Isaac shows, that the revolution of the fluid
in which Jupiter is supposed 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 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 fluid that carries it
along, being then more pressed, ought to have a greater motion; and yet
it is even then that the earth's motion is slower.

He proves that there is no such thing as a celestial matter which goes
from west to east since the comets traverse those spaces, sometimes from
east to west, and at other times from north to south.

In fine, the better to resolve, if possible, 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 Descartes 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 sought, but in vain, by all the
philosophers, whilst the vulgar think there is nothing mysterious in it.
He said to himself; that from what height soever in our hemisphere, those
bodies might descend, their fall would certainly be in the progression
discovered by Galileo; and the spaces they run through 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 centre of the earth, or on the summits of the highest
mountains, 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