NATURAL philosophy, otherwise call ed physics, is that science which considers the powers of nature, the properties of natural bodies, and their actions upon one another. Laws of nature are certain axioms, or general rules, of motion and rest, observed by natural bodies in their actions upon one another. Of these laws Sir 1. Newton has established three : Law 1. That every body perseveres in the same state, either of rest or uniform rec tilinear motion, unless it is compelled to change that state by the action of some foreign force or agent. Thus projectiles persevere in their motions, except so far as they are retarded by the resistance of the air, and the action of gravity : and thus a top, once set up in motion, only ceases to turn round because it is resist ed by the air, and by the friction of the plane upon which it moves. Thus also the larger bodies of the planets and comets preserve their progressive and circular motions a long time, undiminish ed, in regions void of all sensible resist ance. As body is passive in receiving its motion, and the direction of its mo tion, so it retains them, or perseveres in them, without any change, till it be acted upon by something external. Law 2. The motion, or change of motion, is al ways proportional to the moving force by which it is produced, and in the direc tion of the right line in which that force is impressed. If a certain force produce a certain motion, a double force will pro duce double the motion, a triple force triple the motion, and so on. And this motion, since it is always directed to the same point with the generating force, if the body were in motion before, is either to be added to it, as where the motions conspire; or subtracted from it, as when they are opposite ; or combined oblique ly, when oblique : being always com pounded with it according to the denomi nation of each. Law 3. Re-action is al ways contrary and equal to action ; or the actions of two bodies upon one another are always mutually equal, and directed contrary ways, and are to be estimated always in the same right line. Thus,
whatever body presses or draws another is equally pressed or drawn by it. So, if 1 press a stone with my finger, the finger is equally pressed by the stone : if a horse draw a weight forward by a rope, the horse is equally opposed or drawn back towards the weight ; the equal tension or stretch of the rope hindering the pro gress of the one, as it promotes that of the other. Again, if any body, by strik ing on another, do in any manner change its motion, it will, itself, by means of the other, undergo also an equal change in its own motion, by reason of the e9uality of the pressure. When two bodies meet, each endeavours to persevere in its state, and resists any change ; and because the change which is produced in either may be equally measured by the action which it excites upon the other, or by the re sistance which it meets with from it, it follows that the changes produced in the motions of each are equal, but are made in contrary directions : the one acquires no new force but what the other loses in the same direction ; nor does this last lose Any force but what the other acquires ; and hence, though by their collisions motion passes from the one to the other, yet the sum of their motions, estimated in a given direction, is preserved the same, and is unalterable by their mu tual actions upon each other. In these actions the changes are equal ; not those, we mean, of the velocities, but those of the motions or momenta ; the bodies being supposed free from any other impediments. For the changes of ve locities, which are likewise made contrary ways, inasmuch as the mo tions are equally changed, are reci procally proportional to the bodies or masses.