CHAP. CIII.

OF THE MAGNET OR LOADSTONE.

HIS wonderful stone is ufually found in iron

TH

mines, and is produced in most parts of the world. It is a heavy stone, fomething resembling the ore of iron, only clofer and more ponderous.

It is endowed with fome surprising qualities and powers. It attracts iron, which will adhere to it very strongly; and this virtue it alfo communicates to the iron fo attracted.

In every magnet there are two poles, one of which points northward, the other fouthward; and if the magnet be divided into ever fo many pieces, the two poles will be found in each piece.

It is this property which has rendered it fo useful in improving the art of navigation; for this gave rife to the mariner's compafs, by means of which a failor can now conduct his fhip to the Eaft or Weft-Indies, or even round the world, with as much ease, and as little danger, as he could formerly make a voyage to the coaft of Norway or Holland.

The improvements in fhip-building have kept pace with those in navigation. To be convinced

of this, one need only confider the infinite difference there is between a first-rate man of war, and as Indian canoe, which feems to be little better than a large butcher's trough; and yet this was probably the first form of all failing vessels.

Even after men had learnt to build ships with more art, and of a larger fize, they never ventured out of fight of land without fear and trembling, because they could not tell whither they were failing east or weft, fouth or north, or to what part of the world they were going. But now, in the darkeft night, and in the midst of the boundless ocean, they know the courfe they are fteering with as much certainty, as if it were in broad day, and within fight of land.

CHAP. CIV.

OF METEORS, AND THE DIFFERENT REGIONS OF AIR.

WHATEVER is engendered in the air which

furrounds us, and which appears to be be yond the moon, is a meteor. This word fignifies a body raised above the earth we inhabit.

Meteors

Meteors are compofed of vapours and exhalations.

Vapours are particles of water which mingle with the air.

Exhalations are particles of all the different terreftrial bodies, which rife into the air, fuch as fulphur, falts, bitumens, and other bodies of different natures, more or less combuftible, folid or heavy.

The air is compofed of a high, middle, and lower region. The air of the higher region is lighter and colder than that of the middle; and that of the middle still finer than the lower. It is heaviest near the furface the of earth.

The weight of a column of air, one foot square, reaching from the surface of the earth to the top of the atmosphere *, is 2,160 pounds; fo that a man of middling fize, the furface of whose body is about 14 square feet, sustains a preffure of air of thirty thouSand and 240 pounds; a preffure that would-be insupportable, and even fatal to him, were it not that it is equal on every part, and counterbalanced by the fpring of the air within him, which is diffused through the whole body, and re-acts with equal force against the outward preffure.

CHA P. CV.

SOME OF THE PROPERTIES OF AIR.

THE

HE air may be rarefied, or made to occupy more fpace. If you take a bladder, entirely empty, as you think, and tie its neck with a string, and lay it before the fire, the heat will fo rarefy the little air inclofed in it, as to make it extend the bladder to its utmost stretch, and, if continued, will make it break through with the report of a

gun.

The air may be condensed even to a fixtieth part of the space it before occupied.

It is alfo endued with an elaftic power. The air compreffed in the wind-gun, will, by its elaftic force when discharged, drive a bullet through a board at the distance of several yards, in the same manner as if it were with gunpowder.

The air-pump is a curious machine for extracting the air from a large hollow glafs, called a receiver, fomewhat refembling those glaffes that are ufed in gardens for covering tender plants. Into this glafs if any animal is put, and the air drawn from it, it will almoft immediately die; which fhews that air is neceffary to the life of animals.

They

They do not all die in the very fame space of time, but fooner or later, according to the strength of the vital principle within them, or according as they have been accustomed to live with more or less air. Dogs, cats, rats, and mice, die in about half a minute. Infects, fuch as wafps, bees, hornets, and grafshoppers, appear dead in two minutes, and will continue in that state a whole day and night,. and afterwards revive upon the re-admiffion of the air. Earwigs, beetles, and fnails live a long time without air; and frogs will live longer without it than toads.

In the exhausted receiver of an air-pump all bo- dies fall with the fame rapidity. A feather falls as faft as a guinea; that is, the lighteft of all bodies falls as fast as the heavieft.

Common air is impregnated with a certain kind of vivifying spirit or quality, which is neceffary to continue the lives of animals; and this, in a gallon of air, is fufficient for one man during the fpace of a minute, and no longer.

If a man defcend into the deep in a diving-bell, he can only live as many minutes as the number of gallons of air contained in the bell amount to.

Even a burning candle confumes the vivifying fpirit of a gallon of air in a minute, as appears by the following experiment. Set a lighted candle M 6

upon