670. The southern polar current is crossed and checked by the western current passing by Cape Horn, and the Cape of Good Hope. This prevents the ice islands from floating away into warmer latitudes; and probably is the cause of the great accumulation of ice around the south pole.

671. There are various smaller currents in different parts of the ocean. There is one of considerable extent flowing in towards the western coast of Africa, which has sometimes caused the shipwreck of vessels. It is remarkable that a current continually flows from the Atlantic into the Mediterranean Sea, so that vessels cannot come out of the Straits of Gibraltar, unless they are aided by a strong casterly wind.

672. When two opposing tides or currents meet, they often cause whirlpools. That of Charybdis, in the Straits of Messina, was the terror of ancient mariners. The most remarkable known is the Maelstrom, on the coast of Norway. It roars with a tremendous noise, especially in a storm, and its influence is then felt for more than nine miles. Its power is such, that ships, and even whales, have been drawn in, and ingulfed, from a distance of several miles.

673. The passage from New-York Harbour to Long Island Sound, which is called Hurlgate, is remarkable for its numerous whirlpools, produced by the flow of the tides through a rocky channel. It can only be passed with safety at particular times of tide. At other times, the appearance and roar of the whirlpools is terrifying to one who is not familiar with the scene.

674. Besides these motions the waters of the ocean and of lakes circulate from the top to the bottom, in cold seasons and climates. When the water at the surface is cooled more than that below, it becomes denser (or heavier,) and sinks to the bottom; a warmer portion rises in its place; and when it is cooled, sinks again, and gives place to another-until the whole mass assumes an equal temperature.

675. Were this law to continue through all the degrees of cold, the whole ocean would be reduced to the freezing point, and probably be congealed to the bottom; so that the heat of the temperate zones could never melt it. But by a wonderful direction of Providence, when water arrives at the temperature of 40 degrees, or 8 degrees above the freezing point, it expands instead of condensing, and becoming lighter, it remains on the surface, and protects the water below from the influence of the cold air. In this way, the greater part of the ocean is kept above the freezing point.

676. By the saltness of the ocean and its constant movements, it is prevented from corrupting, and preserved from the extremes of heat and cold. Its currents, flowing and returning from the warm to the cold regions, equalize in some degree the temperature of the earth, and render countries habitable, which would otherwise be desolated with frost. The vapours, breezes, and showers which rise from it, refresh those which are parched with heat, and preserve them from becoming

deserts.

[111] Thus examination shows us, that what appears at first sight a useless waste of waters, is an indispensable provision of the Creator for our preservation and comfort. It also furnishes a habitation for innumerable fish, which supply a large part of mankind with food; and renders communication more easy, between the countries it seems to divide.

677. When the air over any place is more heated than that around, it is rarified or expanded, and rises. The surrounding air rushes in to supply its place, and this produces a current called wind.

678. There are some winds which blow constantly in the same direction. Of this kind, there are two general currents of the atmosphere-that which follows the course of the sun in the Torrid Zone-and that flowing from the cold regions round the poles, towards the equator, which is chiefly felt in the Temperate Zones. Other winds are periodical, or blow only at certain periods of the day or the year; but these, as well as the constant winds, are chiefly confined to warm climates. Beyond latitude 400, the winds are generally variable.

679. In the Torrid Zone, as the parts under the sun are hottest, and as the earth turns from west to east, the air moves in an opposite direction, following the sun, and forms constant winds from the east, called trade winds. North and south of the equator, the trade winds become more north-easterly and south-easterly, by uniting with the polar current.

680. The trade winds extend as far as 280 or 300 on each side of the equator, during the summer of each hemisphere; but on the coast of America, they are felt as far as 40°. By the aid [112]

of these winds, the Spanish vessels which sail from Mexico to the Philippine Isles, often finish a voyage, nearly equal to half the circumference of the globe, in 60 days, without altering their Course, or changing a sail. But in returning they are obliged to go north, beyond the limits of the trade winds.

681. In the Indian Ocean, north of 10° south latitude, there are periodical winds called monsoons, which blow half the year north-east, and the other half south-west, They extend over the whole of India and the sea coast of East Persia. The southwest monsoon blows constantly, from April to October, bringing with it floods of rain, and dreadful tempests. During the rest of the year, the north-east monsoon produces a dry and agreeable state of the air. The change from one monsoon to the other is gradual, and accompanied with storms and hurricanes.

682. In islands and places near the sea, in warm climates, there is usually a wind from the land at night, and from the sea in the day, called the land and sea breezes. Without the sea

breeze, the heat of many places in these climates would be almost insupportable.

In the Mediterranean sea, the West Indies, &c. the land breeze usually begins at 6 or 7 o'clock in the evening, and blows until 8 in the morning, when the sea breeze begins, increasing till noon, and gradually dying away in the afternoon. Between the changes, there is a period of stillness, as between the ebbing and flowing of the tide.

683. Hurricanes are violent storms occurring in South America, the West Indies, and other hot countries, in which the wind changes in a short time to every point of compass, and blows with a violence which scarcely any thing can resist.

