the direction and the volume of the current can be changed at pleasure. He can suppose, in addition to the water-wheels before figured, and which will indicate the force of the stream, a pair of hinged valves or gates, which, whether the current be strong or weak, will be moved only by a change in its direction. The former will represent the ordinary magnets, and the latter the polarized magnets.
It is plain that, so far, this is only another form of duplex, sending two messages in the same direction at once. To make it a quadruplex telegraph it is necessary, in the first place, to add to it Stearns's duplex, or a contrivance similar to it. Even then a dead-lock would happen when the currents sent from each end of the line should be of the same intensity, and opposite in direction; that is, when all eight operators were working together. To remedy this, extra batteries are introduced, which are neutralized by part of the current in the main circuit, when that is in a working condition, but are set free to work the instruments when the currents in the main circuit destroy one another. In the diagram the extra batteries, etc., have been omitted, as also the transmitting apparatus of one station and the recording instruments of the other.
Although not strictly coming under its title, because belonging, as yet, rather to the future, this article would hardly be complete without some reference to a scheme of multiplex telegraphy which promises results of the greatest importance. The ingenious magnetic apparatus used by Prof. Helmholtz, of Berlin, in his researches in acoustics, was too suggestive not to have inspired more than one inventor with the idea of turning it to account in telegraphy. Accordingly, several, both here and in Europe, have been trying to realize it, and it is likely that the magnetically-excited tuning-forks, or the sonorous steel bars which may be substituted for them, will shortly be heard in every telegraph-office. There seems, so far, to be no ascertained limit to the number of distinct musical notes which may be propagated on a single wire at one time; and, when that limit is found, it is likely that it may be doubled or quadrupled by means of the former systems. The reduction in the cost of erection and maintenance of wires which this will bring about will be an enormous saving to telegraph companies, especially to any new ones that may be formed, or to the Government, if it should undertake the control and extension of the service.
An interesting experiment of Sir Charles Wheatstone's on the transmission of sound through solid linear conductors has, perhaps, helped to suggest this approaching transformation of the telegraph. An account of it was published in 1831. A narrow wooden rod was attached at one end to the sounding-board of a piano, and, after passing through two empty rooms, was joined at the other end to a sounding-board alone. Any piece of music played on the piano was distinctly heard by means of the sounding-board in the distant room. And not the
