has proved itself highly useful in strengthening the sounds given out by the telephone, and it is probable that we shall soon see those three inventions working unitedly; for the microphone might make the telephone sounds so powerful as to enable them to be printed by phonograph as they are received, and thus a durable record of telephonic messages would be obtained. We can now transmit sound by wire, but it may yet be possible to transmit light, and see by telegraph. We are apparently on the eve of other wonderful inventions, and there are symptoms that before many years a great fundamental discovery will be made, which will elucidate the connection of all the physical forces, and will illumine the very frame-work of Nature.
In 1879, Professor Hughes endowed the scientific world with another beautiful apparatus, his 'induction balance.' Briefly described, it is an arrangement of coils whereby the currents inducted by a primary circuit in the secondary are opposed to each other until they balance, so that a telephone connected in the secondary circuit is quite silent. Any disturbance of this delicate balance, however, say by the movement of a coil or a metallic body in the neighbourhood of the apparatus, will be at once reported by the induction currents in the telephone. Being sensitive to the presence of minute masses of metal, the apparatus was applied by Professor Graham Bell to indicate the whereabouts of the missing bullet in the frame of President Garfield, as already mentioned, and also by Captain McEvoy to detect the position of submerged torpedoes or lost anchors. Professor Roberts-Austen, the Chemist to the Mint, has also employed it with success in analysing the purity and temper of coins; for, strange to say, the induction is affected as well by the molecular quality as the quantity of the
