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stands too easily, 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 otherwise than by impulsion; 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, since this is imperceptible to us; this matter goes from west to east, since all the planets are carried from west to east. Thus from hypothesis to hypothesis, from one appearance to another, philosophers have imagin’d a vast whirlpool of subtile 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, say these, the velocity of the subtile matter that turns round our little vortex must be seventeen times more rapid than that of the earth; or, in case its velocity is seventeen times greater than that of the earth, its centrifugal force must be vaftly greater, and consequently impel all bodies towards the earth. This is the cause of gravity, aecording to the Cartesian system. But the theorist, before he calculated the celltrifugal force and velocity of the fubtile matter, should first have been certain that it existed.
Sir Isaac 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 supposes évery 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 infensible degrees ; it is demonstrated, that if the earth swims in a fluid, its density must be equal to that of the earth ; and in case its density be the same, all the bodies we endeavour to move muft meet with an insuperable resistance.
With regard to the great vortices, they are still more chimerical, and it is impossible to make them agree with Kepler's law, the truth of which has been demonstrated. Sir Isaac shews, that the revolution of the fluid, in which Jupiter is suppos'd to be carried, is not the same 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 ellipses, and consequently being at a much greater distance one from F 3
the other in their Aphelia, and a little nearer in their Perihelia ; the earth’s velocity, for instance, ought to be greater, when it is nearer Venus and Mars, because the fuid that carries it along, being then more press’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 such thing as a celestial matter which goes from west to eaft, since the comets traverse those spaces, fometimes from east to west, and at other times from north to fouth.
In fine, the better to resolve, if porfible, every difficulty, he proves, and even by experiments, that it is impossible there should be a Plenum ; and brings back the Vacuum, which Aristotle and Des Cartes had banished from the world.
HAVING by these and several other arguments destroyed the Cartesian vortices, he despaired of ever being able to discover, whether there is a secret principle in nature, which, at the same time, is the cause of the motion of all celestial bodies, and that of gravity on the earth. But being retired in 1666, upon account of the plague, to a solitude near Cambridge ; as he was walking one day in his garden, and saw some fruits fall from a tree, he fell into a profound meditation on that gravity, the cause of which had so long been fought, but in vain, by all the philosophers, whilst the vulgar think there is nothing mysterions in it. He said to himself, that from what height foever, in our hemisphere, those bodies might descend, their fall would certainly be in the progression discovered by Galileo; and the spaces they run thro’would be as the square of the times. Why may not this power which causes heavy bodies to descend, and is the same without any sensible diminution at the remotest distance from the center of the earth, or on the summits of the highest inountains ; Why, said Sir Isaac, may not this power extend as high as the moon ? And in case its influence reaches so far, is it not very probable that this power retains it in its orbit, and determines its motion ? But in case the moon obeys this principle (whatever it be) may we not conclude very naturally, that the rest of the planets are equally subject to it? In case this power exists (which besides is proved) it must increase in an inverse Ratio of the squares of the distances. All therefore that remains is, to examine how far a heavy body, which should fall
upon the earth from a moderate height, would go; and how far in the same time, a body which should fall from the orbit of the moon, would descend. To find this noF4
thing is wanted but the measure of the earth, and the distance of the moon from it.
Thus Sir Isaac Newton reason'd. But at that time the Englif had but a very imperfect measure of our globe, and depended on the uncertain supposition of mariners, who computed a degree to contain but fixty English miles, whereas it confifts in reality of near feventy. As this false computation did not agree with the conclufions which Sir Isaac intended to draw from them, he laid aside this pursuit. A half-learn’d philosopher, remarkable only for his 'vanity, would have made the measure of the earth agree, any how, with his fyftem : Sir Ifaac, however, chose sather to quit the researches he was then engag'd in. But after Mr. Picart had meatur'd the earth exactly, by tracing that meridian, which redounds so much to the honour of the French, Sir Isaac Newton resum'd his former reflexions, and found his account in Mr. Picart's calculation.
A circumstance which has always appear'd wonderful to me is, that such sublime discoveries should have been made by the sole assistance 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