THE ELECTRIC TELEGRAPH. 



turns the hands A and A' upon the contact pieces u and D, so as to 

 allow the current to pass between the up wire and the down wire, 

 along the hands without interruption, and also without spending any 

 part of its force in needlessly working his telegraphic instrument. 



123. If he find that the dispatch is intended for himself, and 

 that it proceeds from a station on the up line, for example, he 

 places the hand A' upon u, A upon T, and by the two hands behind 

 the disc he connects the wire issuing from the instrument with E. 

 By this arrangement, the current arriving at TJ passes by the 

 hands A' and A to T, thence through the telegraphic instrument to 

 E by the hands behind the disc and to the earth. 



In this case the course of the current is limited to the part of 

 the line wire which is included between the station from which it 

 is transmitted and that to which it is addressed. By connecting 

 the telegraphic instrument with the earth by E, the down 

 line wire is free ; so that while the up line wire is employed in 

 conveying the dispatch in question, other dispatches may be 

 transmitted between any stations on the down line. 



124. If we express for example the chief terminal station by s, 

 and the series of stations upon the line proceeding from it down- 

 wards by s 1} S 2 , S 3 , s 4 , &c., we can conceive various dispatches to 

 be at the same time transmitted between them by the arrange- 

 ment here explained, being made at each station which receives a 

 dispatch. Thus, if s sends a dispatch to s x , and s x cuts off its 

 communication with the down wire by putting its telegraphic 

 instrument in connection with the earth, the current transmitted 

 from s stops at s x . A dispatch may therefore be at the same 

 time sent between s a and S 3 , another between S 4 and S 5 , and so on. 



Thus, the same line of conducting wire may be at the same 

 time engaged in the conveyance of several dispatches, the only 

 limitation ' being that when a dispatch is being transmitted 

 between two stations, no other dispatch can at the same time be 

 transmitted between any of the intermediate stations. 



It follows from this as a necessary consequence that if, as generally 

 happens in thickly peopled tracts of country, the terminal and 

 one or two of the most populous of the intermediate stations keep 

 the telegraph in constant work, separate and independent wires, 

 and instruments must be provided to serve the secondary inter- 

 mediate stations, just as upon railways, second and third-class 

 trains are provided to serve those lesser stations on the line, which 

 are passed by the first-class trains without stopping. 



Every great telegraphic line presents an example of this. Thus 



upon the Dover line separate wires and instruments are appropriated 



to the transmission of dispatches between the terminal stations, 



London and Dover, and the intermediate stations, Tonbridge, 



3 84 



