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body must be globular, which always casfis a circu. lar shadow.

Nor are the little unevennesses on the earth's surface, arising from hills and vallies, any materia objections to its being considered as a round body; because the highest mountains bear less proportion to the bulk of the earth, than the little risings on the coat of an orange bear to that fruit; or a grain of sand to an artificial globe, of nine inches dia

And accordingly, we find that the mountains and vallies on the surface of the earth, cause no irregularities in the shadow, during a lunar eclipse; the circumference thereof being even and regular, and appearing as if cast by a body truly globular.

The roundness of the earth being thoroughly established, a way is naturally opened for the covery of its motion. For while it was considered as a plane, mankind had an obscure notion of its being supported, like a scaffolding, on pillars, though they could not tell what supported these. But the figure of a globe is much better adapted to mom tion.

This is confirmed by considering, that, if the carth does not move round the fun, not only the fun, but all the stars and planets, muft move round the earth. Philosophers, by reckonings founded on the

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fureft observations, have been able to guess pretty nearly at the distances of the heavenly bodies from the earth, and from each other, just as every body, who knows the first eleinents of mathematics, can measure the height of a steeple, or any object placed on it. It appears, therefore, that if we conceive the heavenly bodies to move round the earth, we must suppose them endowed with a motion, or velocity, so immense as to exceed all conception. All the appearances in nature, however, may be as well explained by imagining the earth to move round the sun in the space of a year, and to turn on its own axis once in twenty-four hours.

CH A P. V.

ON THE MOTION OF THE EARTH.

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'HE revolution of the earth round its axis,

every twenty-four hours, or its diurnal motion, alternately causes day and night, as either fide is turned toward, or from the sun; and its periodical revolution round that luminary, in three hundred and fixty-five days fix hours, or its annual motion, produces the four seasons of the year. .

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To form a conception of these two motions of the earth, we may imagine a ball moving upon a billiard-table, or bowling-green. The ball proceeds forward upon the green or table, not by Niding along like a plane upon wood, or a llate upon ice, but by turning round its own axis, which is an imaginary line drawn through the centre or middle of the ball, and ending on its surface in two points, called its poles.

Conceiving the matter then in this way, and that the earth, in the space of twenty-four hours, moves from west to east, the inhabitants on the surface of it, like men on the deck of a ship, who are insensible of their own motion, and think that the banks move from them in a contrary direction, will conceive that the fun and stars move from east to west, in the same time of twenty-four hours, in which they, along with the earth, move from west to east..

This daily or diurnal motion of the earth being once clearly conceived, we may easily form a notion of its annual or yearly motion round the sun. For as that luminary seems to have a daily motion round our earth, which is really occasioned by the daily motion of the earth round its axis, fo, in the course of a year, he seems to have an annual motion in the heavens, and to rise and set in different

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points of them, which is really occafioned by the daily motion of the earth on its orbit or path round the Sun, which it completes in the time of a year.

This double motion of the earth may also be compared to a coach turning round in a court-yard. The wheels go round their own axis, at the fame time that they move round the yard. It travels at the rate of fifty-eight thousand miles every hour, which is one hundred and twenty times swifter than a cannon-ball.; and by its rapid motion on its axis, the inhabitants of London are carried five hundred and eighty miles every hour. Those at the Equator move much faster; those towards the Poles much flower ; and those at the very Poles hardly move at all.

What has been said, with regard to the motion of the earth, the fmallest reflection may lead us to apply to the other planets.

c H A P. VI.

AN OBJECTION ANSWERED.

THE

HE following objection is made by fome to,

the diurnal rotation of the earth on its own axís. If it moyes, fay they, from west to east,

will not a ball, fired perpendicularly upward in the air, fall considerably westward of the place from which it was shot ? By no means. For, as both the gun and ball partake of the earth's motion, the ball will be carried forward with the air, as quick as the earth and air turn, and will therefore fall on the very spot from which it was fired. Thus, if one let fall a stone from the top of a main-maft, it will fall on the deck, as near the foot of the mast, when the ship fails, as when she is at rest.

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BE
ESIDES the fix primary planets already men-

tioned, which move round the sun, there are other ten bodies which move round three of these, in the same manner as they do round the sun. Of these our Earth has one, called the moon ; Jupiter has four, and Saturn has five. These are all called moons from their agreeing with our moon, which was first attended to. They are also called Satellites, and secondary planets, because they seem to be at. tendants of the Earth, Jupiter, and Saturn, about

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