•Hurricanes are attended with a great swelling of the sea, and sometimes with earthquakes. They happen in the rainy season, principally in August, destroying all the produce of the ground, tearing up trees, blowing down buildings, and inundating large tracts of country. The trade winds are interrupted during their continuance.

On the south-eastern coast of Asia, especially in the Gulf of Tonquin, tremendous storms of the same kind occur, which are called Typhons or Tuffoons.

The velocity of the trade winds is estimated by Lalande at 5 or 6 miles an hour; and of a brisk gale, at 12 or 15. High winds move from 30 to 40 miles an hour and storms and hurricanes, from 50 to 100.

Winds are necessary to purify the atmosphere. They raise and transport the clouds from the sea to fertilize the land, and serve to convey us over the ocean, the great highway of the globe.

684. Whirlwinds are formed by opposite winds, meeting and moving swiftly in a circle, raising sand and light bodies into the air. In the deserts of Africa they sometimes draw up the sand

When they ap

into a moving pillar, which buries all in its way. pear on the ocean, they draw up the water, and produce waterspouts. In a water-spout, a column of water is seen hanging from the clouds, and descending until it meets a column rising [113] from the ocean. (Fig. 26.) These unite and often move with rapidity, until they meet with some opposing wind, or other cause, which destroys them. They are not uncommon in warm latitudes.

685. The quality of winds is affected by the countries over which they pass; and they are sometimes rendered pestilential by the heat of deserts, or the putrid exhalations of marshes and lakes. Thus from the deserts of Africa, Arabia, and the neighbouring countries, a hot wind blows, called the Samiel or Simoom, which sometimes produces instant death. A similar wind blows from the Sahara, upon the western coast of Africa, called the Harmattan, producing a dryness and heat, which is almost insupportable, and scorching like the blasts of a furnace.

686. In the southern countries of Europe, particularly Spain and Italy, a warm, unpleasant wind blows from Africa, which is called the Sirocco. It occasions great uneasiness in the human frame, irritating the nervous system, and checking perspiration. Its temperature does not exceed 95 degrees; but a dry, feverish heat is produced in the body, and such debility, that exertion is painful.

The Sirocco sometimes blows only a few hours; occasionally from morning to evening; and very rarely three days. The inhabitants shut themselves up in their houses to avoid its influence. It is singular that to the natives of cold climates, and to strangers in feeble health, it is often reviving.

The various winds of this kind are doubtless only blasts of heat from extensive deserts, under different names, varying in their power and effects according to circhimstances.

687. Between the fourth and tenth degrees of north latitude, a part of the ocean, lying south of Cape Verd and its islands, is remarkable for calms, almost perpetual, attended with dreadful thunder and lightning. The showers are so frequent that it has been called "The Rains."

RAIN

688. When water is dissipated into the air in an invisible man ner, the process is called evaporation. Spontaneous evaporation is continually produced by the agency of the sun and air, from the waters of the ocean and the land. The watery vapours condensed by cold, or rising into the atmosphere to a region of the air lighter than themselves, form visible mists, or fogs, and the strata of visible vapours, called clouds.

689. The vapours thus accumulated in the clouds, at lengthi fall in rain, snow, and hail, and return through the rivers, to the ocean. Snow consists of such vapours as are frozen while the particles are small. It is rarely seen below latitude 30o in America, and 36o on the eastern continent. Hail is a more compact mass of frozen water, which consists of such vapours as are united into drops, and are frozen while they are falling. It is scarcely ever known in latitudes higher than 60o.

[114] 690. The vapours which arise from the earth do not ascend above a certain height, and here neither rain nor snow are known. The ordinary height of clouds does not exceed one or two miles.

691. Various quantities of rain fall in different parts of the earth, according to the climate and situation. The quantity of rain and snow is greatest on mountains, and they contain the principal sources of rivers. Islands, and places near the ocean, are of course more subject to rains and moisture, than the interior of a country.

692. The quantity of rain also varies with the latitude. In the Torrid Zone the evaporation is most abundant on account of the heat, and the greatest quantity of rain falls on this portion of the earth. It descends in one season of the year; and for the rest of the year there is no rain.

From this circumstance the number of rainy days is smallest at the equator; and increases in proportion to the distance from it. From north latitude 120 to 430, the mean number of rainy days is 78-from 43° to 460, the mean number is 103-from 460 to 509, it is 134-from 510 to 60°, 161.

The average number of rainy days in a year, for 20 years, in Salem and Cambridge, Massachusetts, and in 20 cities in Europe, is thus stated, with the number of fair and cloudy days for one or two years.

693. The rainy season in the Torrid Zone is during the summer, when the sun raises the greatest quantity of vapour, and of course it varies with this season on each side of the equator. In the southern regions of Asia, the rains accompany the south-west monsoons, which continue from April to October, and are attended with violent thunder storms.

The effects of these periodical rains are most remarkable in the northern tropical regions of Africa. Showers begin in April, and increase till June; when toirents of rain begin to descend, and continue almost three months without intermission. The face of nature is soon changed; rivulets before dried up swell into mighty streams; the rivers overflow their banks; and the plains become vast