Europe to America, descend to the latitude of the Canary Islands, where they fall into a current and are carried rapidly to the west. In going from America to Asia across the Pacific, a similar effect is observed. It might be supposed that this was due solely to the trade-winds, but such is not the case: for it is quite possible to distinguish their effect from that of the currents, since the progress of the vessel is quicker than it could be with the aid of the wind alone. The origin of the polar currents is, no doubt, in a great measure, to be referred to the centrifugal force which is the result of the earth's rotation. (See Mathematical Geography, chap. 8.) It may be further explained, when we reflect that the water towards the poles, both on account of its lower temperature and its being less attracted by the heavenly bodies, is heavier than the water in the tropical regions, and, moreover, that the heat of the torrid zone occasions a much more powerful evaporation of the sea than is elsewhere experienced: the consequence is, that the waters nearer the poles will move towards the equator, in order to restore the equilibrium which has, in these several ways, been destroyed. The tropical current may also, though in ano ther manner, be explained as proceed ing from the earth's rotation. The waters, as they advance from the polar seas, pass from regions where the rotatory motion of the earth's surface is very slight, to those where it is exceedingly rapid; they cannot immediately acquire the rapid motion with which the solid parts of the earth revolve in the tropical regions, and they are, accordingly, left rather behind, that is, to the westward (the earth turning round from west to east). The ocean, consequently, appears to retreat from the western, and advance upon the eastern coasts of the continents, or, in other words, to have a general movement from east to west; and the effect is very much assisted by the constant blowing of the trade winds.

We will now explain the modifications or changes which this grand movement in the ocean undergoes, in consequence of the obstacles presented by the land to its free progress. When it meets with shores or narrow straits to impede or turn aside its course, it forms strong and even dangerous currents. The eastern coast of America, and the West India Islands, constitute a sort of dyke to the general westward motion of the

Atlantic; and it will be seen, if we refer to a map, that from Cape St. Roche, which has about five degrees of south latitude, the coast of South America stretches away in a continued line to the north-west, as far as the isle of Trinidad. Owing to this shape of the coast, the waters, as far as the tenth degree of south latitude, are, when they approach America, carried away in a current to the north-west. This current afterwards enters the gulf of Mexico, through the strait formed by the western end of Cuba, and the opposite peninsula, (from this part it is called, by navigators, the Gulf-stream,) and follows the bendings of the Mexican coast, from Vera Cruz to the mouth of the Rio del Norte, and thence to the mouths of the Mississippi, and the shoals west of the southern extremity of Florida. It next takes a new direction to the north, and rushes impetuously into the gulf of Florida. M. Humholdt observed in the month of May 1804, in the 26th and 27th degrees of latitude, that its velocity was eighty miles in twenty-four hours, although, at the time, there was a violent wind against it. At the end of the gulf of Florida, (north lat. 28°) it runs to the north-east, at the rate, sometimes, of five miles an hour. It may always be distinguished by the high temperature * and the saltness of its waters, their indigo-blue colour, and the quantity of sea-weed floating on the surface, and also by the heat of the surrounding atmosphere. The rapidity and temperature of the Gulf-stream, diminish towards the north, while, at the same time, its breadth increases. Its further progress northward is at last checked by the southern extremity of the great bank of Newfoundland, in the 42d degree of latitude, where it turns suddenly to the east. It afterwards continues moving towards the east, and the east-southeast, as far as the Azores islands; and

Humboldt observes that "the waters of the Mexican Gulf, forcibly drawn to the north-east, preserve their warm temperature to such a point, that at forty and forty-one degrees of latitude, he found them at seventy-two degrees and a half (Fahrenheit); when,

out of the current, the heat of the ocean at its surface was scarcely sixty-three degrees and a half. In the parallel of New York, (forty-one degrees north) the equal to that of the seas of the tropics in the eighteenth temperature of the Gulf stream is, consequently, degree of latitude."

Its breadth in latitude twenty-eight degrees and a half is seventeen leagues; (3.46 miles to a league) in the parallel of Charles town, (thirty-three degrees, nearly from forty to fifty leagues; and on the meri

dian of Corvo and Flores, the westernmost of the Azores islands, it is one hundred and sixty leagues.

thence it turns towards the straits of Gibraltar, the Isle of Madeira, and the group of the Canaries, till, on reaching the parallel of Cape Blanco, it completes the round by mixing with the grand westerly current of the tropics. It is probable, however, that a branch still keeps on its course to the south and south-east, along the coast of Africa; for it is well known that ships, if they approach too near the shore, are drawn into the gulf of Guinea, and with difficulty get out again. We thus see that between the parallels of 11 and 43 degrees, the waters of the Atlantic are carried on in a continual whirlpool. Humboldt remarks that, supposing a particle of water to return to the same place from which it departed, "we can estimate, from our present knowledge of the swiftness of currents, that this circuit of three thousand eight hundred leagues is not terminated in less than two years and ten months. A boat, which may be supposed to receive no impulse 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: 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." It is a curious fact, that towards the close of the 15th century, before Europeans were acquainted with the existence of America, two bodies belonging to an unknown race of men were cast by the Gulf-stream on the coasts of the Azores, and pieces of bamboo were brought by the same current to the shore of the small island of Porto Santo; by these circumstances, Columbus is said to have been strengthened in his conjectures with respect to the existence of a western continent.

