The advantages proposed to be gained by the copying telegraph, in addition to its increased rapidity of transmission, are the authentication of telegraphic correspondence by the signatures of the writers, freedom from the errors of transmission, and the maintenance of secrecy. As a special means of obtaining secrecy, the messages may be received on paper moistened with a solution of nitrate of soda alone, in which case they would be invisible until brushed over with a solution of prussiate of potass, to be applied by the person to whom the communication is addressed. Professor Wheatstone has recently contrived an improvement in his index telegraph, which was described by Professor Faraday in a lecture at the Royal Institution in June last. Its chief merit, however, consists in the beauty of the mechanism, for it is essentially the same as the index telegraphs he and others have previously invented, with the substitution of magneto-electricity for the moving force. Having now traced the history of the invention of the instruments by means of which messages may be transmitted, it becomes necessary to describe the methods employed for making the electrical connection from one place to another. This part of the electric telegraph system is, after all, the most essential to its efficient working, and bears the same relation to the transmitting instruments that the structure of a railroad does to locomotive engines in the system of railway conveyance. The fact that an electric current might be sent through a long circuit had been established by Dr. Watson, in conjunction with other Fellows of the Royal Society, in 1747, when they sent the charge of a Leyden jar through two miles of wire, supported upon short sticks driven into the ground; the wire at each terminus being connected with the earth for the return current. This method of insulation and conduction fully answered the purpose, and served to determine the great velocity with which electricity is transmitted, for no perceptible interval occurred between the discharge of the Leyden jar at one end of the circuit, and its effect at the other extremity. Mr. Ronalds made the next experiment on an extensive scale, by insulating eight miles of wire in glass tubes, the wire being carried backwards and forwards for that distance on his lawn at Hammersmith. That mode of insulation was found very efficient. It was, indeed, too perfect, for the difficulty arose of discharging the electricity from the wire after the charge had passed through it.