Perpetual Motion does not mean something which is for ever moving, but is a term applied to any machine or contrivance which will go on moving without any external motive power, or will do work in excess of the energy supplied to it. It is, in fact, a creator of energy, and from the very earliest times has absorbed a vast amount of attention from real and pseudo-philosophers. Though equally ancient, this problem has scarcely the respectability of the search for the Philosopher's Stone, and has certainly been less fruitful in its results. The chief addition to science which its search has produced is the denial of itself. The law of the conservation of energy, which experience has proved, can, in fact, be stated in the form that perpetual motion is impossible. Since it was invariably the custom of perpetual motors to stand still when called upon to carry out the designs of their inventors, scientists doubted from very early times the possibility of their existence. So great a nuisance did these self-imagined creators become that in 1775 the Parisian Academy of Science refused to receive any more descriptions of their schemes, and lamented the fact that many good workmen were lost by thus developing into misguided enthusiasts. Many examples of perpetual motion were worked by means of balls which alternately became nearer the inside or outside of a wheel. The balls on the outside were to overbalance those on the inside and so turn the wheel, this happy result being brought about by means of curved spokes or pivoted levers. Something of this sort was exhibited by the Marquis of Worcester in the Tower before a royal audience. The wheel of Offyreus was reported to have been moving for eight weeks in a locked room of a castle belonging to the Landgrave of Hesse-Cassel. Its works were, however, concealed, and Offyreus destroyed their hidden portion because the mathematician's Gravesande (who had at first been induced to believe in it) became rather curious as to its construction. Water-wheels were also - on account of their utility - the subject of much deliberation, and many a one was constructed (on paper) which considerately pumped up the water for its own use, sometimes embellished by a water-screw, but as often without. One form of perpetual motion was founded on the hydrostatic paradox: A B is a large vessel of water. B C is a narrow tube leading from its base and bent round at the top; the weight of water in A B should naturally (!) overbalance that in B C, and so water would be sent up to C, where it would drop back into the tank and go on for ever. The simplicity of this design would recommend it to all but for the trifling circumstance that it does not work. Magnetism, being little-understood, was naturally invoked to act the part of creator of energy. A steel ball, B, on an inclined plane, P H, was attracted upwards by the magnet M, but reaching the hole H it fell through, rolled back to P, and again started on its upward path, continuing the cycle for ever - in the mind of its inventor. Any number of examples might be quoted of historical interest, but space does not permit it. The more modern development is that of a gas-engine which drives a dynamo machine; this generates electricity which decomposes water, and this decomposition heats the gas-engine. A simple application of the laws of dynamics and the dynamical theory of heat at once shows the impossibility of this circular and efficient combination. There is in America an alleged perpetual motor, which is, however, currently reported to derive its energy from a small boy in the next room.