An arm of the Gulf-stream in the 45th and 50th degrees of latitude, runs to the north-east, towards the coasts of Europe, and becomes very strong when the wind has blown long from the west. The fruit of trees which belong to the American torrid zone is every year deposited on the western coasts of Ireland and Norway; and on the shores of the

Hebrides are collected seeds of several plants, the growth of Jamaica, Cuba, and the neighbouring continent. The most striking circumstance, perhaps, is that of the wreck of an English vessel, burnt near Jamaica, having been found on the coast of Scotland.

There are various currents in the Pacific and Indian oceans. The general westward motion of the former is impeded by a numerous archipelago, and hence it receives different directions. A strong currrent sets to the west, through each of the two straits which respectively separate New Holland from New Guinea and from Van Diemen's Land. It then gets diverted, and flows northward along the coast of Sumatra, till it reaches the bottom of the Bay of Bengal. The following appears to be the reason of its taking this course :the general impetus of the Pacific towards the west, being encountered by New Holland and the numerous East India Isles, is broken and dispersed ; while the westerly motion of the Indian sea has not, in so early a stage, acquired much strength; the polar current from the south, at the same time, presses upon the wide opening which the Indian sea presents to that quarter, and the waters on the eastern verge of that sea are, therefore, pushed into the Bay of Bengal. In the neighbourhood of Čeylon and the Maldive islands, however, the tropical motion has become powerful enough to resist the polar current. The westerly current then recommences, but is again turned out of its line and made to flow to the south-west, by the chain of islands and shallows, which reaches from the extremity of the Indian peninsula to Madagascar. After passing Madagascar, it dashes against Africa, and at the termination of that continent, mingles with the general mo. tion of the waters.

A current afterwards sweeps from the Atlantic into the Pacific ocean, through the straits of Magellan. There can be little doubt that this is a branch of the general current from the south pole; though, at the same time, it may be partly the result of the westerly movement of the Atlantic, which, being checked by the shores of Brazil, flows to the south-west, along the South American coast.

There is a question connected with the currents of the Arctic ocean, which has engaged a good deal of attention, and been considered difficult to explain:

stroyed by the monsoons*, which belong to the class of periodical winds. These blow half the year from one quarter, and the other half from the opposite direction: when they shift, variable winds and violent storms prevail for a time, which render it dangerous to put to sea. They of course suffer partial changes in particular places, owing to the form and position of the lands, and to other cireumstances, but it will be sufficient to give their general limits and directions. Northward from the third degree of south latitude, a south-west wind blows from April to October-from October to April a north-east; these monsoons extend over the China sea, but here they incline more to the direction of north and south. Between the 3d and 10th degrees of south latitude, a north-west wind blows from October to April, and a south-east during the other six months of the year: the former is seldom steady in the open sea, but in December and January it sometimes extends northwards a degree or two beyond the equator. These two monsoons have the greatest strength and regularity in the Java Sea, and thence eastward towards New Guinea. The facts above exhibited may be thus summed up :from April to October a south-west wind prevails north of the equator, southward of this a south-east windfrom October to April, a north-east wind north of the equator, and a northwest between the equator and 10° of south latitude; south of this the usual trade wind, which is in motion through the whole year.

In attempting to account for these movements of the atmosphere over the Indian Ocean, the first thing which strikes us is, that the north-east and south-east monsoons, which are found the one on the north and the other on the south side of the equator, are nothing more than the trade-winds blowing for six months, and then succeeded for the remainder of the year by winds directly opposite. It is also to be noticed that the south-west monsoon in the northern, and the north-west monsoon in the southern hemisphere, each prevails while the sun is perpendicular to their respective regions: they are, therefore, connected with the immediate presence of that luminary.. If the Indian Ocean were not bounded as

• From the Malay word moossin, which signifies a

season.

it is by land on the north, the tradewinds would blow over it (at least in the central parts) as they do in the Atlantic and Pacific Oceans. But it is well known that water, owing to its transparency, is very little warmed by the sun's rays, whereas the land is powerfully heated by them; consequently, when the sun is between the equator and the tropic of Cancer, India, Siam, and the adjacent countries, become much hotter than the ocean; the air over them gets rarefied and ascends; colder air then rushes in from the Indian ocean, and a south-west wind is produced. When the sun, however, has crossed to the south of the equator, these countries become gradually cool, and the north-east trade-wind resumes its course. At the same time the northwest monsoon commences in the southern hemisphere, in consequence of the air over New Holland being rarefied by the presence of the sun.

The monsoons in the Red Sea blow in the direction of the shores; and a similar effect is observed in the Mozambique channel, between Africa and Madagascar, where these winds follow the line of the channel. On the coast of Brazil, between Cape St. Augustine and the island of St. Catharine, and in the bay of Panama, on the west of the isthmus of that name, periodical winds occur somewhat similar to the monsoons of Asia.

The land and sea-breezes, which are common on coasts and islands situated between the tropics, are another kind of periodical winds. During the day, the air, over the land, is strongly heated by the sun, and a cool breeze sets in from the sea; but in the night the atmosphere over the land gets cooled, while the sea, and consequently the air over it, retains a temperature nearly even at all times: accordingly, after sunset, a land-breeze blows off the shore. The sea-breeze generally sets in about ten in the forenoon, and lasts till six in the evening; at seven the land breeze begins, and continues till eight in the morning, when it dies away. These alternate breezes are, perhaps, felt more powerfully on the coast of Malabar than anywhere-their effect there extends to a distance of twenty leagues from the land. During summer, the sea-breeze is very perceptible on the coasts of the Mediterranean, and sometimes even as far north as Norway.