As effect and cause are necessarily equal, so is motion the measure of
impulse, in time. Therefore as long as a body is in motion it is being
impelled, however insensible the impelling cause. Motion must be in the
direction of impulse; for, as a body cannot move itself, and is the
passive patient of impulse, so must its direction be the same as that
of impulse; therefore when the direction of motion is changed, it must
be by a novel impulse in the novel direction.
From all matter being in motion, and all effect consisting in motion,
and because like effects everywhere are attributable to the like
or same cause, so must there be a cause of motion as universal as
matter; rather than that there should be a distinct impelling cause
for every individual motion following after the body, to put and keep
it in motion. In all philosophic research the golden rule of nature
should be held in mind, which prescribes "the shortest mode and fewest
materials:" _to mistake on the side of simplicity is more wise than
censurable in the search after natural physical truths_.
A universal cause of motion, it would seem, can be no other than a
universal medium, a medium of pressure, one occupying the regions of
planetary space, competent to keep the planets in interminable motion
and effect all terrestrial minor motion: only by such means is it
conceivable how the earth can be under endless, ever-varying impulse,
productive of ever-changing direction. When impulsively pressed into
motion by such a medium, the direction of a planet must be orbicular,
on account of the pressure on the solar side being always less than
on the opposite, by which the projectile direction is diverted from
rectilinear to curvilinear.
Newton imagined that a medium, and however rare, occupying the regions
of space, must retard, in time destroy, and eventually require the hand
of Deity to restore the primeval order of planetary motion: no very
bright idea of the great mathematician, considering the Omniscience of
the Projector of a _self-going_, _self-regulating_ Universe. Whereas
a medium as dense as molten gold, could produce no such disorder as
long as impulse is greater than resistance; which the long-continuance
and order of planetary motion strongly seem to indicate is the case.
Were there no medium in space, the planets must be at rest; one could
not possibly affect another but by its shadow: Uranus being agitated
by the greatly remote presence of Neptune, is proof of there being a
connecting medium between. Gravitation is supposed to move the body
possessed of the property, forwards,--why not every way?--to the sun or
towards some neighbouring planet, but not to send that body or planet
an agitating warning of its presence. How is gravitation within one
planet to keep another in a state of agitation; which agitation being
motion--a mechanical effect--is proof of there being a medium by which
mediate connection is maintained between the two, Uranus and Neptune.
Without a planetary medium there could be no _system_ of planets.
Suppose the existence of such a medium, then its sudden removal,--must
not every subordinate system, which makes part of the universal system,
become disjointed the same instant? Besides, from the laws of vision,
rather of optics, there is equal proof that space contains a medium.
There is no light to come from a star to the eye; there is nothing of
sight belonging to the eye-balls; and there must be something between
a star and the sense to connect the star with the sense; or how is the
sense or brain to be so affected by the star, as that the perception
or sensation shall be always the same when the eye-ball lenses are
directed to the same star; and only by a universal medium can all the
stars of the hemisphere be in connection with the eye at the same
time, or the time of a few winks of the eye. Therefore until it is
proved that constant planetary motion can be without constant and equal
corresponding impulse, as to direction; and that a star can affect
the sense of itself, immediately or with nothing between, all denial
of planetary space being occupied by a medium of pressure, is utterly
untenable.