No sooner had the world got fairly settled down to submarine telegraphy than the dynamo came along. Up to that time electricity had always been procured from chemical batteries. To obtain it mechanically by moving a coil of wire in front of a magnet was a great step in advance. The infant was now striding along lustily. Batteries are expensive, inconvenient, and of very small power. Once get electricity from a machine, and there is no limit to the amount to be got. The arc-light had been produced by means of joining many hundreds of batteries together, but that was a brilliant experiment--there was nothing practical or commercial about it. But with an electric machine it was different, and once the machine was in existence the electric light was something to think about. [Illustration: AN ELECTRIC LIGHTING PLANT.] The evolution of the electric motor followed, as a natural thing, from the evolution of the dynamo, for a motor is simply a dynamo reversed. In the dynamo you revolve the armature--as the coils that move between the magnets are called--and the machine gives out current. In the motor you feed current into the armature, and it revolves and gives out mechanical power. There is a very pretty story to the effect that this action was discovered quite by chance. In some accidental way the wires leading from a dynamo at work were connected to another dynamo, and this second one at once began to turn merrily round, as if by magic. However this may be, the dynamo had been in existence for some time before any practical work was done in sending power from place to place along a slender wire. The electric motor, as a commercial machine, is barely ten years old. Yet now its busy cheerful hum may be heard under thousands of street cars in hundreds of towns. It is used to work all sorts of machinery, from the sewing-machine and the dentist's drill (beastly thing!) to heavy factory machines of all kinds. Ten years ago the electric motor was in its swaddling-clothes, and was never placed out of sight of its nurse, the dynamo. Nowadays electrical engineers think nothing of building motors of several hundreds of horse-power, and of placing them many miles from the dynamos that supply them with current. In this way a factory may be run by the power of a waterfall ever so many miles distant. The waterfall drives the dynamo, the dynamo sends its current along wires carried on poles up hill and down dale until they reach the motor, and the motor drives the machinery of the mill. At Niagara Falls work of this kind will be done on a very large scale, and many places round about will be supplied with light and power from the huge dynamos that are to be placed there.