UNIVERSAL GRAVITATION.
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.