding facts are doubtless referable to the action of the equatorial current and the gulf stream. Previously, the inhabitants of the Canaries had considered the vegetable productions thrown upon their shores as coming from the enchanted isle of St. Borondon, which, according to the reveries of the pilots, and certain legends, was placed towards the west in an unknown part of the ocean, enveloped with eternal fogs.

It is little more than half a century since the oceanic currents began to be accurately investigated. We are indebted to M. Rossel, Colonel Sabine, and Major Rennell, for the principal part of the information which has been collected concerning them. Though tolerably well acquainted with their site, direction, and velocity, the causes in which they originate are not thoroughly understood. In all probability they are chiefly due to the influence of permanent winds, to a difference in temperature or saltness between two parts of the sea, to the annual melting of the polar ice, to the unequal evaporation which the surface of the ocean experiences in high and low latitudes, and to the greater velocity with which the equatorial regions are carried round in the daily rotation of the globe. The sea-currents have been compared to the continental rivers, and both exhibit the phenomena of volumes of water moving in a certain direction, but in extent of surface and depth they are utterly disproportionate; and if the former were transferred to the land, they would constitute great inland seas, or arms of the ocean.

There are two great currents flowing from the poles towards the equator, north and south, which preserve their direction through a considerable space. The drifting of the ice from the polar regions into the temperate seas, on each side of the line, evidences the existence of streams following that course. The north polar current appears to strike the shores of Asia, and, passing round the north cape of Europe, it crosses the upper part of the Atlantic, running to the south-west till it reaches the east coast of Greenland. It then traverses the narrow sea between that country and Iceland, turns round Cape Farewell, the southern extremity of Greenland, and proceeds northward into Davis's Strait. It follows the eastern side of the strait as far to the north as Holsteinborg, in latitude 67°, where, from causes of which we are ignorant, it abruptly turns to the west, and strikes the opposite shore at Cape Walsingham. From thence its course is southward to Labrador, and south-east to the north bank of Newfoundland, where it mingles with the Gulf stream, which will hereafter be noticed. The breadth of the arctic current is in some places from 250 to 300 miles. Its velocity varies, in different parts of its course,

from eight or nine to fifteen or sixteen miles per day. The icy masses it bears along are supposed to be about two months in making the before-mentioned circuit from Cape Farewell to the coast of Labrador. We are not so familiar with the antarctic or south polar current, but have similar evidence of its action in the transportation of the ice from high to low latitudes, where its course is checked by a counter stream passing westward by the Cape of Good Hope and Cape Horn. Various circumstances operate to put these streams in motion. The greater intensity of the centrifugal force at the equator, which is the result of the earth's rotation, and the greater evaporation of the sea between the tropics, owing to the powerful heat of the torrid zone, must necessarily produce a movement of the waters from the poles towards the equator, in order to restore the equilibrium which the preceding causes are perpetually destroying. It is however a singular fact, only recently ascertained, with reference to the arctic current, and quite impossible to explain at present, that it appears to be annually suspended for about three months, from the middle of October to the middle of January; and no perceptible evidence whatever of its action at that period is found at Cape Farewell, as at other seasons, in the accumulation of ice around it. Upon the inhabitants of Iceland this great oceanic stream confers no unimportant benefit in the drift timber which it casts upon their shores, affording them an abundant supply of wood for fuel, and for the construction of boats, their own forests having been improvidently exhausted. The immense quantity of it, amounting to whole forests of pines and firs, has attracted much attention; and it has been deemed difficult to explain from what country it can have been derived. Captain Parry found a large quantity thrown by the sea upon the coasts of Spitzbergen; and Crantz informs us, that the masses of floating wood thrown upon the island of Jan Mayen often equal the whole of the island in extent. The most probable solution of its origin is, that the rivers of Northern Asia carry the timber of the Siberian forests into the Arctic Ocean, from whence it is borne by the current to mitigate the cold of the Icelandic winter. It was the north polar current that presented the most formidable obstacle to Parry in his attempt to reach the pole by means of boat-sledges and reindeer, and led to the failure of the daring enterprise. Having travelled over the frozen surface of the deep to nearly latitude 83°, which seemed to be the utmost limit of animal life, the adventurers found that when, according to their reckoning, they had made ten or eleven miles of direct northing, they had actually gone four miles to the south, owing to the current carrying the snow-fields in that direction. An invisible power was thus continually undoing what they were labouring to accomplish, which rendered the success of the expedition hopeless.

The grandest movement of the ocean in the form of a stream-current proceeds from east to west, on each side of the equator, and is therefore called the equatorial or tropical current. In the Pacific Ocean it sweeps westward from the coast of Peru in one immense volume, till, reaching Australia and the islands of the Asian Archipelago, it is broken into 'smaller branches of different divergence; and hence the numerous and variable currents prevailing in the Indian Ocean, which render the navigation so dangerous. A great branch passes to the south of Australia, preserving a generally uniform westerly direction, till it is obstructed by the island of Madagascar and the east coast of Africa, from which it glides off to the south round the Cape of Good Hope. In the Atlantic Ocean the equatorial current is very perceptible, the westward flow of the waters commencing in the Bay of Benin and at the Canaries, and proceeding across its basin to the opposite shore of America, where, off Cape St Roque, on the Brazilian coast, it is separated into two branches. One branch, called the Brazil current, proceeds southward along its coast-line, crosses the mouth of the La Plata, passes through the Straits of Magellan and round Cape Horn, and mingles with the westward flow of the Pacific.

