In the next place we shall treat of Monsoons, or Trade- WINDS.

"Trade-winds, observing well their stated course,

To human good employ their pow'rful force;

The loaded ships across the ocean fann'd

By steady gales, spread commerce through the land:

These you observe - but have you no desire

The hidden spring of such effects t'inquire ?

Or, when contending winds around you blow,

Do you ne'er wish the cause of them to know?"

Monsoons are those winds which blow six months con stantly the same way, and the contrary way the other six months.

Mr. Olinthus Gregory observes, that "though the winds in a temperate zone of the earth are very inconstant and changeable, yet this is not the case in every part of the terrestrial globe; for,in the torrid zone, and some other parts, the winds are generally very uniform and constant in their direction, as will appear from the following facts relative thereto:"1. Over the Atlantic, and Pacific oceans, particularly between thirty degrees of north and thirty degrees of south latitude, the trade-winds, as they are called, blow uniformly from east to west, all the year round, with a small variation in the different seasons.

"2. When the sun is on the equator, the trade-winds, in sailing northward, veer more and more from the east towards the north; so that about their limit they become nearly northeast; and vice versa in sailing southward, they become at last nearly south-east. 3. When the sun is near the tropic of Cancer, the trade-winds north of the equator become more nearly east than at other times, and those south of the equator more nearly south; and vice versa, when the sun is near the tropic of Capricorn. 4. The trade-winds are not due east upon the equator, but about four degrees to the north of it.

"To account for these facts relative to the winds, is a most curious and important, though mysterious, inquiry; having employed the pens of several very eminent philosophers: but amongst all the explanations I have seen, there is none in my opinion more agreeable to nature than one given by Mr. John Dalton, of Manchester, in his "Meteorological Observations and Essays." The method of reasoning applied to the subject in that work, I shall here make use of.

"The inequality of heat in the different climates and places, and the earth's rotation on its axis, appears to be the principal causes of all winds, regular and irregular. It may be observed, that whenever the heat is greatest, there the air will ascend, and a supply of colder air will be received from the neighbouring parts: it will be willingly allowed, that the heat is at all times greatest in the torrid zone, and decreases gradually in proceeding northward or southward; also that the poles may at all times be considered as the centres of cold Hence it reasonably follows, that abstracting from accidental circumstances, there will be a constant ascent of air over the torrid zone, which air will afterwards fall north-ward and southward, whilst the colder air below is determined by a continual impulse towards the equator.

"When the effects of the earth's rotation are taken into consideration, our reasoning must be as follows: the air over any part of the earth's surface, when apparently at rest, or calm, will have the same rotatory velocity as that part; but if a quantity of air in the northern hemisphere receive an impulse in the direction of the meridian, either northward or southward, its rotatory velocity will be greater in the former case, and less in the latter, than that of the air into which it moves; consequently, if it move northward, it will have a greater velocity eastward than the air, or surface of the earth over which it moves, and will therefore become a south-west wind, or a wind between the south and west; and, vice versa, if it move southward, it becomes a north-east wind. From similar considerations it will appear, that in the southern hemisphere the winds will be north-west and south-east respectively.

"The trade-winds may therefore be explained thus . The two general masses of air proceeding from both hemispheres towards the equator, as they advance are constantly deflected more and more towards the east, by reason of the earth's rotation; that from the southern hemisphere, originally a south wind, is made to veer more and more towards the east; in like manner, that from the northern hemisphere is made to change its directions from the north towards the east: these two masses meeting near the equator, their velocities south and north destroy each other, and they proceed afterwards with their common velocity from east to west round the torrid zone, excepting the irregularities produced by the continents. The equator is not in reality the place of concourse, but the northern parallel of four degrees; because the centre of heat is thereabouts, the sun being longer on the north side of the equator, than on the south side. Moreover, when the sun is near one of the tropics, the centre of heat upon the earth's surface is then nearer that tropic than usual, and therefore the winds about the tropic are more nearly east at that time, and those about the other tropic more nearly north and south.

"If all the terrestrial globe were covered with water, or if the variations of the earth's surface in heat were regular and constant, so that the heat was the same in every part of the same parallel of latitude, the winds would then be very nearly regular also: but this is not the case; for we find the irregularities of heat, arising from the interspersion of land and sea, are such, that though all the parts of the atmosphere in some measure conspire to produce regular winds about the torrid zone, yet very striking irregularities are often found to take place. A remarkable instance we have in monsoons, which are winds that in the Indian ocean, 8cc. blow for six months together one way, and the next six months the contrary way: these, with sea and land breezes, do not seem easily to be accounted for on any other principle than that of rarefaction.