much that the local relay no longer avails, and recourse must be had to another device. This consists in adding a derived or loop circuit to the bridge wire, consisting of a condenser, c, electro-magnet, m, and adjustive resistance, r. The electro-magnet is set so as to act upon the other end of the tongue of the neutral relay, as shown. In this way, when a change of current takes place, the discharge from the condenser through m serves to keep the tongue in its place until the critical time is past; r is merely for adjustment. By these ingenious contrivances Messrs. Prescott and Edison have rendered their quadruplex system a practical success. Within the last few years other systems, more or less similar, have been devised, but as yet theirs is the only one of note.

TELEPHONIC TELEGRAPHY.

a. The Tone Telephone.

The last five years have seen the practical development of this interesting branch of telegraphy. For many years back attempts have been made to transmit musical tones or articulate sounds to a distance by means of electricity, but the results were at best only hopeful until, in 1860, Herr Phillip Reis, of Homburg, profiting by the researches of Wertheim, Marian, and Henry, invented the first telephone. In Reis's telephone a stretched membrane is set into vibration in unison with the sound to be transmitted, and, by a little contact-piece which it carries, this vibrating membrane is caused to interrupt the electric current circulating in the line. The current so interrupted is utilised at the distant station to produce a sound similar to the original sound at the near or sending station. To produce such a sound Reis made use of Page's discovery, that an audible "click" accompanies the demagnetisation of a bar of iron inserted in an electro-magnetic helix. He surrounded an iron wire with a helix, and caused the interrupted current from the sending station to pass through the helix to earth. At every interruption a distinct sound was given out by the iron core, and the joint effect of these reproduced a note of the same pitch as that sounded at the sending station.

In telegraphy, however, the telephone only acquires special importance when it is regarded as a means of multiplex transmission. A single message by telephone can have small advantage over the ordinary methods; so Reis's telephone was neglected as a kind of scientific curiosity.

Ten years later, in 1870, Mr. Cromwell Fleetwood Varley, F.R.S., appears to have first clearly designed the employment of the telephone in the transmission of several messages simultaneously on one line wire, by means of separate notes. Varley's patent of that year is full of most ingenious plans and contrivances, not only for the sending and receiving of telephone messages, but for rendering the vibratory electric signals visible, both temporarily, by light, and permanently, by recorded marks on moving paper. The principles of later inventions are to be found here, and even some of the details. But Varley seems to have left the carrying out of his system in abeyance; and the merit of practical success belongs to the systems of M. Paul la Cour, of Copenhagen, and Mr. Elisha Gray, of Chicago.

The principle of all these systems is that a vibrating body, such as a tuning-fork or membrane, emitting a certain note, shall be caused at each vibration to interrupt an electric

current in the telegraph line; and the current so interrupted shall, at the distant station, be made to set a corresponding body in vibration, so as to reproduce the original note there. Of course it is possible, by having a number of vibrators at the sending station and a corresponding number of vibrators in unison with them at the receiving station, to give rise simultaneously to several distinct sets of electric vibrations in the line wire, and to reproduce several distinct notes at the receiving station. In this way is multiplex transmission rendered feasible.

M. Paul la Cour's English patents bear date 1874 and 1876. Briefly, his system is as follows:-Fig. II represents his sending arrangement. F is a tuning-fork which is put into vibration, and makes contact once every vibration with the contact-point P. The fork is connected, through the signalling-key, with the sending-battery, and the contactpoint P is connected to the line. Here the fork is simply shown, and must be started by hand; but in his later patent

M. la Cour provides, by means of electro-magnets, that the fork shall be maintained constantly in a state of vibration. In either case the current sent into the line by depressing the signalling-key is interrupted by the fork an equal number of times per second that the fork vibrates. This intermittent current traverses the line to the distant station, where it is passed "to earth" through the receiving apparatus shown in Fig. 12. This consists of a fork similar to the first, and vibrating the same note. Each leg is surrounded by a helix of wire. Two other helices are placed upright, one on each side of the ends of the legs; they are fitted with iron cores, and adjustible poles, n, s, and are in fact electro-magnets. These four helices are joined up in series, so that the line-current passes through each in turn.

magnetises oppositely the legs of the fork, which are attracted by the contiguous poles of the electro-magnets. The legs of the fork are thus pulled apart, and let go with each pulsation of the intermittent current. In this way the fork is put into audible vibration.

These vibrations are also turned to the actual recording of the message in permanent marks, by means of a local circuit and Morse or other recorder. A fine metal point, P, is brought very near to one leg of the fork, so that when the fork vibrates this leg comes into contact with the point, and completes the local circuit through the fork, local battery, and recording instrument.

M. la Cour's first experiments were made in June, 1874, on a short line in the neighbourhood of Copenhagen, and in November of the same year he succeeded in working

successfully from Fredericia, in Jutland, to Copenhagen, a distance of 390 kilometres.

Mr. Elisha Gray's first English patent bears date of 1874, a few months before M. la Cour's; but in it there is no mention of multiplex telegraphy. Subsequent patents, in 1875 and 1876, describe the development of his system and its application to multiplex signalling. His principal methods of sending are represented in Figs. 13 and 14. In Fig. 13 the vibrator is a tuning-fork or reed, R, placed between two electro-magnets, A and B, which can be adjusted by the screws D and G to or from the reed, so as to have greater or less power over it. The reed is barbed with

two short springs, which play against the adjustible contactpoints a and b, making contact with them alternately as the reed vibrates. The reed is perpetually maintained in vibration by means of a local battery and the electro-magnets in the following way :-The local battery is connected up, as shown, through the electro-magnets and the fixed end of the reed. When the reed vibrates so as to make contact, by means of its spring, with b, and break contact with a, the electro-magnet A is in circuit and actuated by the current, while the magnet в is cut out of circuit by the shortcircuit wire w. The magnet A therefore exercises a pull upon the reed R, which assists the reed in vibrating towards

it, and breaking contact with b while making contact with a. This, while keeping the electro-magnet A still in circuit, also puts the magnet в in circuit. In this way the electromagnet в is alternately thrown in and out of circuit, and

the effect of this on the reed is to keep up its vibration. The reed, thus kept vibrating by the electro-magnets and local battery, is used to interrupt the line circuit and sending current. The sending battery is connected up, as shown, through the signalling-key to the reed, and by the reed

to Earth