_Gravity or Gravitation_ is that species of attraction, or force by
which, all bodies or particles of matter in the universe tend towards
each other; it is also called _attraction of gravitation and universal
gravity_; gravity, in a more limited, sense is the tendency of a mass
of matter toward a center of attraction especially the tendency of a
body toward the center of the earth.
This influence is conveyed from one body to another without any
perceptible interval of time. If the action of gravitation is not
instantaneous, it comes very nearly to it by moving more than fifty
millions of times faster than light.
Gravity extends to all known bodies in the universe, from the smallest
to the greatest; by it all bodies are drawn toward the center of the
earth, _not because there is any peculiar property or power in the
center_, but because the earth being a sphere, the _aggregate_ effect
of the attractions exerted by all its parts upon any body exterior to
it, is such as to influence that body toward the center.
This property manifests itself, not only in the motion of falling
bodies, but in the _pressure_ exerted by one portion of matter upon
another which sustains it; and bodies descending freely under its
influence, whatever be their figure, dimensions or texture, all are
_equally accelerated_ in right lines perpendicular to the plane of
the horizon. The apparent _inequality_ of the action of gravity upon
different species of matter near the surface of the earth arises
entirely from the resistance which they meet with in their passage
through the air. When this resistance is removed (as in the exhausted
receiver of an air-pump), the inequality likewise disappears.
_The law of gravity_, discovered by Sir Isaac Newton, toward the end of
the seventeenth century, may be stated as follows:
_Every particle of matter in the universe attracts every other
particle with a force whose direction is that of a line joining the
two particles considered, and whose magnitude is directly as the
product of the masses and inversely as the square of the distance
between them._
As a groundwork for this great generalization, Newton employed the
results of two of the greatest astronomers who preceded him, Copernicus
and Kepler. About 1500 A. D. Copernicus perceived and announced that
the apparent rotation of the heavens about the earth could be explained
by supposing the earth to rotate on an axis once in twenty-four hours.
Previous to this time the earth had been regarded as the center of the
universe. He also showed that nearly all the motions of the planets,
including the earth, could be explained on the assumption that these
revolved in circular orbits about the sun, whose position in the
circle, however, was slightly eccentric.
Thus, building somewhat upon the labors of the two parties named,
Newton was the first to prove the law of the forces which would account
for the motions of all the bodies in the solar system.