PJXGIXG THE ALARUM. 



hands, which are behind the disc, are placed one on B' and the 

 other on D. In this case, if a current comes down the line to tr, 

 it will pass by the hands A and A' to B, and thence through the 

 alarum wire to B', whence it passes by the hands at the back of 

 the disc to D, and thence along the down wire. 



If, on the other hand, the current arrive by D, it passes in the 

 same manner through the alarum to r, and so along the up wire. 



From whatever part of the line the current may be transmitted, 

 whether on the up or the down line, it must therefore pass through 

 the alarum, and give notice. 



120. In some cases a station is provided with two distinct 

 alarums, one for the down and the other for the up line, having 

 different tones, so that the agent, on hearing them, knows from 

 which direction the signals are about to come. 



In that case the wire of the up line alarum is attached to B, and 

 that of the down line to B', the wires which issue from the two 

 alarums being always in such case connected with the earth. 



"When the agent is not engaged in transmitting, he places the 

 hands A' and A on u and B, and the hands behind the disc on D 

 and B'. If a current arrive by r, it passes by B through the alarum 

 to the earth, and gives notice. If it arrive by D, it passes in like 

 manner through the alarum B' to the earth, and gives notice. 



It is, however, more usual to have a single alarum at each 

 station, acting as above described. 



The connections being so arranged that the current shall pass 

 along the entire line from terminus to terminus, all the alarums 

 at all the stations will be rung the moment the current is trans- 

 mitted. General notice is therefore given that a dispatch is about 

 to be sent from some one station along the line to some other. 



121. It is necessary, however, to inform the agents at each 

 station of the place from whence the dispatch is about to be sent, 

 and the place to which it is to be addressed. To learn this, the 

 agent transfers the connections from the alarum to his telegraphic 

 instrument. This is accomplished by removing the hand A from 

 B to T, and connecting the wire coming from the telegraphic 

 instrument by the hands at the back of the disc with D. By this 

 change the current passes from u to T, from T through the 

 telegraphic instrument to D, and from thence down the line. 

 The signals transmitted appear upon the telegraphic instrument, 

 informing the agent whence the dispatch will come, and where it 

 is desired to transmit it. 



122. If he find that it is not to be addressed to himself, his 

 arrangements will depend on the position which his own station 

 holds in relation to the two stations between which the dispatch 

 is about to be transmitted. If his station lie between them, he 



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