it is from what quarters the timber can come which is found floating on the polar seas in such large quantities, and so much of which is thrown ashore on the northern side of Iceland*. The few specimens seen of the growth of Mexico and Brazil, must have travelled to the north by means of the Gulf-stream of which we have spoken; the rest (principally pines and firs) most likely comes from Siberia and North America, along the shores of which it is drifted till it arrives at the opening into the Atlantic, in the midst of which Iceland is placed.

The existence of under currents different from, and even opposite in their direction to those on the surface is, by no means, improbable in some cases, though it is a matter not admitting of proof. It has been thought that the Mediterranean, which has a strong flow always setting into it through the straits of Gibraltar, sends back a portion of its waters into the Atlantic, by a concealed current. Contrary currents, passing along side by side, are not uncommon. In the Kattegat, a northern current flows out of the Baltic, along the coast of Sweden, while a southern one enters the Baltic along the coast of Denmark. When two opposite currents of about equal force meet one another, they sometimes, especially in narrow channels, turn upon a centre and assume a spiral form, giving rise to eddies or whirlpools. The most celebrated of

Velocity of the Wind.

Miles in an hour.

1

4

5)

10

15

30

35

50

80

these are the Euripus, near the island of Euboea, in the Grecian Archipelago; Charybdis, in the strait between Italy and Sicily; and the Maelstrom, off the coast of Norway. The most violent of them, when agitated by tides or winds, become very dangerous to navigation. On Currents in the Atmosphere. It does not belong to our present subject to investigate the properties and component parts of the atmosphere, but it will be proper to notice the agitations or movements which are constantly taking place in that fluid. A change in the temperature of a portion of air; an increase or a diminution of the quantity of water which it holds in a state of vapour; in short, any circumstance which causes it either to contract or to expand, destroys the equilibrium subsisting among the different parts of the atmosphere, and occasions a rush of air, that is, a wind, towards the spot where the balance has been destroyed.

Every one knows that the velocity and force of winds are exceedingly various; accordingly, several methods have been suggested, and instruments invented, in order to determine their amount with some degree of exactness. In the fifty-first volume of the "Philosophical Transactions," there is a table of the different velocities and forces of winds, drawn up from a considerable number of facts and experiments; the following particulars are extracted from it:

100

Winds may be divided into three classes-those which flow constantly in the same direction, those which are periodical, and those which are variable. It must be observed that the terms which express the direction of winds are employed in a sense quite

From the account which Captain Parry has given of his last voyage, it appears that there is also a great quantity of timber which, has been cast by the sea upon the northern coast of Spitzbergen.

A storm.
A hurricane.

contrary to that in which they are used when we speak of the direction of currents in the ocean; a westerly current, for example, signifies a current flowing towards the west, but a westerly wind signifies a wind coming from that quarter.

The permanent winds are those which blow constantly between, and a few degrees beyond, the tropics, and are called trade-winds. On the north of the

equator, their direction is from the north-east (varying at times a point or two of the compass each way); on the south of the equator, they proceed from the south-east. The origin of them is this: the powerful heat of the torrid zone rarefies or makes lighter the air of that region; the air, in consequence of this rarefaction, rises, and to supply its place a colder atmosphere from each of the temperate zones moves towards the equator. But (as in the case of the polar currents in the ocean) these north and south winds pass from regions, where the rotatory motion of the earth's surface is less, to those where it is greater. Unable at once to acquire this new velocity, they are left behind, and instead of being north and south winds, as they would if the earth's surface did not turn round, they become north-east and south-east winds. The space included between the second and fifth degrees of north latitude, is the internal boundary of the two winds; and this space experiences calms, frequently interrupted, however, by violent storms. The reason why it is situated to the north of, instead of exactly at, the equator, seems to be that the northern hemisphere is warmer than the southern; for since the trade-winds are the result of the continual ascent of heated air in the equatorial parts, their internal boundary will be where the principal ascent is going on,-that is, where the annual temperature is the highest; which, on account of the above-mentioned inequality of temperature in the two hemispheres, will not be at the equator, but somewhat to the north of it. The external limits of the tradewinds are at a medium in about the 30th degrees of north and south latitude respectively; but each limit, as the sun approaches the neighbouring tropic, declines further from the equator. The position of the sun has an influence also on their strength and direction; for when that luminary is near the tropic of Cancer, the south-east wind bee mes gradually more southerly, and stronger, and the north-east weaker, and more easterly; the effect is reversed when he gets towards the tropic of Capricorn.

