The principle of the conservation of energy is well illustrated by the fact that as the dynamo generates an electric current if made to revolve, conversely it may be made to revolve itself if an electric current is sent through it from an exterior source. It is, therefore, available not only as a source of light in the former case, but as a direct source of mechanical power in the latter. It is on this principle that electric engines are constructed and electric railways are worked. Here also it is a question of cost and convenience, for you can only get electricity enough either to light a street or to drive an engine, by an original steam-engine or other motive power to work the dynamo, and a system of conducting wires to convey the electricity to the place where the light or power is wanted. Where the motive power is supplied by nature, as in the case of tidal or river currents or waterfalls, it is quite possible that power may be obtained in this way to compete with that obtained directly from the steam-engine; but there are as yet considerable practical difficulties to be overcome in the transmission of any large amount of energy for long distances. To overcome some of these difficulties the accumulator has been invented, which affords yet another remarkable instance of the transformation of energy. It consists of two lead plates immersed in acidulated water. When a strong electrical current is sent through the water, it decomposes it, the oxygen going to one lead plate and the hydrogen to the other. The oxygen attacks the lead plate to which it goes, forming peroxide of lead; while the hydrogen reduces any oxide in the other plate, producing pure lead, and leaving a film of surplus hydrogen on the surface. The charging current is then reversed, so that the latter plate is now attacked and the former one reduced, when the current is again reversed. By continuing this process the surfaces of both lead plates become porous, so that they present a large surface, and can therefore hold a great deal of peroxide of lead. The charging current being now broken, the oxygen which has been forcibly separated from the liquid seeks to recombine with hydrogen; and if the two lead plates are joined by a wire, this effort of the oxygen generates an electrical current in the opposite direction to the original one, which is the current utilised. Electricity is thus stored up in a portable box, where it can be kept till wanted, when it is drawn out by connecting the plates, and as a large amount of energy has been accumulated the current which is produced lasts for a considerable time.