developed in the future; but, setting aside these theories, we may possibly say that we know as much about electricity as we know about matter; for while we are conversant with many of the properties of each, we _know_ nothing of the ultimate nature of either. But while the theory of electricity has scarcely advanced beyond the point at which it was left by Clerk Maxwell, the practical applications of the science have experienced great developments of late years. Less than a century ago the lightning-rod was the only practical outcome of electrical investigations which could be said to have any real value. [OE]rsted's discovery, in 1820, of the action of a current on a magnet, led, in the hands of Wheatstone, Cooke, and others, to the development of the electric telegraph. Sir William Thomson's employment of a beam of light reflected from a tiny mirror attached to the magnet of the galvanometer enabled signals to be read when only extremely feeble currents were available, and thus rendered submarine telegraphy possible through very great distances. The discovery by Arago and Davy, that a current of electricity flowing in a coil surrounding an iron bar would convert the bar into a magnet, at once rendered possible a variety of contrivances whereby a current of electricity could be employed to produce small reciprocating movements, or even continuous rotation, where not much power was required, at a distance from the battery. An illustration of the former is found in the common electric bell; it is only necessary that the vibrating armature should form part of the circuit of the electro-magnet, and be so arranged that, while it is held away from the magnet by a spring, it completes the battery circuit, but breaks the connection as soon as it moves towards the magnet under the magnetic attraction. To produce continuous rotation, a number of iron bars may be attached to a fly-wheel, and pass very close to the poles of the magnet without touching them; when a bar is near the magnet, and approaching it, contact should be made in the circuit, but should be broken, so that the magnet may lose its power, as soon as the bar has passed the poles; or the continuous rotation may be produced from an oscillating armature by any of the mechanical contrivances usually adopted for the conversion of reciprocating into continuous circular motion. But all such motors are extremely wasteful in their employment of energy. Faraday's discovery of the rotation of a wire around a magnetic pole laid the foundation for a great variety of electro-motors, in some of which the efficiency has attained a very high standard. About ten years ago, Clerk Maxwell said that the greatest discovery of recent times was the "reversibility" of the Gramme machine, that is, the possibility of causing the armature to rotate between the field-magnets by sending a current through the coils. The electro-motors of to-day differ but little from dynamos in the principles of their construction.