The other and principal branch, known by the name of the Guiana current, is properly a direct continuation of the equatorial. It runs from off Cape St. Roque, across the mouth of the Amazon; and after skirting the low coast of Guiana, and passing through the Caribbean Sea, it enters the Gulf of Mexico, where a course is commenced in a fresh direction. Mention has previously been made of the Gulf stream. This originates in the Mexican Gulf, and is the efflux of the waters accumulated there by the equatorial current. The stream is first clearly perceptible to the north-west of the island of Cuba, where it flows weakly to the east; but upon being turned from that direction by the opposition of immense sand-banks, it proceeds northward, and, owing to the narrowness of the channel, rushes with great velocity through the Strait of Florida. Obeying the impulse there given to its waters, the Gulf stream runs along the coast of the United States; and, being there free from obstruction, it gradually expands in volume, and diminishes in rapidity. On striking the banks of Newfoundland it sets again to the east, and to the south-east upon joining the north polar current. It then traverses the basin of the Atlantic to the Azores, enters the equatorial current on the coast of Africa, and is conducted again to the west, to re-enter into itself in the Gulf of Mexico.

The equatorial current and the Gulf stream thus constitute a whirlpool of prodigious extent in the Atlantic Ocean, which cuts laterally the gorge between Africa and the Brazils, scours round the indentation of Central America, recrosses the bed of the Atlantic following an opposite direction, and is perpetually circulating in the same route. Humboldt remarks, that supposing a particle of water returns to the same place from which it departed, our present knowledge of the swiftness of currents will enable us to estimate, that this circuit of 3800 leagues will require not less than two years and ten months for its accomplishment. "A boat which may be supposed to receive no impression from the winds, would require thirteen months from the Canary Islands to reach the coast of Caraccas; ten months to make the tour of the Gulf of Mexico, and reach Tortoise Shoals opposite the port of the Havannah; while forty or fifty days might be sufficient to carry it from the Straits of Florida to the bank of Newfoundland. It would be difficult to fix the rapidity of the retrograde current from this bank to the coasts of Africa: but estimating the mean velocity of the waters at seven or eight miles in twenty-four hours, we find ten or eleven months for this last distance. A short time," he continues, “before my arrival at Teneriffe, the sea had left in the road of St. Croix a trunk of a Cedrela odorata covered with the bark. This American tree vegetates exclusively under the tropics, or in the neighbouring regions, and it had no doubt been torn up on the coast of the continent, or of that of Honduras. The nature of the wood, and the lichens which covered its bark, were evident proofs that this trunk did not belong to those submarine forests which ancient revolutions of the globe have deposited in lands transported from the polar regions. If the Cedrela, instead of having been thrown on the strand of Teneriffe, had been carried farther south, it would probably have made the whole tour of the Atlantic, and returned to its native soil with the general current of the tropics." The conjecture in the last passage is supported by a fact recorded in the history of the Canaries by the Abbé Viera. In the year 1770 a small vessel laden with corn, and bound from the island of Lancerotte to Vera Cruz in Teneriffe, was driven to sea while none of the crew were on board, and was carried by the westward motion of the waters across the bed of the Atlantic, where it went ashore at La Guayra, near Caraccas. It was the equatorial current that conveyed the fragment of the vessel to Guadaloupe which Columbus found floating near the island; and the Gulf stream brought those productions of the New World to the Canaries, upon which he seized as indications of the existence of western regions, before the discovery of them had been effected. A complete view of this great water-course would require a notice of several subordinate currents; but it may be sufficient to observe, that the

western flow of the Pacific, mentioned as sweeping round the Cape of Good Hope, mingles with it as a perpetual feeder, while, from its northern region, a branch is sent off towards the coasts of Great Britain and Norway. This arm of the gulf stream leaves it in latitude 45° and 50°, near the bank of Bonnet-Flamand. It runs to the north-east, and becomes very strong when the winds have blown a long time from the west. By this current, plants, seeds, and the fruit of trees, which belong to the torrid zone of America, are annually borne through the North Atlantic, and deposited on the western coasts of Ireland, Scotland, and Norway. On the shores of the Hebrides, the seeds are collected

of several plants belonging to Jamaica, Cuba, and the neighbouring continent; and the remains of cargoes of vessels wrecked in the West Indian seas have been drifted to the same quarter. The fragments of the English vessel, the Tilbury, burnt near Jamaica, reached the coast of Scotland; and tortoises inhabiting the waters of the Antilles have undergone a similar transportation. A most remarkable case is related by Wallace, that twice, in 1682 and 1684, American savages of the race of the Esquimaux, driven out to sea in their leathern canoes during a storm, and left to the guidance of the currents, reached the Orkneys-an example of involuntary migration, which shows how, when the art of navigation was yet in its infancy, the motion of the waters of the ocean may have contributed to disseminate the different races of men over the face of the globe.

It is natural to inquire concerning the origin of this extraordinary and vast whirl of the waters of the ocean. The westerly movement in equatorial regions is in a direction contrary to that of the earth's rotation, and appears to be connected with this last phonomenon, along with the trade winds, which follow the same course. Owing to the action of the current and the wind, the waters of the Atlantic are crowded into the great American bay which terminates with the Gulf of Mexico, entering it on the south, where, upon being repulsed by the shores of the continent, they accumulate, and flow off to the north-east, forming the gulf stream. It has been calculated that the waters of the gulf, 4000 miles distant from the Cape de Verde islands on the coast of Africa, are elevated 325 feet above the level of the ocean in the latter locality. This arises from the pressure exerted by the particles of water upon each other under the influence of the wind and current, and the resistance offered to their farther westward progress by the shores of Central America. It has frequently been observed, that a prolonged wind