GRAVITY, a term used by physical writers to denote the cause by which all bodies move toward each other, unless prevented by some other force or obsta cle. The most familiar effect, and that which continually obtrudes itself on our notice, is the weight of bodies, or their tendency toward the centre of the earth. It has not been ascertained, or rendered probable, that gravity is a secondary pro perty of matter ; that is to say, that it flows from any of the other known origi nal properties. Sir Isaac Newton, how ever, was of opinion, that our reasonings on the subject might be simplified, by supposing it to depend on a prodigiously elastic and rare fluid, by him called ether, and assumed to possess an increasing de gree of condensation, in parts of space more and more remote from the various masses of matter. According to this doc trine, a falling body moves, because it is pressed toward the rarer parts of this ex tended fluid. We shall leave this theory to its merits, as being neither very per spicuous, nor much related to our subject. Bergman, and others, have considered the chemical and cohesive attractions to be one and the same with the attraction of gravity, but modified in its laws, by variations in the masses,' densities, and distances of the particles of bodies. Ma ny difficulties appear at firstsightto offer themselves against this supposition. But in truth it cannot be examined at first sight ; and requires to be submitted to the rigour of mathematical investigation, which has not yet been done.
The phenomena of particular gravity, or that which respects the earth, or by which bodies descend or tend towards the centre of the earth, are as follow ; 1. All circumterrestrial bodies do here by tend towards a point, which is either accurately, or very nearly, the centre of the magnitude of the terraqueous globe. Not that it is meant that there is any vir tue or charm in the point called the cen tre, by which it attracts bodies ; but be cause this is the result of the gravitation of bodies towards all parts of which the earth consists.
2. In all places equidistant from the centre of the earth, the force of gravity is nearly equal. Indeed, all parts at the
earth's surface are not at equal distances from the earth's centre, because the equatorial parts are higher than the polar parts by about seventeen miles ; as has been proved by the necessity of making the pendulum shorter in those places, be fore it will swing seconds. In the new "Petersburg Transactions," vol. 6 and 7. M. Krafft gives a formula for the pro portion of gravity in different latitudes on the earth's surface, which is this : y = (1+0.0052848 sine 'a)g; where g denotes the gravity at the equa tor, and y the gravity under the other la titude a.
3. Gravity equally affects all bodies, without regard either to their bulk, figure, or matter: so that, abstracting from the resistance of the medium the most com pact and the most loose, the greatest and the smallest bodies would all descend through an equal space in the same time, as appears from the quick descent of every light body in an exhausted receiv er. The space which bodies do actually fall in vaeuo, is 16A feet in the first se. cond of time, inthe latitude of London ; and for other times, either greater or less than that, the spaces descended from rest are directly proportional to the squares of the times, while the falling body is not far from the earth's surface.
4. This power is the greatest at the earth's surface, from whence it decreases both upwards and downwards; but not both ways in the same proportion ; for, upwards, the force of gravity is less, or decreases as the square of the distance from the centre increases ; so that at a double distance from the centre above the surface, the force would be only one fourth of what it is at the surface ; but be low the surface, the power decreases in such sort, that its intensity is in the direct ratio of the distance from the centre ; so that at the distance of halfa semi-diameter from the centre, the force would be but half what it is at the surface ; at one-third of a semi-diameter the force would be but one-third, and so on.