upon the air-pump, or indeed upon any other place fo fmooth, that, when covered with a receiver, no air can enter. If the receiver holds a gallon, the candle will burn a minute; and then, after having gradually decayed from the firft inftant, it will go out; which fhews that a conftant supply of fresh air is as neceffary to feed flame, as it is to fupport the lives of animals.

The vivifying spirit of air may be destroyed in a variety of ways. First, by paffing through the lungs of animals, as is evident from what we have just now faid. Secondly, by paffing through fire, particularly charcoal fire, or the flame of fulphur. Hence finokey chimnies must be very unwholesome, especially if the rooms they are in be small and clofe. Thirdly, by being long pent up in any close place, fuch as the holds of fhips, oil-cifterns, or wine-cellars. The air in many of thefe is fometimes fo corrupted, as to prove instant death to any animal that comes into it.

This air is called damp; not only because it is filled with humid or moift vapours, but because it deadens fire, extinguishes flame, and destroys life. The dreadful effects of damps are fufficiently known to those who work in mines.

Air is abfolutely necessary for the propagation of found. If the clapper be made to ftrike ever fo

hard

hard against the bell, it will make no found at all, when the air is exhausted out of the receiver.

The elaftic air, which is contained in many bodies, and is kept in them by the weight of the atmosphere, may be got out of them either by boiling, or by the air-pump; but the fixed air, which is by much the greater quantity, cannot be got out but by diftillation, fermentation, or putrefaction.

This is a wife law of providence; for if fixt air did not come out of bodies without difficulty, and spend some time in extricating itself from them, it would tear them to pieces. Trees would be rent

by the change of air from a fixt to an elastic state, and animals would be burft in pieces by the explofion of air in their food.

Dr. Hales found, by experiment, that the air in apples is fo much condenfed, that, if it were let out into the common air, it would fill a space of 48 times as great as the bulk of the apples themselves. If, therefore, the air were let loose at once in these fubftances, they would tear every thing to pieces about them, with a force fuperior to that of gunpowder. Hence, in eating apples, it is well that they part with the air by degrees, as they are chewed, and ferment in the ftomach, otherwife an apple would be immediate death to him that eats it.

CHA P.

T

CHAP. CVI.

OF THE WIND.

'HE wind is nothing else but the air put violently in motion; and this is occafioned chiefly by means of heat. For, when any part of the air is heated by the fun, or otherwise, it will fwell,. and thereby affect the adjacent air; and fo, by various degrees of heat in different places, there will arife various motions of the air.

When the air is much heated, it will afcend towards the upper part of the atmosphere, and the adjacent air will rush in to fupply its place; and therefore there will be a ftream or current of air, from all parts, towards the place where the heat is. And hence we see the reason, why the air rushes with fuch force into a glafs-house, a tile-kiln, or towards any place where a great fire is made; and alfo why smoke is carried up a chimney, and why the air rushes in at the key-hole of a door, or any fmall chink, where there is a fire in the room. In general, we may take it for granted, that the air will prefs towards that part of the world, where it is most heated.

The

The winds are divided into four principal ones, the north, fouth, east, and weft, which receive their names from the four quarters of the world.

The Frigid Zone is the parent of the north wind,. which is confequently the coldeft.

The fouth wind is the warmeft, and particularly in the fummer, becaufe it comes from the Torrid Zone, over countries hotter than ours.

The east wind is the dryeft, because it comes across the vast continent of Afia, which is but little watered by rivers or feas.

The west wind often blows us rain; because, as it croffes the great Atlantic ocean, it attracts a great quantity of vapours.

When these impetuous winds happen to meet, the greatest inconveniencies follow. The fulphureous exhalations from the fouth, torrents of nitre from the north, and watery vapours from every fide, become, indifcriminately, blended together in one confused mass.-From hence proceed tempefts, thunder, rain, hail, and wirlwind.

The velocity of wind is at the rate of 50 or 60 miles an hour, in a great storm; that of a common brisk wind is about 15 miles an hour; and some winds move not even one mile in that space of time.

A per

A perfon, therefore, on horfeback, and even fometimes on foot, may be said to outstrip the wind; for, if he moves faster than the wind, which is very poffible, he will have a wind in his face, though the motion of the air be really the contrary way. The velocity of found is thirteen times as great as that of the strongest wind.

THERE

CHA P. CVII.

ON THE TROPICAL WINDS.

HERE are certain winds, called tropical winds, which blow almost always from the fame point of the compafs. They are of three kinds. 1. The general trade winds, which extend to near thirty degrees of latitude on each fide of the equator, in the Atlantic, Ethiopic, and Pacific oceans. On the north fide of the equator, they blow from north-east, · on the fouth fide from the south-east, and near the

equator from almost due east. 2. The monfoons, or shifting trade winds, which blow fix months in one direction, and the other fix months in the oppofite direction. These are moftly in the Indian, or Eastern ocean, and do not reach above two hundred

leagues