pand, project a ball with great force, and answer the purpose of powder. This is called the air-gun.

WIND.

WIND is a stream of air; or air in motion. The natu ral state of this fluid, as well as of all others, is rest; which it endeavours to keep, or to retrieve, when lost, by a universal equilibrium. When this natural equilibrium is lost, by any cause whatever, there must of consequence be a motion of all the circumjacent air towards the point where it is destroyed, to restore it again. And this motion is what we mean by wind.

Various are the causes of particular winds: but the most common and principal causes are heat and cold; the first, by rarefying the air and making it lighter; and the last, by condensing and making it heavier. When the atmosphere is more rarefied by heat in one place than in another, the heated air will ascend, and the colder and more condensed air will rush in from every other quarter. When the air in a room is heated, the external air, which is cooler, will force itself into the room through any aperture, and thereby produce a stream.* The pressure on the outside will be greater than on the inside; and the air, being a fluid, will move in the direction of the greatest pressure, until the equilibrium be restored. Therefore, when any part of the atmosphere is more rarefied than another, the denser air will rush in to that part, as it will press more in that direction

When the air is heated in a room without a chimney, its elasticity being increased, will produce a current outwards, through any aperture or crevice; but when heated in a room having a chimney, the heated air will ascend, and its place will be supplied from the external air.-Ed.

than in any other, and thereby produce a wind. The current of air through a broken pane of glass, into a heated room, is a sufficient proof of this position. It may, however, be rendered still more evident by an easy experiment. If a water-plate be filled with warm water, and placed in a tub of cold water; and if a lighted candle be blown out, and the smoking wick be held over the cold water, the smoke will move towards the heated air over the plate and ascend there. But you will see a contrary motion of the smoke, if the experiment be reversed, by filling the plate with cold water, and placing it in a tub of warm water.

The whole torrid zone, which bounds the apparent annual course of the sun, will have the air in it considerably rarefied at different seasons of the year, according as the sun is north or south of the equator, by receiving a greater quantity of his direct rays, than any other part of the atmosphere. The point of greatest rarefaction will not be directly under the sun, but about three hours to the eastward of him, and in his diurnal path; because every body requires some time to be heated, and does not instantaneously receive the greatest degree of heat it is capable of sustaining. This point of greatest rarefaction following the sun in his daily motion, will continually shift its place towards the westward, heating the western air, and leaving the eastern air to cool. To this point, wherever it is, the colder air will continually rush in, to restore the equilibrium of the atmosphere, from the north and south, the east and west. But the current from the south.will be overborne by the current from the north, while the sun is north of the equator; as the air is more heated between the tropics and south of the point of greatest rarefaction, than on the north side of the tropics. The contrary hap

pens, when the sun is on the south side of the equator, and for the same reason; the current of air from the south prevailing over that from the north; so that from the vernal to the autumnal equinox there will be a north wind, and from the autumnal to the vernal, there will be a south wind towards the point of greatest rarefaction. And as this point follows the apparent motion of the sun from east to west, while the sun gradually heats the western air and suffers the eastern air to cool, it follows, that the eastern current will prevail over the western, and thereby generate an east wind. But the north and east winds, when compounded together, will produce a northeast wind, while the sun is on the north side of the equator, which will blow for six months. In like manner, the currents of air from the south and east, when compounded together, while the sun is south of the equator, will generate a southeast wind for the other six months of the year. These are called trade winds, from their great utility in trade and navigation.

This theory we find verified in the Atlantic, Pacific, and Ethiopic oceans. In these seas, we find a wind blowing either from the northeast or southeast through the year, which extends to about 30 degrees on each side of the equator. We cannot expect, however, that these tropical winds will be always regular all over the world, unless the whole surface of the globe were uniformly covered with water; so that there might not be any extraordinary rarefaction produced in any place by any other cause than the direct heat of the sun. The land is susceptible of a greater degree of heat than the water, and will be generally more heated, as it constantly presents the same surface to the rays of the sun, while the heated surface of the water is, from its continual motion, constantly changing. As the sun, there

fore passes over islands and continents, the atmosphere over them will be more heated, than that over the open ocean, and consequently the trade winds will vary their direction, and set in upon the land; as we find on the coast of Guinea, where the trade wind blows from the westward for a considerable distance in the Atlantic ocean, and sets in upon the coast. And, indeed, in every tropical island, we find the sea breezes setting in upon the land in all directions, as soon as the morning sun has produced a greater rarefaction in the air over the land, than over the water. And, in general, the winds are more variable in all places, and at all times, near the shores, than in the open seas, on the same account.

As some variations in the direction of the wind may arise from the land in general, so the particular nature of the soil may make these variations more considerable.

A flat sandy country is generally more heated by the sun than any other, and therefore will occasion a stronger current of air towards it.

Great ranges of mountains, in any country, may reflect the wind and alter its direction; and the beds of large and extensive rivers open a channel in which the wind will generally blow: as we find the river St. Laurence, which runs upwards of two thousand miles from its source to the sea, and filling many large and extensive lakes in its course, opens an extensive channel for the wind in an eastwardly direction; while the Mississippi, rising from the same country, flows southward through a no less extent of land, and thereby opens a southern channel for the wind; and from these two directions compounded together, we account for the general northwest winds, which prevail in North America.

When the snows cover the tops of mountains, for a

considerable time before they begin to melt, the air will be greatly condensed, and afterwards it will be rarefied, and thereby the wind may blow in one direction while the snow continues, and in a contrary direction when it melts.

From these, and a thousand other causes, which cannot be particularly enumerated, the air must be subject to various perturbations, and have a tendency to motion in different directions, depending on the situation of countries, the degrees of heat and cold in the climate, the nature of the soil, the degree of cultivation, together with many other causes producing fermentations, eruptions of volcanos, the extrication of fixed and subterraneous air, and the like; by which means we must be subject to variable winds almost perpetually. Yet near the equator, where the cause which produces the general trade winds operates with the greatest force, these variations will be less frequent and more regular, especially in the open sea.

In the Atlantic ocean, there is more land on the north side of the equator, than on the south, especially on the African side: and the air over it will therefore be more rarefied than at the equator, and consequently the limit between the northeast and southeast winds will not be the equator, but some parallel to the northward of it: and we find in fact that the limit between them is about four degrees north of the equator.

The place where the eastwardly' and westwardly winds commence in the Atlantic ocean, near the coast of Guinea, is called by the seamen, the calms, rains, and tornados. The reason is, because the atmosphere in that region is divided between the two currents, which blow from that place, one upon the African coast, and the other to the westward, making the trade wind;