7 8 THE POPULAR SCIENCE MONTHLY. 



the direction and the volume of the current can be changed at pleas- 

 ure. 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 or- 

 dinary 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 du- 

 plex, 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 omit- 

 ted, as also the transmitting apparatus of one station and the record- 

 ing 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 with- 

 out some reference to a scheme of multiplex telegraphy which prom- 

 ises 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 in- 

 ventor with the idea of turning it to account in telegraphy. Accord- 

 ingly, several, both here and in Europe, have been trying to realize it, 

 and it is likely that the magnetically-excited tuning-forks, or the so- 

 norous steel bars which may be substituted for them, will shortly be 

 heard in every telegraph-office. There seems, so far, to be no ascer- 

 tained 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 main- 

 tenance 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 trans- 

 mission of sound through solid linear conductors has, perhaps, helped 

 to suggest this approaching transformation of the telegraph. An ac- 

 count 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 



