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and being in some measure carried off by them, it becomes too light to bear the clouds, which are brought from every quarter by the air, rushing in, to restore the equilibrium of the atmosphere; so that they descend there in perpetual rains, with the most terrible thunder and lightning. The air about that place being acted upon, by these contrary forces, in all directions, will therefore be in equilibrio, and will not move in any direction, so that there will be little or no wind in that place, which is for that reason denominated the calms.

If the air be carried off from this place by opposite currents, it may be asked, how fresh air is supplied? But the answer is easy,-from opposite currents above the place which bring the clouds and produce the perpetual rains.

By the evacuation of the air, its pressure there will be diminished, and consequently the neighbouring air will rush in, from all quarters above, which will form a kind of eddy where the opposite currents meet, and thereby occasion those violent storms, which are called tornados.

On the American side of the Atlantic ocean, the trade winds will become more and more easterly as they approach the American continent. Because, as they are compounded of a north and east wind, or a south and east wind, in different seasons of the year; instead of blowing from the northeast or southeast, they will verge towards the east; the eastern current will be stronger than either the north or south current; because the land of America will be more heated than the water, and the eastern current will gradually prevail over that from the north or south.

In the Atlantic and Ethiopic seas, the trade wind will be more easterly in north latitudes, and more southerly

in south latitudes, when the sun is near to the tropic of cancer; and when he is in the tropic of capricorn, the trade winds in north latitudes will be more northerly, and in south latitudes more easterly than usual. When the sun is near the tropic of cancer, the air under it will be as much rarefied as about the equator, which will diminish the force of the current from the north, and strengthen the current from the south, towards the point of greatest rarefaction; by which means the easterly current will prevail in north latitudes, and the southerly current will prevail in south latitudes; so that on the north of the equator, the trade winds will blow more easterly than usual, and in south latitudes, more southerly, as being in a direction nearer to the stronger current. But when the sun is near the tropic of capricorn, similar changes must take place. For as the air is considerably rarefied from the equator to the tropic, there will be scarcely any southern wind in southern latitudes, and consequently an easterly wind will prevail in these latitudes, while the northern current in north latitudes will, from the coldness of the air on the north side of the equator, prevail over the eastern, and cause the trade winds, on that side, to be more northerly than what they would otherwise be.

In the Indian ocean, from Madagascar to New Holland, between the latitudes of ten and thirty degrees south, there are the same trade winds that prevail in the same latitudes in the Atlantic and Pacific oceans. And besides these, we have periodical winds called monsoons, which blow for six months in one direction, and in the other six months, from the opposite point of the compass. In the northern parts of the Indian ocean the mon soon is northeast from October till April, and from April till October it is southwest. The first is the re

gular trade wind accounted for already. And the southwest wind, in the summer months, is occasioned in the same manner, as the same wind is produced on the coast of Guinea, by the great rarefaction of the air on land.

The air over the continent of India and China being greatly heated, ascends, and the colder air from the southwest rushes in to restore the equilibrium. There is, indeed, this difference, that the land on the coast of Guinea lies under the equator, and therefore remains heated through the whole year, so that the southwest wind blows there continually. But the land of India and China lies north of the equator, and there is no continent to the south, so that the southwest wind will blow in the Indian ocean only for half of the year, or while the sun is over the land. The position of the islands in this ocean, with respect to each other, may occasion either one or other of the currents, of which these monsoons are compounded, to be stronger than the other, and consequently cause the general direction of the wind to vary a little from the regular course. As we find to the east of Sumatra and Java, the northeast wind in winter is more northerly, and the southwest wind in summer is more southerly, than in the other monsoons to the west.

In the southern parts of the Indian ocean, from three to ten degrees south, the regular south trade wind blows from the end of May to the beginning of December, and a northwest wind blows for the other six months; which is probably occasioned by the position of New Holland or Zealand, over which the air being rarefied, may cause a current from the northwest to set in towards it.

These are the general causes and affections of the

trade winds and monsoons, which we find to be most regular in the great Pacific ocean; however, none of them are without exception, arising from different circumstances of heat and cold, land and water, situation and direction.

The velocity of wind is very variable, from one to fifty or sixty miles per hour. Various machines have been invented to determine this velocity; but without any of them, it may be found with sufficient precision, by observing how far a light feather is carried by it in a second. From many experiments it is found that even in violent storms and hurricanes, it seldom moves with greater velocity than sixty miles per hour.

If a current of air strike against any obstacle, such as the sail of a ship or of a mill, the force of the stroke will be proportional to the area of the obstacle and the velocity of the wind. And as the velocity of the wind may be measured by the space that a heavy body must fall to acquire that velocity, the force of the stroke will be proportional to the weight of a column of air whose base is the area of the obstacle, and whose altitude is the distance from which a heavy body falls to acquire the velocity with which the wind blows. This is also true of water in motion.

When the area of the sail is given, the force will be proportional to the square of the velocity. The force is as the quantity of matter and velocity conjointly; but the quantity of matter is as the velocity; for the quicker or slower that the wind moves, the more or fewer particles strike against a given surface in a given time. Therefore, the force will be as the square of the velocity. This is true of any fluid whether compressible or incompressible; and so is the last proposition.

Hence, it will be four times as difficult to swim or

row against a stream, that flows with double the ve locity of another; and a wind that blows with double the velocity of another will give four times the momentum to a given sail, to move the ship. Now, if the obliquity of the sail to the wind be given, its perpendicular area is also given, and the force of the wind to move the sail will therefore be as the square of the velocity.

The momentum of a current of wind to move a sail, when it blows with a given velocity, will always be as the square of the cosine of the angle of incidence, or the angle between the direction of the wind and the perpendicular to the surface of the sail.

The momentum of a stroke from a given body will always be in proportion to the cosine of the angle of incidence, as we have seen in mechanics. But the momentum of a stroke is also proportional to the quantity of matter in the striking body: and the quantity of matter in a stream of wind against a given sail will increase as the cosine of the angle of incidence increases; there fore, the momentum will be as the cosine of the angle of incidence, on a double account; first, because an oblique stroke is to a direct one in this proportion, and secondly, because the quantity of air, which strikes the oblique sail, is to the quantity, which strikes a direct one, in the same proportion. Consequently, the momentum of the wind, to move a given sail, will be as the square of the cosine of the angle of incidence, when the wind blows with a given velocity.

OF SOUNDS.

Ir is the business of metaphysicians to contemplate the idea of sound in the mind, and of anatomists to describe the construction of the ear, and the manner of conveying the impressions made upon it, by the motion

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