which they move, and which are called primary Saturn has, befides, a thin, broad ring that furrounds his body, without touching it, in the fame manner as a horizon does an artificial globe. It is twenty-one thoufand miles in breadth, and is as far from Saturn on every fide.

T

CHAP. VIII.

ON THE SIZE OF THE PLANETS.

HE Earth is twenty-seven times as big as Mercury, very little bigger than Venus, and five times as big as Mars. But Jupiter is more than a thousand times as big as the Earth, Saturn five hundred and eighty-fix times as big, exclufive of his ring; and the Sun is near nine hundred thousand times as big as the Earth.

The Moon is, at leaft, forty-three thousand times lefs than the Sun, and fifty times lefs than the Earth. The reafon of her appearing as that fhe is much nearer the

big as the Sun is,

Earth. Her diftance from the Earth is only two

hundred and forty thousand miles; whereas that of the Sun is ninety-five millions.

I

СНАР.

CHA P. IX.

OF COMETS.

OMETS are certain dark or opaque bodies,

COME

like the planets, and move round the fun, but in very eccentric orbits, being fometimes fo far from him, that their cold must be exceffive, and fometimes fo hear him, that their heat must be fo intenfe, as would prove altogether intolerable to an inhabitant of this earth; and would even deftroy, or at least vitrify, the earth itfelf.

Sir Ifaac Newton computed the heat of the comet that appeared in the year 1680, when nearest the fun, to be two thousand times hotter than redhot iron, and that, being thus heated, it must retain its heat till it comes round again, although its period fhould be more than twenty thoufand years; and it is computed to be only five hundred and seventy-five..

It is believed that there are at least twenty-one comets belonging to our fyftem, moving in different directions. All thofe which have been obferved, have moved through the etherial regions and the orbits of the planets, without fuffering the leaft fenfible refiftance in their motions, which

fufficiently

fufficiently proves that the planets do not move in folid orbs.

Of all the comets, the periods of three only are known with any degree of certainty, being found to return at intervals of 75, 129, and 575 years; and of these, that which appeared in 1680 is the moft remarkable. This comet, at its greatest diftance, is about eleven thoufand two hundred millions of miles from the fun, while its leaft distance from the centre of the fun is about four hundred and ninety thousand miles. In that part of its orbit, which is neareft the fun, it flies with the amazing velocity of eight hundred and eighty thoufand miles in an hour; and the fun, as feen from it, appears one hundred degrees in breadth, and confequently forty thousand times as large as he pears to us.

ap

Our earth was out of the way, when this comet laft paffed near her orbit; but it requires a more perfect notion of the motion of the comet, to be able to judge, if it will always pass by us, with fo little effect. The comet, in one part of its orbit, approaches very near to the orbit of our earth; fo that, in some revolutions, it may approach near enough to have very confiderable, if not fatal effects upon it. One of the comets was expected to return in 1789, but it has not yet appeared.

Comets

Comets are always attended with long tranfparent trains or tails, iffuing from that fide of them which is turned away from the fun.

Comets were formerly supposed to be prodigies or portents, and to foretel fome great event or revolution, fuch as the fall of empires, or the death of fome eminent and distinguished perfonage; but they are now known to have no more connection with the civil or political affairs of this world, than any other of the heavenly bodies.

CHAP. X.

OF THE FIXED STARS.

THE fixed flars comprehend all the other hea

venly bodies, except the fun, planets, and comets. They are diftinguished by the naked eye from the planets, by being lefs bright and luminous, and by continually exhibiting that appearance, which we call the twinkling of the stars. This arifes from their being fo extremely finall, that the interpofition of the least body, of which there are many constantly floating in the air, deprives us of the fight of them. When the interpofed body changes

changes its place, we again fee the ftar; and this fucceffion being perpetual, occafions the twinkling.

But a more remarkable property of the fixed stars, and that from which they have obtained their name, is their never changing their situation, with regard to each other, as the planets, from what we have already faid, muft evidently be always changing theirs.

The ftars which are nearest to us feem largeft, and are therefore called of the firft magnitude. Thofe of the second magnitude appear less; and fo proceeding on to the fixth magnitude, which includes all the fixed ftars that are vifible without a telefcope.

CHAP. XI.

NUMBER OF THE FIXED STARS.

A

S to their number, though in a clear winter's night, without moonfhine, they seem to be innumerable, which is owing to their strong sparkling, and our looking at them in a confufed manner, yet when the whole firmament is divided, as it has been done by the ancients, into figns and con

fellations,