GRAVITY

(Lat. gravis, heavy.) These terms are often used synonymously to denote that mutual tendency which all the bodies of nature have to approach each other, with forces which are directly as their ma es, and inversely proportional to the squares of their distances. That every particle of matter in the universe has a disposition to pre towards, and if not opposed to approach to every other, is a fact of which we derive the knowledge partly from our constant experience of what tukes place at the earth's surface, and partly by reasoning from the observed motions of the celestial bodies. This mutual tendency of all the particles of matter to each other is called the attraction of gravitation. In reference to any particular body, or ma of matter, the aggregate attraction of all its particles is usually called simply lis gravity. Of Terrestrial Gravity. — Universal experience demonstrates that all heavy bodies, when unsupported, fall towards the surface of the earth. The direction of their motion may be ascertained by a plumb-line; nnd it is found to be always perpendicular to the level surface of the earth, that is, to the surface of stagnant water. But the earth is very nearly spherical, and a line perpendicular to the surface ofa sphere must pa through its centre; hence the direction of a body moving in consequence of the force of terrestrial graviry is towards the centre of the earth. And this la the direction in which it must move if the force of gravity is the resultant of the attraction of all the particles of terrestrial matter on the falling body; for it has been demonstrated (by Newton) that a sphere attracts an exterior body in the same manner as if all its matter were condensed into a single point at its centre. As bodies when left without support fall from all heights to which they may be carried, it may be inferred that gravity acts on them during the whole time of their descent and is therefore a uniformly accelerating force. This might also be inferred from the fact, which is easily rendered sensible, that bodies which fall from a greater height arrive at the earth with a greater velocity. But Galileo was the first who proved by experiment that the acceleration of falling bodies is uniform, and that the spaces descended through are consequently as the squares of the times of descent. Experiments of this kind are attended with some difficulty on account of the resistance of the air. In order to render this [s. 543]