bodies attract one another in proportion to the quantity of matter contain'd in them, it can only be in proportion to the quantity of their parts; and if this power is found in the whole, it is undoubtedly in the half, in the quarter, in the eighth part, and fo on in infinitum.

THIS is Attraction, the great spring by which all nature is mov❜d. Sir Ifaac Newton, after having demonftrated the existence of this principle, plainly forefaw that its very name wou'd offend; and therefore this philofopher in more places than one of his books, gives the reader fome caution about it. He bids him beware of confounding this name with what the Ancients called occult qualities; but to be fatisfied with knowing that there is in all bodies a central force which acts to the utmost limits of the univerfe, according to the invariable laws of mechanicks.

IT is furprifing, after the folemn proteftations Sir Ifaac made, that fuch eminent men as Mr. Sorin and Mr. de Fontenelle, fhould have imputed to this great philofopher the verbal and chimerical way of reafoning of the Ariftotelians; Mr. Sorin in the memoirs of the academy of 1709, and Mr. de Fontenelle in the very elogium of Sir Ifaac Newton.

MOST of the French, the learned and others, have repeated this reproach. These

are

are for ever crying out, why did he not employ the word Impulfion, which is fo well understood, rather than that of Attraction, which is unintelligible?

SIR Ifaac might have answer'd these criticks thus: Firft, you have as imperfect an idea of the word Impulfion as of that of Attraction; and in cafe you cannot conceive how one body tends towards the center of another body, neither can you conceive by what power one body can impel another.

SECONDLY, I could not admit of Impulfion, for to do this, I must have known that a celeftial matter was the agent; but fo far from knowing that there is any fuch matter, I have prov'd it to be merely imaginary.

THIRDLY, I use the word Attraction for no other reason, but to exprefs a defect which I difcover'd in nature; a certain and indifputable effect of an unknown. principle; a quality inherent in matter, the caufe of which perfons of greater abilities than I can pretend to, may, if they. can, find out.

WHAT have you then taught us? Will thefe people fay further: And to what purpose are fo many calculations to tell us what you your felf do not compre

hend?

A

7

I HAVE taught you, may Sir Ifaac rejoin, that all bodies gravitate towards one another in proportion to their quantity of matter; that thefe central forces alone keep the planets and comets in their orbits, and cause them to move in the proportion before fet down. I demonftrate to you, that it is impoffible there fhould be any other cause which keeps the planets in their orbits, than that general phænomenon of gravity. For heavy bodies fall on the earth according to the proportion demonftrated of central forces; and the planets finishing their courfe according to thefe fame proportions, in cafe there were another power that acted upon all those bodies, it would either increase their velocity, or change their direction. Now not one of those bodies ever has a fingle degree of motion or velocity, or has any direction but what is demonftrated to be the effect of the central forces; confequently it is impoffible there fhould be any other principle.

GIVE me leave once more to introduce Sir Ifaac fpeaking: Shall he not be allow'd to fay, My cafe and that of the Ancients is very different? These faw, for instance, water afcend in pumps, and faid, the water rifes because it abhors a vacuum. But with regard to my felf, I am in the cafe of a man who fhould have first observ'd

that

that water afcends in pumps, but should leave others to explain the cause of this effect. The anatomist who firft declar'd, that the motion of the arm is owing to the contraction of the mufcles, taught mankind an indifputable truth; but are they lefs obliged to him because he did not know the reason why the muscles contract? The caufe of the elafticity of the air is unknown, but he who first discover'd this fpring perform'd a very signal service to natural philofophy. The fpring that I difcover'd was more hidden and more univerfal, and for that very reason mankind ought to thank me the more. I have dif cover'd a new property of matter, one of the secrets of the Creator; and have calculated and difcover'd the effects of it. After this fhall people quarrel with me about the name I give it?

VORTICES may be call'd an occult quality because their exiftence was never prov'd: Attraction on the contrary is a real thing, because its effects are demonftrated, and the proportions of it are calculated. The caufe of this caufe is among the Arcana of the Almighty.

Procedes buc, & non amplius.

Hither thou fhalt go, and no farther.

LET

LETTER XVI.

Ο Ν

Sir Ifaac Newton's OPTICK S.

TH

HE Philofophers of the laft age found out a new univerfe; and a circumftance which made its discovery more difficult was, that no one had so much as fufpected its existence. The most fage and judicious were of opinion, that 'twas a frantick rashness to dare fo much as to imagine, that it was poffible to guess the laws by which the celeftial bodies move, and the manner how light acts. Galileo, by his aftronomical discoveries, Kepler by his calcalation, Des Cartes (at least in his diopticks) and Sir Ifaac Newton (in all his works) feverally faw the mechanism of the fprings of the world. The geometricians have fubjected infinity to the laws of calculation. The circulation of the blood in animals, and of the fap in vegetables, have changed the face of nature with regard to us. A new kind of existence has been giv

en