MOTION is defined to be the continued and successive change of place. There are three general laws of motion. 1. That a body always perseveres in its state of rest, or of uniform motion in a right line, till by some external force it be made to change its state ; for,as body is passive in receiving its motion, and the direction of its motions, so it retains them, or per severes in them, without any change, till it be acted on by something exter nal. From this law it appears, why we inquire not, in philosophy, concerning the cause of the continuation of motion or rest in bodies, which can be no other than their inertia ; but if a motion begin, or if a motion already produced is either accelerated or retarded, or if the direction of the motion is altered, an inquiry into the power or cause that produces this change is a proper subject of philoso phy. 2. The second general law of mo tion ia, that the change of motion is pro.. portionsl to the force impressed, and is produced in the right line in which that force acts. When a fluid acts upon a bo dy, as water or air upon the vanes of a mill, or wind upon the sails of a ship, the acceleration of the motion is not propor tional the whole force of those fluids, but to that part only which is impressed upon the vanes or sails, which depends upon the excess of the velocity of the fluid above the velocity which the vane or sail has already acquired: for if the velocity of the fluid be only equal to that of the vane or sail, it just keeps up with it, but has no effect either to advance or retard its motion. Regard must always be had to the direction in which the force is im pressed, in order to determine the change of motion produced by it : thus, when the wind acts obliquely with re.. spect to the direction of a ship, the change of her motion is first to be esti mated in the direction of the force im pressed: and thence, by a proper appli cation of mechanical and geometrical principles, the change of the motion of the ship in her own direction is to be de duced. 3. The third general law of mo tion ia, that action and re-action is equal.
with opposite directions, and are to be estimated always in the same right line. Body not only never changes its state of itself, but resists, by its inertia, every ac tion that produces a change in its motion: hence, when two bodies meet, each en deavours to persevere in its state, and re sists any change ; the one acquires no new motion but what the other loses in the same direction ; nor does this last lose any force but what the other ac quires: and hence, though by their col lision motion passes from the one to the other, yet the sum of their motions, esti mated in a given direction, is preserved the same, and is unalterable by their mu tual actions upon each other.
All motion may be considered absolute ly or relatively. Absolute or real motion, says Mr. Maclaurin, is when a bodychanges its place in absolute space ; and relative motion is, when a body changes its place only with relation to other bodies. From the observation of nature, every one knows that there is a motion; that a body in motion perseveres in that state, till, by the action of some power, it is ne cessitated to change it ; that it is not in relative or apparent motion in which it perseveres, in consequence of its inertia, but in real or absolute motion. Thus the apparent diurnal motion of the sun and stars would cease, without the least pow er or force acting upon them, if the mo tion of the earth was stopped ; and if the apparent motion of any star was de stroyed by a contrary motion impressed upon it, the other celestial bodies would still appear to persevere in their course. See brawn..
To make this matter still plainer, Mr. Martin observes, that space is nothing but an absolute and infinite void, and that the place of a body is that part of the immense void which it takes up or possesses : and this place may be consi dered absolutely, or in itself, in which case it is called the absolute place of the body ; or else with regard to the place of some other body, and then it is called the relative or apparent place of the body. as motion is only the change of place in bodies, it is evident that it will come under the same distinction of abso lute, and relative or apparent. All mo tion is in itself absolute, or the change of absolute space ; but when the mo tions of bodies are considered and com pared with each other, then are they relative and apparent only; they are relative, as they are compared to each other ; and they are apparent, only insomuch that not their true or abso. lute motion, but the sum or difference of the motions only, is perceivable to us.