body must be globular, which always cafts a circular fhadow.

Nor are the little unevenneffes on the earth's furface, arifing from hills and vallies, any materia objections to its being confidered as a round body; because the highest mountains bear less proportion to the bulk of the earth, than the little rifings on the coat of an orange bear to that fruit; or a grain of fand to an artificial globe, of nine inches diameter. And accordingly, we find that the mountains and vallies on the furface of the earth, caufe no irregularities in the fhadow, during a lunar eclipfe; 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 confidered as a plane, mankind had an obfcure notion of its being fupported, like a scaffolding, on pillars, though they could not tell what fupported these. But the figure of a globe is much better adapted to motion.

This is confirmed by confidering, that, if the earth does not move round the fun, not only the fun, but all the flars and planets, must move round the earth. Philofophers, by reckonings founded on the

B 4

fureft

fureft obfervations, 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 elements of mathematics, can meafure 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 muft fuppofe them endowed with a motion, or velocity, fo immenfe as to exceed all conception. All the appearances in nature, however, may be as well explained by imagining the earth to move round the fun in the space of a year, and to turn on its own axis once in twenty-four hours.

CHAP. V.

ON THE MOTION OF THE EARTH.

THE revolution of the earth round its axis,

every twenty-four hours, or its diurnal motion, alternately caufes day and night, as either fide is turned toward, or from the fun; 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.

To

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 fliding along like a plane upon wood, or a flate 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 furface 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 weft to east, the inhabitants on the furface of it, like men on the deck of a ship, who are infenfible of their own motion, and think that the banks move from them in a contrary direction, will conceive that the fun and ftars move from eaft to weft, in the fame 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 eafily form a no-tion of its annual or yearly motion round the fun. For as that luminary feems to have a daily motion round our earth, which is really occafioned 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 rife and fet in different.

points of them, which is really occafioned by the daily motion of the earth on its orbit or path round the fun, 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 fwifter 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 fafter; thofe towards the Poles much flower; and thofe at the very Poles hardly move at all.

What has been faid, with regard to the motion of the earth, the fmalleft reflection may lead us te apply to the other planets.

CHAP. VI.

AN OBJECTION ANSWERED.

THE HE following objection is made by fome to, the diurnal rotation of the earth on its own axis. If it moyes, fay they, from west to east,;;

will not a ball, fired perpendicularly upward in the air, fall confiderably weftward of the place from which it was fhot? 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 ftone from the top of a main-mast, it will fall on the deck, as near the foot of the mast, when the fhip fails, as when she is at rest.

CHAP. VII.

OF THE SECONDARY PLANETS, OR MOONS.

BES

ESIDES the fix primary planets already mentioned, which move round the fun, there are other ten bodies which move round three of these, in the fame manner as they do round the fun. Of thefe 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 alfo called fatellites, and fecondary planets, because they feem to be attendants of the Earth, Jupiter, and Saturn, about B 6

which