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

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, exclusive of his ring; and the Sun is near nine hundred thousand times as big as the Earth.

The Moon is, at least, forty-three thousand times less than the Sun, and fifty times less than the Earth.

The reason of her appearing as big as the Sun is, that she is much nearer the Earth. Her distance from the Earth is only two hundred and forty thousand miles; whereas that of the Sun is ninety-five millions.

CH A P. CH A P. IX.

OF COMETS.

COME
OMETS are certain dark or opaque bodies,

like the planets, and move round the sun, but in very eccentric orbits, being sometimes so far from him, that their cold must be excessive, and fometimes fo hear him, that their heat must be fo intense, as would prove altogether intolerable to an inhabitant of this earth ; and would even destroy, or at least vitrify, the earth itself.

Sir Isaac Newton computed the heat of the comet that appeared in the year 1680, when nearest the sun, to be two thousand times hotter than rede hot iron, and that, being thus heated, it must retain its heat till it comes round again, although its period should be more than twenty thousand 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 system, moving in diffesent directions. All those which have been observed, have moved through the etherial regions and the orbits of the planets, without suffering the least sensible resistance in their motions, which

sufficiently 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 most remarkable. This comet, at its greatest diftance, is about eleven thoufand two hundred millions of miles from the sun, while its least distance from the centre of the sun is about four hundred and ninety thousand miles. In that part of its orbit, which is nearest the sun, it flies with the amazing velocity of eight hundred and eighty thousand miles in an hour; and the fun, as feen from it, appears one hundred degrees in breadth, and consequently forty thousand times as large as he appears to us.

Our earth was out of the way, when this comet lait passed 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 considerable, 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 transpa. rent trains or tails, issuing from that side of them which is turned away from the sun.

Comets were formerly supposed to be prodigies or portents, and to foretel some great event or revolution, fuch as the fall of empires, or the death of some eminent and distinguished personage ; 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.

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THE fixed fars comprehend all the other hea.

venly bodies, except the sun, planets, and comets. They are distinguished by the naked eye from the planets, by being less bright and luminous, and by continually exhibiting that appearance, which we call the twinkling of the stars. This arifes from their being fo extremely fmall, that the interposition of the least body, of which there are many constantly floating in the air, deprives us of the fight of them. When the interposed body

changes changes its place, we again see the star; and this succession being perpetual, occasions the twinkling.

But a more remarkable property of the fixed stars, and that froin 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 said, muft evidently be always changing theirs.

The stars which are nearest to us feem largest, and are therefore called of the first magnitude. Those of the second magnitude appear lefs ;. and so proceeding on to the

fixth magnitude, which includes all the fixed stars that are visible without 'a telefcope.

1

C H A P. XE.

- NUMBER OF THE FIXED STARS.

AS

S to their number, though in a clear winter's

night, without moonshine, they seem to be innumerable, which is owing to their strong sparkling, and our looking at them in a confused manner, yet when the whole firmament is divided, as it has been done by the ancients, into signs and con-,

stellations,

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