PERPETUAL NOTION, TirE. According to Newton's first law (sec licerrox, LAWS unresisted motion continues forever unchanged. Thus, if friction could be avoided, it top or a gyroscope spinning in vacim is an instance of motion which would be unchanged forever, and which, therefore, might be called perpetual. The motion of the sun in space, thetearth's rotation about its axis, and numerous other common motions, are in this popular sense perpetual. [It is necessary to remark here, that even these motions arc subject to retard.atiou; for instance, those of the bodies of the solar system, by the resistance of the luminiferous medium, which we know to be matter, and which tills all space. This was remarked by Newton himself, for lie says, "The larger bodies, planets, and comets, preserve their motions longer (than terrestrial objects), because they move in less resisting media." The same cause influences the motion of the gyroscope, but in its case there is another retarding influence at work, due to the production of elec tric currents by the magnetism of the earth.] But this is not what is technically under stood by the title The Perpetual Motion. It means an engine which, without any supply of power from without, can not only maintain its own motion forever, or as long as its materials last, but can also be applied to drive machinery, and therefore to do external work. In other words, it means a device for creating power energy without corre sponding expenditure. This is now known to be absolutely impossible, no matter what physical forces lie employed. In fact, the modern physical axiom, the conservation of energy (see FoncE), founded on experimental bases as certain as those which convince us of the truth of the laws of motion may te expressed, in the negative, thus: The per petual motion i.1 impassible. Ilehnholtz's beautiful investigations regarding conservation of energy (referred to in FoncE), are founded on this axiom. So is the recent application, by Mush's, of Carnot's remarkable investigation of the " motive-power of fire," to the true theory of heat. Other instances will be mentioned at the cud of the article.
TLe complete statement of the impossibility of procuring the perpetual motion with the ordinary mechanical arrangements, in which it was most commonly sought, is to be found in the Principle" (q.v.), its a deduction from Newton's third law of motion. The equivalent principle of conservation of energy is there stated in a manner winch leaves nothing to be desired; although not given in anything like the modern phraseology. Yet it is usually said, in works on perpetual motion; that De La Ilire (in 1678) gave the first proof of its impossibility in ordinary mechanics. This proof, published long after Newton's, is by no means so complete, as it exposes only some of the most patent absurd ities which had been propounded for the solution of the problem. It is certain, and worthy of particular notice, that Newton was far in advance of the greatest of his con temporaries and their immediate successors, in even the fundamental notions of mechanics. Thus we find John Bernouilli seriously propounding a form of the perpetual motion, depending upon the alternate mixture, and separation by a filter, of two liquids of differ ent densities; an arrangement which is as preposterous as the very common suggestion of a water-wheel which should pump up its own supply of water; and whose absurdity must be evident to any one acquainted with Newton's chapter on the laws of motion.
It is curious that, long before Newton's time, the physical axiom that tile perpetual motion is impossible was assumed by Sterinus as a foundation for the science of statics. This is particularly interesting when we compare it with the magnificent discoveries which have been evolved in our own day from the same principle applied to the •phys hxd forces generally% and not to gravitation alone, as contemplated by Stevinus. His process is as follows: Let an endless chain of uniform weight be passsed round a smooth triangular prism ABC, of which the face BC is horizontal.