This, like the celebrated automaton chess-player, was evidently a case of clever imposi tion; and but for its strange effect on 'S Gravesande, would probably have been forgotten long ago. Tricks of this kind, more or less ingenious, such as that of Spence of Linlith gow (1818), which many of our readers may recollect, are still common, especially in America.
Bishop Wilkins's third form is a good example of the second class of contrivances above mentioned. Three water-wheels, driven by the descending water, are intended to turn an Archimedean screw, so as constantly to replenish a tank above. Wilkin's calm investigation of the reasons why his device will not succeed is very interesting creditable.
As a contrast, let us take a ease of special absurdity, that of Norwood. In the figure. it is supposed that, as the weight of the water or mercury in the large vessel immensely exceeds that in the neck, it will preponderate, and drive the liquid through the spout into the vessel again; thereby furnishing, not only an admirable perpetual motion, but a conclusive disproof of one of the fundamental laws of hydrostatics.
The second of Wilkins's cases is an instructive one. It depends on magnetism, and will be readily understood from the cut. AB is a loadstone, which draws the iron ball, C, up the inclined plane to H, where there is a hole through which the ball falls down the curved incline, pushes open a trap at F, and is dragged again up the plane by the loadstone. The error of this is the neglect of the action of the loadstone on the falling ball. There would be an admirable case of the perpetual motion if we could remove or annihilate (without expentliture of work) the action of the loadstone during the descent. Unfortunately, the law of magnetic attraction is the same as that of gravitation, and what is impossible with the one must be eqcally so with the other. A good illustration The spokes . the center. (S) of four of this is Addeley's perpetual motion, represented in the annexed sketch. projecting from the wheel are magnets, whose south poleS are all turned from These are attracted by the north poles (N). and repelled by the south pole: fixed magnets; and blocks of wood (A) are interposed, to prevent magnetic action where it would tend to stop the machine! If it were possible to find a substance which would deal with gravitation or magnetism as an opaque body does with light (casting a shadow), the perpetual motion'Would be obtained with the greatest ease.
It would be tedious and unprofitable to go through the various physical forces, showing how a misconcep tion of their laws has led to hundreds of patented schemes for the production of perpetual motion. We may merely hint at Magneto-electric machines turned by electromagnetic engines, to which they supply the electric currents; electric machines, driven by a gas engine, the fuel for which is supplied by the decom position of water by the electricity produced, etc.; the absurdity of all of which may be imagined from the perfectly analogous case of a steam-engine to which heat might be supposed to be supplied by the friction of bodies driven by the engine itself. An excellent example of this absurdity is furnished by the writiugs of one of our ablest geologists. Ile considers that the internal heat of the earth may be due to chemical com bination, that the heat so produced may develop thermo-electric currents, and that these iu their turn may decompose the compounds formed, so that the process may go on indefinitely.
But the third class of attempts above described merits a few words. It certainly does not give the perpetual motion, hut it is capable of furnishing prime-movers which will work uninterruptedly for perhaps hundreds of thousands of years. This is done, how ever, as we should expect, at the expense of other stores of energy in the universe. Thus, the tide-wheel, or tidal engine, a little-used but most effective source of power. derives its energy entirely front the earth's diurnal rotation. Engines driven by collected rain-water, .such as mill-wheels, etc., and others driven by power stored up from winds, etc., depend upon energy radiated from the sun, mainly in the form of heat. None of these can, therefore, in strictness be called the perpetual motion, since the energy of the earth's rotat on, or of the sun's beat, is drawn upon in their production.