fall to the Earth by gravity, in that case, if there were no resisting medium, the body would not fall to the Earth at all; but continue to circulate round the Earth, keeping always the same path, and returning to the point from whence it was projected with the same velocity with which it moved at first. We find that the Moon therefore must be acted upon by two powers, one of which would cause her to move in a right line, another bending her motion from that line into a curve. This attractive power must be seated in the Earth, for there is no other body within the Moon's orbit to draw her.* The attractive power of the Earth therefore extends to the Moon, and in combination with her projectile force, causes her to move round the Earth in the same manner, as the circulating body above supposed.

The Moons of Jupiter, Saturn and Herschel, are observed to move around their primary planets; therefore there is an attractive power in these planets, operating on their Satellites in the same manner, as the attraction of the Earth operates on the Moon. All the planets and Comets move round the Sun, and respect it as their centre of motion, therefore the Sun must be endowed with an attracting power, as well as the

*If the Moon revolves in her orbit in consequence of an attractive power residing in the Earth, she ought to be attracted as much from the tangent of her orbit in a minute, as heavy bodies fall at the Earth's surface in a second of time. It is accordingly found by calculation, that the Moon is deflected from the tangent 16,09 feet in a minute, which is the very space through which heavy bodies descend in a second of time at the Earth's surface.

Earth and planets.

Consequently all the bodies, or matter of the Solar System are possessed of this attractive power, and also all matter whatsoever.

As the Sun attracts the planets with their Satellites, and the Earth the Moon, so the planets and Satellites re-attract the Sun, and the Moon the Earth. This is also confirmed by observation; for the Moon raises tides in the Ocean; the satellites and planets disturb each other's motions. Every particle of matter being possessed of an attracting power, the effect of the whole must be in proportion to the quantity of matter in the body.

Gravity also, like all other virtues, or emanations, either drawing or impelling a body towards a centre, decreases as the square of the distance increases; that is, a body at twice the distance, attracts another with only a fourth part of the force; at four times the distance, with a sixteenth part of the force.

By considering the law of gravitation which takes place throughout the Solar System, it will be evident that the Earth moves round the Sun in a year. It has been stated and shown, that the power of gravity decreases as the square of the distance increases, and from this it follows with mathematical certainty, that when two or more bodies move round another as their centre of motion, the squares of the time of their periodical revolutions, will be in proportion to each other, as the cubes of their distances from the central body. This holds precisely with regard to the planets round

the Sun, and the satellites round their primaries, the relative distances of which are well known.

All Globes which turn on their own axis, will be oblate spheroids, that is, their surfaces will be further from their centres in the equatorial, than in the polar regions; for as the equatorial parts move with greater velocity, they will recede farthest from the axis of mo tion, and enlarge the equatorial diameter. That our Earth is really of this figure, is demonstrable from the unequal vibrations of a pendulum, and the unequal length of degrees in different latitudes.

Since then the Earth is higher at the equator than at the poles, the Seas naturally would run towards the polar regions, and leave the equatorial parts dry, if the centrifugal force of these parts, by which the waters were carried thither, did not keep them from returning. Bodies near the poles are heavier than those nearer the Earth's centre, where the whole force of the Earth's attraction is accumulated. They are also heavier, because their centrifugal force is less on account of their diurnal motions being slower. For both these reasons, bodies carried from the poles towards the Equator, gradually lose part of their weight.

Experiments prove that a pendulum which vibrates seconds near the poles, vibrates slower near the Equator, which shows that it is lighter, or less attracted there. To make it oscillate in the same time, it is found necessary to diminish its length. By comparing the different lengths of pendulums vibrating seconds at the equator, and at London; it is found that a pen

dulum must be 2,542 lines

shorter at the Equator

than at the poles.

Interrogations for Section Third.

What is GRAVITY?

Do falling bodies strike the surface of the Earth at right angles?

Do falling bodies near the Earth, always direct their course to its centre?

Where is the centre of Gravity situated?

When bodies are projected in a right line, what brings them to the Earth?

If there were no attractive power at the centre of the Earth, what would be the consequence were a body so projected, and not meeting any resistance from the air?

We find that the Moon moves round the Earth in an orbit nearly circular. Why is it so ?

Where is that attractive power situated ?

Have the other planets attractive powers also?
How is it known?

* A line is 1-12th part of an inch.

G

Where is the centre of attraction of the Solar Sys

tem placed?

How is it known?

Do the planets attract the Sun as well as the Sun the planets ?

Has every particle of matter an attractive power? In what proportion does Gravity increase?

How far is the Moon deflected by Gravity from a tangent in one minute of time?

How far does a falling body descend in one second? In what proportion are the squares of the times of the periodical revolutions of all the planets?

What will be the form all planets which revolve on their own axis ?

Why will they be of that form?

How is it ascertained to a certainty, that our Earth is of that form?

Why are bodies near the poles heavier than those at the Equator?

Why is a pendulum vibrating seconds shorter at the Equator than at the poles?

What is the length of a pendulum vibrating seconds at the Equator?

Ans. 39,2 inches.