CHAP, in.] ELECTRIC TELEGRAPHY. 565 



cross to the letter A the current passes along the line, enters the 

 indicator and the electro-magnet, which draws into contact the 

 armature M. The motion of the latter causes the rotation in the 

 opposite direction of the shaft ab of the pallet^, which lets the tooth 1 

 escape, and the tooth 2 comes to a stop against the pallet^? when the 

 wheel has been made to move by the escapement-motion having 

 turned it. The indicating needle has then advanced through one 

 division and stops at A. 



When the current ceases the armature returns to its first position 

 under the action of the spring r, the pallet^? lets tooth 2 escape, the 

 wheel moves again, and the pallet p stops in its turn at the tooth 2 ; 

 the needle lias turned through another division ; a very simple 

 arrangement allows the needle to be returned to the cross without 

 sending a current (which is sometimes necessary). By means of the 

 rod li, seen on the right, the shaft which carries the pallets, and the 

 pallets themselves, can be lowered ; these no longer catch against the 

 teeth of the escapement-wheel, and the wheel moves until a roller F 

 encounters a stoppage suitably placed, which corresponds to the 

 position at which the needle is at the cross. 



The little dial seen on the left hand at the top of the indicator, 

 serves to regulate the spring r. If this spring were not suitably fixed, 

 the magnitude of the oscillations of the armature might be too ,great 

 or too little ; in the first case the pallets would be liable to go out of 

 the plane of the escapement-wheel, and the wheel would move 

 without interruption ; in the second case the pallets could not disen- 

 gage the teeth, and there would be no escape: the indicator would not 

 work. It remains to show how the apparatus at a station are arranged, 

 and we will take for example an intermediate station which can corre- 

 spond along the line with two neighbouring stations, one situated- at 

 the right, the other to the left of the first. 



Take the station at Sevres, on the telegraph line between Paris and 

 Versailles. Fig. 364 represents the manipulating and receiving 

 apparatus. The manipulator is fixed on a table, and on either side 

 are seen the galvanometers which give notice of the transmission of 

 currents over each wire of the line. Above on the same horizontal 

 table are placed the indicator, and on-each side the alarum which gives 

 notice of the sending off a message, whether to the side of Paris or to 

 that of Versailles. We shall see in a minute how these alarums act. 



