ftands too eafily, as much as of those he does not understand.

GRAVITY, the falling of accelerated bodies on the earth, the revolution of the planets in their orbits, their rotations round their axes, all this is mere motion. Now motion cannot perhaps be conceiv'd any otherwife than by impulfion; therefore all those bodies must be impelled. But by what are they impelled? All space is full, it therefore is filled with a very subtile matter, fince this is imperceptible to us; this matter goes from west to east, fince all the planets are carried from west to eaft. Thus from hypothefis to hypothefis, from one appearance to another, philofophers have imagin'd a vaft whirlpool of fubtile matter, in which the planets are carried round the fun: They also have created another particular vortex which floats in the great one, and which turns daily round the planets. When all this is done, it is pretended that gravity depends on this diurnal motion; for, fay thefe, the velocity of the fubtile matter that turns round our little vortex must be feventeen times more rapid than that of the earth; or, in cafe its velocity is seventeen times greater than that of the earth, its centrifugal force must be vaftly greater, and confequently impel all bodies towards the earth. This is the caufe of gravity,

accord

according to the Cartefian fyftem. But the theorist, before he calculated the centrifugal force and velocity of the fubtile matter, fhould firft have been certain that it existed.

SIR Ifaac Newton feems to have destroy ed all these great and little vortices, both that which carries the planets round the fun, as well as the other which fuppofes every planet to turn on its own axis.

FIRST, with regard to the pretended little vortex of the earth, it is demonftrated that it must lose its motion by infenfible degrees; it is demonftrated, that if the earth swims in a fluid, its density must be equal to that of the earth; and in cafe its density be the fame, all the bodies we endeavour to move must meet with an infuperable refiftance.

WITH regard to the great vortices, they are ftill more chimerical, and it is impoffible to make them agree with Kepler's law, the truth of which has been demonftrated. Sir Ifaac fhews, that the revolution of the fluid, in which Jupiter is fuppos'd to be carried, is not the fame with regard to the revolution of the fluid of the earth, as the revolution of Jupiter with respect to that of the earth. He proves, that as the planets make their revolutions in ellipfes, and confequently being at a much greater diftance one from F 3

the

the other in their Aphelia, and a little nearer in their Perihelia; the earth's velocity, for inftance, ought to be greater, when it is nearer Venus and Mars, because the fluid that carries it along, being then more prefs'd, ought to have a greater motion; and yet it is even then that the earth's motion is flower.

He proves that there is no fuch thing as a celestial matter which goes from west to eaft, fince the comets traverse those fpaces, fometimes from eaft to weft, and at other times from north to fouth.

In fine, the better to refolve, if poffible, every difficulty, he proves, and even by experiments, that it is impoffible there Thould be a Plenum; and brings back the Vacuum, which Ariftotle and Des Cartes had banished from the world.

HAVING by thefe and feveral other arguments deftroyed the Cartefian vortices, he despaired of ever being able to discover, whether there is a fecret principle in nature, which, at the fame time, is the caufe of the motion of all celeftial bodies, and that of gravity on the earth. But being retired in 1666, upon account of the plague, to a folitude near Cambridge; as he was walking one day in his garden, and faw fome fruits fall from a tree, he fell into a profound meditation on that gravity, the caufe of which had fo long

been

been fought, but in vain, by all the philofophers, whilft the vulgar think there is nothing myfterions in it. He faid to himfelf, that from what height foever, in our hemisphere, those bodies might defcend, their fall would certainly be in the progreffion discovered by Galileo; and the spaces they run thro' would be as the fquare of the times. Why may not this power which caufes heavy bodies to defcend, and is the fame without any fenfible diminution at the remoteft diftance from the center of the earth, or on the fummits of the highest mountains; Why, faid Sir Ifaac, may not this power extend as high as the moon? And in cafe its influence reaches fo far, is it not very probable that this power retains it in its orbit, and determines its motion? But in cafe the moon obeys. this principle (whatever it be) may we not conclude very naturally, that the rest of the planets are equally fubject to it? In cafe this power exifts (which befides is proved) it must increase in an inverse Ratio of the fquares of the distances. All therefore that remains is, to examine how far a heavy body, which fhould fall upon the earth from a moderate height, would go; and how far in the fame time, a body which fhould fall from the orbit of the moon, would defcend. To find this no

thing is wanted but the measure of the earth, and the distance of the moon from it.

THUS Sir Ifaac Newton reafon'd. But at that time the English had but a very imperfect measure of our globe, and depended on the uncertain fuppofition of mariners, who computed a degree to contain but fixty English miles, whereas it confifts in reality of near seventy. As this false computation did not agree with the conclufions which Sir Ifaac intended to draw from them, he laid afide this purfuit. A half-learn'd philofopher, remarkable only for his vanity, would have made the measure of the earth agree, any how, with his fyftem: Sir Ifaac, however, chofe rather to quit the researches he was then engag'd in. But after Mr. Picart had meafur'd the earth exactly, by tracing that meridian, which redounds fo much to the honour of the French, Sir Isaac Newton refum'd his former reflexions, and found his account in Mr. Picart's calculation.

A circumftance which has always appear'd wonderful to me is, that fuch fublime discoveries should have been made by the fole affiftance of a quadrant, and a little arithmetic.

THE circumference of the earth is one hundred twenty three millions, two hundred forty nine thousand fix hundred

feet.