The trade-winds would blow regularly round the whole globe within the distance of about 30 or 40 degrees from the equator each way, if the space within those limits were all covered

with water; but the uneven surface and unequal temperature of the land divert and derange them: it is on this account that the trade-winds are constantly experienced only over the open ocean. The larger the expanse of ocean over which they range, the more steadily they blow: thus in the Pacific they are commonly more steady than in the Atlantic ocean, and in the south than in the north Atlantic. In sailing from the Canaries to Cumana, on the north coast of South America, it is hardly ever requisite to touch the sails of the vessel. The voyage across the Pacific, from Acapulco on the west coast of Mexico, to the Philippine islands, is performed with equal facility; and if there were a channel through the isthmus of Panama, a westward passage from Europe to China would be more speedy and safe than the usual navigation thither round the Cape of Good Hope; the only interruption to the evenness of this voyage would be in the Caribbean Sea and the Gulf of Mexico, where the trade wind blows impetuously, and is sometimes interrupted by westerly winds. It would not be possible, however, to return by the same route, because in sailing east way must be made to the northward, in order to get beyond the region of the trade into that of the variable winds. Both in the Atlantic and in the Pacific Ocean, the current of the trade-winds becomes broader and more directly east in its course, as it advances from one side to the other of those extensive basins. On the west coast of Africa, owing to the rarefaction which the air undergoes over that continent, the wind is mostly turned towards the shore; from Cape Bojador to Cape Verd it is generally north-west, and thence to the island of St. Thomas, under the equator, it bends gradually first to the west and then to the south-west. Along the coasts of Chili and Peru a south wind prevails. These are two instances of the interruption which the trade-winds experience in the neighbourhood of large masses of land.

In the Indian Ocean the south-east trade-wind prevails between 25° and 16° of south latitude, from within a few de grees of the east side of Madagascar, nearly to the coast of New Holand; but from the 10th degree of sou hatitude to the nor hein shores of that ocean, the unitoimity of the tropical movements of the atmosphere is de

We thus perceive that within the limits of from 28 to 30 degrees on each side of the equator, the movements of the atmosphere are carried on with great regularity; but beyond these limits, the winds are extremely variable and uncertain, and the observations made have not yet led to any satisfactory theory by which to explain them. It appears, however, that beyond the region of the trade-winds, the most frequent movements of the atmosphere are from the south-west, in the north temperate zone, and from the north-west, in the south temperate zone, This remark must be limited to winds blowing over the ocean and in maritime countries; because those in the interior of continents are influenced by a variety of circumstances, among which, the height and position of chains of mountains are not the least important. These south-west and north-west winds of the temperate zones are most likely occasioned in the following manner ::-In the torrid zone there is a continual ascent of air, which, after rising, must spread itself to the north and south in an opposite direction to the trade-winds below: these upper currents, becoming cooled above, at last descend and mix themselves with the lower air; part of them may perhaps fall again into the trade-winds, and the remainder, pursuing its course towards the poles, occasion the north-west and south-west winds of which we have been speaking. It has also been conjectured that these winds may frequently be caused by a decomposition of the atmosphere towards the poles, from part of the air being at times converted into

water.

Hurricanes have been supposed to be of electric origin. A large vacuum is suddenly created in the atmosphere, into which vacuum the surrounding air rushes with immense rapidity, sometimes from opposite points of the compass, spreading the most frightful devastation along its track, rooting up trees, and levelling houses with the ground. They are seldom experienced beyond the tropics, or nearer the equator than the 9th or 10th parallels of latitude; and they rage with the greatest fury, near the tropics, in the vicinity of land or islands, while far out in the open ocean they rarely occur. They are most common among the West India islands, near the east coast of Madagascar, the islands of Mauritius and Bourbon, in the Bay of Bengal at the changing of the monsoons, and on the coasts of China.

Whirlwinds sometimes arise from winds blowing among lofty and precipitous mountains, the form of which influences their direction, and occasions gusts to descend with a spiral or whirling motion. They are frequently, however, caused by two winds meeting each other at an angle, and then turning upon a centre. When two winds thus encounter one another, any cloud which happens to be between them is of course condensed and turned rapidly round; and all substances sufficiently light are carried up into the air by the whirling mo tion which ensues. The action of a whirlwind at sea occasions the curious phenomenon called a water-spout, which is thus described by those who have witnessed it. From a dense cloud a cone descends in the form of a trumpet with the small end downwards; at the same time, the surface of the sea under it is agitated and whirled round, the waters are separated into vapour, and ascend with a spiral motion till they unite with the cone proceeding from the cloud; frequently, however, they disperse before the junction is effected. Both columns diminish towards their point of contact, where they are not above three or four feet in diameter. In the middle of the cone forming the water-spout, there is a white transparent tube, which becomes less distinct on approaching it, and it is then discovered to be a vacant space in which none of the small particles of water ascend; and in this, as well as around the outer edges of the waterspout, large drops of rain precipitate themselves. In calm weather, waterspouts generally preserve the perpendicular in their motion; but when acted on by winds they move on obliquelysometimes they disperse suddenly, at others they pass rapidly along the surface of the sea, and continue a quarter of an hour or more before they disappear. A notion has been entertained that they are very dangerous to shipping, owing to the descent, at the instant of their breaking, of a large body of water sufficient to sink a ship; but this does not appear to be the case, for the water descends only in the form of heavy rain. It is true, that small vessels incur a risk of being overset if they carry much sail; because sudden gusts of wind, from all points of the compass, are very common in the vicinity of water-spouts.

D