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Letters on England

15. On Attraction
The discoveries which gained Sir Isaac Newton so universal a reputation, relate
to the system of the world, to light, to geometrical infinities; and, lastly, to
chronology, with which he used to amuse himself after the fatigue of his severer
studies.
I will now acquaint you (without prolixity if possible) with the few things I have
been able to comprehend of all these sublime ideas. With regard to the system of
our world, disputes were a long time maintained, on the cause that turns the
planets, and keeps them in their orbits: and on those causes which make all
bodies here below descend towards the surface of the earth.
The system of Descartes, explained and improved since his time, seemed to give
a plausible reason for all those phenomena; and this reason seemed more just,
as it is simple and intelligible to all capacities. But in philosophy, a student ought
to doubt of the things he fancies he understands 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 axis, all this is mere motion. Now
motion cannot perhaps be conceived 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 imagined a vast whirlpool of subtile matter, in which the
planets are carried round the sun: 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
vastly greater, and consequently impel all bodies towards the earth. This is the
cause of gravity, according to the Cartesian system. But the theorist, before he
calculated the centrifugal force and velocity of the subtile matter, should first
have been certain that it existed.
Sir Isaac Newton, seems to have destroyed all these great and little vortices,
both that which carries the planets round the sun, as well as the other which
supposes every planet to turn on its own axis.
First, with regard to the pretended little vortex of the earth, it is demonstrated that
it must lose its motion by insensible degrees; it is demonstrated, that if the earth
 
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