ON SOME MARVELS IN TELEGRAPHY. 247 



from its mean position in one direction, a and b are connected, 

 as are c and d as shown in the figure while the connection 

 between b and d is broken. Thus the current passes from p 

 by a and b, round the needle n ; thence to station B, round 

 needle ', and by b' and d', to the earth plate G' ; and so 

 along the earth to G, and by d c, to the negative pole N. 

 The upper end of the needle of both stations is deflected to 

 the right by the passage of the current in this direction. 

 When the handle of the commutator at A is turned in the 

 other direction, b and c are connected, as also a and d; the 

 current from p passes along a d to the ground plate G, thence 

 to G', along d &, round the needle n', back by the wire to 

 the station A, where, after circuiting the needle n in the same 

 direction as the needle ', it travels by b and c to the nega- 

 tive pole N. The upper end of the needle, at both stations, 

 is deflected to the left by the passage of the current in this 

 direction. 



It is easily seen that, with two wires and one battery, 

 two needles can be worked at both stations, either one 

 moving alone, or the other alone, or both together ; but for 

 the two to move differently, two batteries must be used. 

 The systems by which either the movements of a single 

 needle, or of a pair of needles, may be made to indicate the 

 various letters of the alphabet, numerals, and so on, need 

 not here be described. They are of course altogether 

 arbitrary, except only that the more frequent occurrence of 

 certain letters, as e, t, a, renders it desirable that these 

 should be represented by the simplest symbols (as by a 

 single deflection to right or left), while letters which occur 

 seldom may require several deflections. 



One of the inventions to which the title of this paper 

 relates can now be understood. 



In the arrangement described, when a message is trans- 

 mitted, the needle of the sender vibrates synchronously with 

 the needle at _the station to which the message is sent. 

 Therefore, till that message is finished, none can be received 

 at the transmitting station. In what is called duplex tele- 



