Page:Encyclopædia Britannica, Ninth Edition, v. 18.djvu/579

P E R P E R 553 more or less injured and eighty-two buildings set on fire ; the tower of the church of St Jean was also burnt, its roofing and timber-work destroyed, and the bells melted by the flames. This damage has since been repaired. PEROUSE. See LA PEROUSE, vol. xiv. p. 298. PERPETUAL MOTION, or PERPETUUM MOBILE, in its usual significance does not mean simply a machine which will go on moving for ever, but a machine which, once set in motion, will go on doing useful work without draw ing on any external source of energy, or a machine which in every complete cycle of its operation will give forth more energy than it has absorbed. Briefly, a perpetual motion usually means a machine which will create energy. The earlier seekers after the "perpetuum mobile" did not always appreciate the exact nature of their quest ; for we find among their ideals a clock that would periodically rewind itself, and thus go without human interference as long as its machinery would last. The energy created by such a machine would simply be the work done in over coming the friction of its parts, so that its projectors might be held merely to have been ignorant of the laws of friction and of the dynamic theory of heat. Most of the perpetual motionists, however, had more practical views, and ex plicitly declared the object of their inventions to be the doing of useful work, such as raising water, grinding corn, and so on. Like the exact quadrature of the circle, the transmutation of metals, and other famous problems of antiquity, the perpetual motion has now become a vener able paradox. Still, like these others, it retains a great historical interest. Just as some of the most interesting branches of modern pure mathematics sprang from the problem of squaring the circle, as the researches of the alchemists developed into the science of modern chemistry, so, as the result of the vain search after the perpetual motion, there grew up the greatest of all the generalizations of physical science, the principle of the conservation of energy. There was a time when the problem of the perpetual motion was one worthy of the attention of a philosopher. Before that analysis of the action of ordinary machines which led to the laws of dynamics, and the discussion of the dynamical interdependence of natural phenomena which accompanied the establishment of the dynamical theory of heat, there Ava.s nothing plainly unreasonable in the idea that work might be done by the mere concatenation of machinery. It had not then been proved that energy is uncreatable and indestructible in the ordinary course of nature ; even now that proof has only been given by in duction from long observation of facts. There was a time when wise men believed that a spirit, whose maintenance would cost nothing, could by magic art be summoned from the deep to do his master s work ; and it was just as reasonable to suppose that a structure of wood, brass, and iron could be found to work under like conditions. The disproof is in both cases alike. No such spirit has ever existed, save in the imagination of his describer, and no such machine has ever been known to act, save in the fancy of its inventor. The principle of the conservation of energy, which in one sense is simply a denial of the possibility of a perpetual motion, rests on facts drawn from every branch of physi cal science ; and, although its full establishment is not half a century old, yet so numerous are the cases in which it has been tested, so various the deductions from it that have been proved to accord with experience, that it is now regarded as one of the best -established laws of nature. Consequently, on any one who calls it in question is thrown the burden of proving his case. If any machine were produced whose source of energy could not at once be traced, a man of science (complete freedom of investi gation being supposed) would in the first place try to trace its power to some hidden source of a kind already known ; or in the last resort he would seek for a source of energy of a new kind and give it a new name. Any assertion of creation of energy by means of a mere machine would have to be authenticated in many instances, and established by long investigation, before it could be received in modern science. The case is precisely as with the law of gravitation ; if any apparent exception to this were observed in the case of some heavenly body, astro nomers, instead of denying the law, would immediately seek to explain the occurrence by a wider application of it, say by including in their calculations the effect of some dis turbing body hitherto neglected. If a man likes to indulge the notion that, after all, an exception to the law of the conservation of energy may be found, and, provided he submits his idea to the test of experiment at his own charges without annoying his neighbours, all that can be said is that he is engaged in an unpromising enterprise. The case is otherwise with the projector who comes for ward with some machine which claims by the mere in genuity of its contrivance to multiply the energy sup plied to it from some of the ordinary sources of nature and sets to work to pester scientific men to examine his supposed discovery, or attempts therewith to induce the credulous to waste their money. This is by far the largest class of perpetual-motion-mongers nowadays. The interest of such cases is that attaching to the morbid anatomy of the human mind. Perhaps the most striking feature about them is the woful sameness of the symptoms of their madness. As a body perpetual-motion seekers are ambi tious, lovers of the short path to wealth and fame, but wholly superficial. Their inventions are very rarely char acterized even by mechanical ingenuity. Sometimes in deed the inventor has simply bewildered himself by the complexity of his device ; but in most cases the machines of the perpetual motionist are of child -like simplicity, remarkable only for the extraordinary assertions of the inventor concerning them. Wealth of ideas there is none ; simply assertions that such and such a machine solves the problem, although an identical contrivance has been shown to do no such thing by the brutal test of standing still in the hands of many previous inventors. Hosts of the seekers for the perpetual motion have attacked their insoluble problem with less than a schoolboy s share of the requisite knowledge ; and their confidence as a rule is in proportion to their ignorance. Very often they get no farther than a mere prospectus, on the strength of which they claim some imaginary reward, or offer their precious discovery for sale ; sometimes they get the length of a model which wants only the last perfection (already in the inventor s brain) to solve the great problem ; sometimes fraud is made to supply the motive -power which their real or pretended efforts have failed to discover. It was no doubt the barefaced fallacy of most of the plans for perpetual motion that led the majority of scien tific men to conclude at a very early date that the "per petuum mobile" was an impossibility. We find the Parisian Academy of Sciences refusing, as early as 1775, to receive schemes for the perpetual motion, which they class with solutions of the duplication of the cube, the trisection of an angle, and the quadrature of the circle. Stevinus and Leibnitz seem to have regarded its impossibility as axi omatic ; and Newton at the beginning of his PrincApia states, so far as ordinary mechanics are concerned, a prin ciple which virtually amounts to the same thing (see MECHANICS, vol. xv. p. 715). The famous proof of De la Hire simply refers to some of the more common gravitational perpetual motions, to which we shall refer shortly. The truth is, as we have said already, that, if proof is to be given, or considered XVIII. 70