make a Body run through, increased as it ought to be towards the Center of the Earth.

The Fall of Bodies then towards the Earth is an effect of this fame Force; whence we gather that the gravity of Bodies is the greater, the nearer they are to the Center, and fo on inverfely; tho' the greatest Distances we can make Experiment upon are too inconfiderable for us to be fen fible of this difference of Gravity.

.

Particular Experiments, have taught that the gravity of Bodies, caufed by this attraction, at equidistance from the Center of the Earth, is as their quantities of Matter.

This Force then which attracts Bodies towards the Center of the Earth, acts proportionably upon all the parts of Matter.

Now attraction is always mutual or reciprocal; one Body cannot attract another without being proportionably attracted thereby. If the attraction of the Earth, upon every part of Matter be equal, every part of Matter attracts the Earth in its turn; and an Atom cannot fall towards the Earth but the Earth must rife to meet it. 16

Thus it is that the Motions of the Planets are amply illuftrated by the Principle of attraction; and the gravity of Bodies is but a Confequence of the fame.

3

I do not here fpeak of the very inconfiderable Anomalies, that may be either

paffed

paffed over, or explained by this Prin ciple.

For Example, the Sun is fuppofed to be immoveable in the Focus of the Ellipfes defcribed by the Planets; and yet he is not abfolutely immoveable; attraction being always mutual, the Sun cannot attract the Planets but he must be attracted by them: Strictly speaking, then the Sun continually changes Place according to the different pofitions of the Planets; fo that it is only the Center of gravity of all the Planets and the Sun that is immoveable; but the enormity of the Sun is fuch to the Planets, that were they all together on one fide, the diftance of the Center of the Sun from the common Center of Gravity, which is then the greatest that can be, would not be one only of his Diameters.

The fame is to be understood of every Planet that has Satellites: The Moon, for Example, fo attracts the Earth that her Ellipfis about the Sun is not described by her Center, but by the common Center of Gravity of the Earth and Moon, while each of thefe Planets performs a Revolution about this Center of Gravity in the space of a Month.

The mutual attraction of the other Planets cause no fenfible change in their Course; Mercury, Venus, the Earth, and Mars, are not fo large as to be able to act upon each other to a degree of Senfibility. Their revo

lutionary

lutionary Motion can only be difturbed by Jupiter and Saturn, or fome of the Comets who might caufe fome diverfion in the Aphelia of these Planets, but fo very inconfiderable and fo flow as not to be minded.

But it is not fo with the mutual attraction between Jupiter and Saturn; these two mighty Planets reciprocally disturb each other, when in Conjunction, and that, to a degree confiderable enough to be observed by Aftronomers.

Thus is it that attraction and its Law being once fettled with regard to the Area's the Planets defcribe about the Sun, and the Times; and with regard to the Periodical times of the Planets, and their distances, the other Phanomena are but neceffary Confequences thereof. The Planets must defcribe the Curves they do defcribe; Bodies muft fall towards the Center of the Earth, and their fall must be as violent as it is; and in fhort, the Planets are fubject to fuch difturbances in their Course as muft naturally refult from this attraction.

One of the effects of attraction, namely, the fall of Bodies, is plain enough; but this effect even is what prevents us from difcoverthe attraction of Bodies towards each other. The Power or Force of attraction being as the quantities of Matter in Bodies, the attraction of the Earth upon every Body whatsoever, hinders our feeing the effects of

C c

their

their own attraction; for urged towards the Center of the Earth by an immenfe Force, their mutual attractions become infenfible, as a Storm destroys the lightest Breath.

But if we extend our View to what can ma-. nifeft their attraction upon each other, we shall find the effects of attraction as continually repeated as thofe of Impulfion. The Motions of the Planets declare themselves every Inftant, while impulfion is a Principle which Nature feems to employ but in fmall.

Attraction being no lefs poffible in the Nature of Things than Impulfion, the Phanomena which indicate Attraction being as frequent as thofe that plead for Impulsion; when we fee one Body tend towards another, to fay that it is not attracted, but that it is pushed along by an invifible Matter, were much the fame as if a favourer of Attraction, who fhould fee one Body put in motion by another, fhould fay it does not move by the effect of Impulfion, but by the attraction of fome invifible Body.

I now leave the Reader to judge whether or no Attraction be fufficiently made out by Facts, or whether it is but a mere gratis dictum we may do without.

For my part I confefs I know not what this gravity of Matter may be, and that I am as ignorant with reference to its impulfive Force. If it could be made out that the one depends upon the other, it would moft

affuredly

affuredly fimplify the Syftems; but in the mean time I believe that without preferring the one to the other, we may make ufe of both.

CH A P. VI.
CHA

Of the Differences wherewith the various Nature of Gravity must affect the Figure of Fluids,

I

that turn about an Axis.

Return now to confider what the feveral Systems may alter in the application I make of the following Problems to the Phenomena of Nature.

In these Problems I determine the Figure that ought to be affumed by a Mafs of homo geneous and fluid Matter, which revolves about an Axis, or a torrent of the fame Matter which turns around an Axis which is not its own, or taken without itself.

For Bodies to arrive at permanency of Figure, all their parts must be in perfect equilibrio. Now thefe parts are animated by two Forces, on which this Equilibrium muit depend; the one, the Centrifugal Force they acquire by their Revolution, tending to reĊ c 2

move