CHAP, iv.] ELECTRIC TELEGRAPHY. 583 



from their normal position upon the circumference of the cylinder 

 corresponding to the particular letter of the alphabet represented by 

 the depressed key. The cylinder, upon the depression of each key, 

 rotates to an extent corresponding to the length of the signal given, 

 and the deplaced pins in their protruding position are carried round 

 with the cylinder, causing contacts to be made by which the necessary 

 succession of . currents to form the signal are passed into the 

 line-wire. 



II. PRINTING TELEGKAPHS. HUGHES'S SYSTEM. 



The various telegraphic systems we have hitherto studied, in spite 

 of their different constructions and methods, have all employed in 

 producing signals a common principle which we can enunciate as 

 follows : the sending by a dispatching office of a determinate series 

 of currents and of interruptions of currents, which produce at the 

 receiving station a series of movements constituting the preconcerted 

 signals. 



The movements of the manipulator and indicator may be the same 

 or not, but it is essential that there should be a relation *between 

 them, if not of absolute simultaneousness, at least of synchronism 

 so that there should be a perfect identity between the signal sent 

 and that reproduced. This last condition, the synchronism of the 

 movements of the manipulator and indicator, is quite indispensable 

 also in the Hughes printing telegraph which we are now 7 about to 

 describe. 



The idea of getting the message printed is not new. From the 

 origin of the invention of the electric telegraph (1841) Wheatstone 

 patented a system of printing in ordinary letters on a band of paper, 

 the words of a message. Afterwards several inventors have followed 

 the same idea and have realized it with greater or less success : we 

 rnay mention the systems of Vail, Bain, Brett, Du Montcel, Freitel, 

 Theyler, Dujardin, Thomson, Digney, &c. But the most perfect of 

 all these systems which more than all has solved the problem, of 

 great rapidity of transmission is the printing telegraph of the 

 American professor Hughes. It is a more complicated and expensive 

 apparatus than the Morse, more difficult to work and keep in order, 



