Amongst the numerous inventions to which railways have given rise, or have aided in bringing to maturity, the Electric Telegraph stands pre-eminent, transcending as far all previously known means of transmitting intelligence, as the railway exceeds all previously known methods of transport. The Semaphore or old telegraph, although capable of communicating with considerable celerity under favourable circumstances, might yet be deemed snail-paced in its action when compared with the electric telegraph, and from the lengths of the nights and the state of the atmosphere in this country, it was inefficient for three-fourths of the 21 hours on an average of the year: the electric telegraph, on the contrary, in addition to its action being so rapid as to defy calculation, and therefore being practically speaking instantaneous, is by night and by day, and under all circumstances of weather, constantly ready for instant use. Considered in connexion with railways, it affords to the latter advantages at least equivalent to that which it derives from them; for if railways from their security from intrusion afford the most favourable situations for the establishment of these telegraphs, the latter, from the instantaneous notice which they are capable of giving of what is occurring at any point of the line, not only obviate to a great extent the chances of accident, or facilitate the remedy of their effects when they do take place, but in many situations, they render it practicable to employ a single line of rails for a double line, and, by the economy thus effected, are calculated to add greatly to the extension of railways.

The idea of an electric telegraph is not so recent as is generally supposed, but it was not until the discovery of the connexion between electricity and magnetism that it was found reducible to practice, and for this discovery we are indebted to Professor Oersted, of Copenhagen, who first suspecting it in 1806, at length succeeded in proving it in 1819. The facts upon which the action of the electro-magnetic telegraph in its various modifications depends, are the following: -

1st. If a magnetic needle be brought near a wire, the ends of which are in contact with the poles of a galvanic battery, the needle will be deflected from its magnetic position, turning to the right or left, according to the course of the electric current.

2d. If a wire carrying an electric current be coiled many times round a soft bar of iron, a powerful magnetism is developed in the iron. This magnetism however continues only so long as the electric current continues to pass, the iron becoming instantly demagnetised on the current being interrupted. Thus, if round a bar of iron a long copper wire be wound, (the wire being coated throughout its entire length with some non-conducting substance, in order to prevent metallic contact between the coil,) upon the ends of the wire being brought in contact the one with the positive, and the other with the negative pole of a galvanic battery, the iron becomes powerfully magnetic, attracting with great force any pieces of iron presented to it. On removing the wire from the battery the iron becomes demagnetised, and so rapidly does this magnetising and demagnetising process go on, that if the bar of iron be miles distant from the battery, and the connexion and interruption of the current be repeated with the utmost activity, a corresponding electric or non-electric state instantly appears in the distant iron bar.

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The third fact (which may be considered as the converse of the second) is, that if a wire be moved near a magnet, an electric current is induced by the magnet in the wire, and by using a powerful magnet, and large coils of wire, very powerful electric currents are produced. We shall now proceed to describe three modifications of the electric telegraph, based upon the preceding facts, which have been, or are at present, in use. We commence with one or two telegraphic arrangements, invented by Messrs. Cooke and Wheatstone, employed on the Great Western railway, between the Paddington terminus and the station at Slough.

This arrangement comprises two distinct parts, namely, the "Communica tor," which is stationed at that end of the line from which the message is transmitted, and the " Indicator," on which the message is read off, which is situated at the opposite end of the line, the two being connected by wires extending the whole distance between the two stations. Fig. 1 represents a plan of the communicator.

a a is a powerful horseshoe magnet, supported on blocks of wood b b, secured to the wooden frame c. d d are two extensive coils of copper wire, covered with silk and varnished. The coils are supported on a forked bar of soft iron, attached to a vertical axis. The axis is turned by a pinion/on its end, which engages with the large toothed wheel g. The face of this wheel is divided into 24 compartments, one of which is marked with a cross, to indicate the starting and finishing point, and the remainder of the divisions are each marked with a letter of the alphabet, the letters J U and X being omitted. The number of teeth in this wheel is such that there occurs between each letter as many teeth as there are teeth in the pinion, so that in moving the wheel through an arc equal to one compartment or division, the pinion performs one revolution; and as the electric current produced in the coils when rotating occurs in only one position, (that is, when the forked bar of soft iron, upon which they are placed, is in contact with the under side of the magnets,) and in any other position they are non-electric, at each revolution of the coils a current of electricity through the wires will be once established and once broken; and in like manner, in turning the wheel through the space of ten letters there would be ten electric currents through the wires, and ten interruptions.