impelled towards one another, will be twenty-six times as fast as in the earth; that is, the moon will move twenty-six miles toward the earth, for every mile the earth moves towards the moon.

Hence it is, that, in this natural tendency of bodies towards one another, that in the lesser is considered as gravitation; and that in the bigger as attraction; because the motion of the lesser body (by reason of its much greater swiftness) is alone taken notice of.

This attraction is the strongest, the nearer the attracting bodies are to each other; and, in different distances of the same bodies, is reciprocally in the duplicate proportion of those distances. For instance, if two bodies, at a given distance, attract each other with a certain force, at half the distance, they will attract each other with four times that force; at one third of the distance, with nine times that force; and so on.

Two bodies at a distance will put one another into motion by the force of attraction; which is inexplicable by us, though made evident to us by experience, and so to be taken as a principle in natural philosophy.

Supposing then the earth the sole body in the universe, and at rest; if God should create the moon, at the same distance that it is now from the earth; the earth and the moon would presently begin to move one towards another in a straight line by this motion of attraction or gravitation.

If a body, that by the attraction of another would move in a straight line towards it, receives a new motion any ways oblique to the first; it will no longer move in a straight line, according to either

of those directions; but in a curve that will partake of both. And this curve will differ, according to the nature and quantity of the forces that concurred to produce it; as, for instance, in many cases it will be such a curve as ends where it began, or recurs into itself; that is, makes up a circle, or an elipsis or oval very little differing from a circle.

CHAP. II.

OF THE UNIVERSE.

To any one, who looks about him in the world, there are obvious several distinct masses of matter, separate from one another; some whereof have discernible motions. These are the sun, the fixt stars, the comets and the planets, amongst which this earth, which we inhabit, is one. All these are visible to our naked eyes.

Besides these, telescopes have discovered several fixt stars, invisible to the naked eye; and several other bodies moving about some of the planets; all which were invisible and unknown, before the use of perspective glasses were found.

The vast distance between these great bodies, are called intermundane spaces; in which though there may be some fluid matter, yet it is so thin and subtile, and there is so little of that in respect of the great masses that move in those spaces, that it is as much as nothing."

These masses of matter are either luminous, or opake or dark.

Luminous bodies, are such as give light of themselves; and such are the sun and, the fixt stars.

Dark or opake bodies, are such as emit no light of themselves, though they are capable of reflecting of it, when it is cast upon them from other bodies; and such are the planets.

There are some opake bodies, as for instance, the comets, which, besides the light that they may have from the sun, seem to shine with a light that is nothing else but an accension, which they receive from the sun in their near approaches to it, in their respective revolutions.

The fixt stars are called fixt, because they always keep the same distance one from another.

The sun, at the same distance from us that the fixt stars are, would have the appearance of one of the fixt stars.

CHAP. III.

OF OUR SOLAR SYSTEM.

OUR solar system consists of the sun, and the planets and comets moving about it.

The planets are bodies, which appear to us like sars; not that they are luminous bodies, that is, have light in themselves; but they shine by reflecting the light of the sun.

They are called planets from a Greek word, which signifies wandering; because they change their places, and do not always keep the same distance with one another, nor with the fixt stars, as the fixt stars do.

The planets are either primary, or secondary. There are six primary planets, viz. Mercury, Venus, the Earth, Mars, Jupiter, and Saturn.

All these move round the sun, which is, as it were the centre of their motions.

The secondary planets move round about other planets. Besides the moon, which moves about the earth; four moons about Jupiter, and five about Saturn, which are called their satellites. The middle distances of the primary planets from the sun are as follows:

The orbits of the planets, and their respective distances from the sun, and from one another, together with the orbit of a comet, may be seen in the figure of she solar system hereunto annexed. The periodical times of each planet's revolution about the sun are as follows:

The planets move round about the sun from west to east in the zodiac; or, to speak plainer, are always found amongst some of the stars of those

constellations, which make the twelve signs of the zodiac.

The motion of the planets about the sun is not perfectly circular, but rather elliptical. The reason of their motions in curve lines, is the attraction of the sun, or their gravitations towards the sun, (call it which you please); and an oblique or sidelong impulse or motion.

These two motions or tendencies, the one always endeavouring to carry them in a straight line from the circle they move in, and the other endeavouring to draw them in a straight line to the sun, makes that curve line they revolve in.

The motion of the comets about the sun is in a very long slender oval; whereof one of the focusses is the centre of the sun, and the other very much beyond the sphere of Saturn.

The moon moves about the earth, as the earth dces about the sun. So that it hath the centre of motion in the earth; as the earth hath the centre of its revolution in the sun, about which it moves. The moon makes its synodical motion about the earth, in 29 days, 12 hours, and about 44 mi

notes.

It is full moon, when the earth being between the sun and the moon, we see all the enlightened part of the moon; new moon, when the moon be ing between us and the sun, its enlightened part is turned from us; and half moon, when the moon being in the quadratures, as the astronomers call it, we see but half the enlightened part.

An eclipse of the moon is, when the earth, being between the sun and the moon, hinders the light of the sun from falling upon, and being re