• But the complete proof of the impossibility of procuring the perpetual motion by any arrangement whatever, involving any known forces, was arrived at mainly by the experi ments of Joule (q.v.), who showed that the principle of the conservation of energy extends, not alone to the forces for which it was enunciated by Newton, but to every known form of physical action. The date 1840-45 may thus be said to have finally- settled this long-disputed question; at all events, until new forms of physical forces may happen to be discovered; and we are now in a position to do generally, what was wisely done French academy in 1775 for ordinary mechanical contrivances alone—viz., refuse to consider any scheme whatever which pretends to give work without corresponding and equivalent expenditure. The language in which this decision of the French academy ts recorded (Mstoire de C Academie, 1775), is well worthy of being quoted, for its calm scien tific clearness and brevity. and for its present applicability to physical science in general: "The construction of a perpetual motion is Even if the effect of the motive power were not in the long run destroyed by friction and the resistance of the medium [in which the motion takes place], this power could produce merely an effect equivalent to itself. In order, therefore, to produce a perpetual effect from 'a finite cause that effect must be infinitely small in any finite time. Neglecting friction and resistance, a body to which motion has been given will retain it forever; but only on condition of its not acting en other bodies, and the only perpetual motion possible, on this hypothesis (which, besides, cannot occur in nature), would be useless for the object which the devisers of perpetual motion in view. ' This species of research has the incon venience of being costly; it has ruined many a family; and numerous mechanics, who might have done great service, have wasted on it their means, their time, and them talents.
"These are the principle motives which have led the academy to its decision. In resolving that it will no longer notice such' speculations, it simply declares its opinion of the uselessness of the labors of those who are devoted to therm" It has been asserted that the infatuation of the perpetual motionists, who (as may be seen by a glance at the specifications of patents in Britain, France, Belgium, America, etc.), are perhaps more numerous now than ever, is due to two causes—one, the idea.
that the perpetual motion is it lost, but recoverable invention; the other, that some immense government reward has been for-years laid aside for the successful discoverer. But, unhappily, these ideas are as fallacious as the grand delusion itself; and any one who, in the present state of science, allows himself to be carried away by this fascinating inquiry, loses his time and wastes his talents, more hopelessly than even a "squarer of the circle." In conclusion, we may mention a few of the cases already hinted at, in which the impos sibility of the perpetual motion formed the basis of an investigation. These will show the great use which may be made of even a negative proposition. Helmholtz has shown from it that the ultimate, particles of matter must exert upon each other forces, whose direction is that of the line pining each pair of particles, and whose.magnitude depends solely on their distance. J. Thomson employed it to show that the freezing-point of water is lowered by pressure, as otherwise work might be created by the freezing of ice cold water. W. Thomson has employed it to show that a diamagnetic (see DIAMAGNET isn) body does not take the opposite magnetism to iron, when iu similar circumstances; for if it did, and if, like iron, it took time for the full development of the action, a per petual motion might he produced.
The literature of this subject is very extensive, but scattered mainly through patent records and ephemeral pamphlets. The Journal des Savants, and Moutuela's Histoire des 3Ththematiques may be consulted; but especially we would refer the curious to an inter esting work by Mr. Dircks (of Patent-Ghost notoriety) entitled Perpetuunt Jlobile (Spon, London, 1861); to which we have been indebted for some of our historical notices. The tenor of the work is such that We cannot easily discover whether the author is a perpetual. motionist or not; but, however this may be, it is extremely complete and interesting as a history.