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. LAWS OF FALLING BODIES. Since a body falls to the ground in consequence of the earth’s attraction on _each_ of its molecules, it follows that, all other things being equal, all bodies, great and small, light and heavy, ought to fall with equal rapidity, and a lump of sand without cohesion should during its fall retain its original form as perfectly as if it were compact stone. The fact that a stone falls more rapidly than a feather is due solely to the unequal resistances opposed by the air to the descent of these bodies; _in a vacuum all bodies fall with equal velocity_.