(Vol. v., p. 344.)
"When shall we three meet again?" Let no one smile at your
correspondent's question, for the common mode of stating Newton's claim
makes it natural enough to ask whether the ancients were aware that
bodies fall to the earth, and to produce proof that they had such
knowledge. But Cicero had more: he not only knew the fall of bodies, but
he had a _medius locus mundi_, or _centrum mundi_, as it was afterwards
called, to which bodies must fall. This was his law of gravitation, and
that of his time. Without describing the successive stages of the
existence of this centre, it may be enough here to state, that a part of
Newton's world-wide renown arises from his having cashiered this
immovable point from the solar system, and sent it on its travels in
search of the real centre of gravity of the whole universe. Newton
substituted, for the old law of gravitation _towards a centre_, his law
of _universal_ gravitation, namely, that _every_ particle gravitates
towards every other. There had been some idea of such a law in the minds
of speculative men: it was Newton who showed that one particular law,
namely, that of the inverse square of the distance, would entail upon a
system, all whose particles are subject to it, those very motions which
are observed in our system. Cicero would have been startled to know
that, when a body falls towards the earth, the earth rises towards it,
_medius locus_ and all: not quite so fast, it is true, nor so far. But
it must not be supposed that we could move our earth any distance in
course of time by continually dropping heavy weights upon it; for the
truth is, that when the weight is raised the earth is a little lowered,
or at least made to move the other way. Archimedes said that, with a
place to stand on, he could move the earth; not aware that he was doing
it at the time he spoke, by the motion of his arm.