152 NINTH ANNUAL REPORT OF 



Station. How shall the' alarm be given from that centre to the public? 

 From the central station proceed also several circuits of wires, called 

 alarm circuits, which go to the various fire-bells throughout the city, 

 and which are connected with striking machines similar in character 

 to the striking machinery of a clock, but liberated by idcgra'ph. The 

 operator at the central station is enabled, by the mere touch of his fin- 

 ger upon a key, to throw all the striking machines into simultaneous 

 action, and thus give instantaneous public alarm. 



By what precise mechanism is this effected at the alarm-bell sta- 

 tions ? The heavy hammers may be raised above the bells by any 

 force which can be conveniently applied, as by a weight which may be 

 wound up by hand. But in all cities where the water is confined under 

 pressure in the mains, it will supply, by means of the eccentric water 

 engine, known familiarly under the name of the " water meter," the 

 power necessary to wield the heavy hammers with the greatest facility. 

 But liow are hammers of one or two hundred pounds weight to be 

 tripped by telegraph? To effect this readil}^ Mr. Farmer invented his 

 electro-magnetic escapement, one of the most beautiful and original of 

 recent mechanical applications. In this escapement the electro-mag- 

 net, when it becomes charged by the galvanic influence received from 

 the central station, attracts the little piece of soft iron or armature in 

 front of it, which supports a small lever poised nearly verticall}^ and 

 weighted with a little ball at its upper end. This lever and ball, when 

 tripped by the withdrawal of the armature, acquires sufficient mo- 

 mentum to strike up the detent of the train of wheels whicli, in their 

 revolution, raise the hammer and then allow it to fall. A single blow 

 of the hammer follows each electrical impulse sent from the central 

 station, and the revolution of the train of wheels raises also the falling 

 lever into its place and catches it again on tlie armature lever, ready 

 to be disengaged or tripped for another blow. 



At the central station, connected with the alarm circuit, is a galvanic 

 battery and an instrument for completing the circuit of that battery, 

 called the district keyboard. This is constructed with several keys, 

 corresponding to the numbers of the fire districts in the city. If you 

 depress any of these the machinery inside commences to move, and the 

 circuit is completed at such intervals as to strike and repeat on the 

 distant alarm .bells the district number represented by that key with 

 suitable pauses between. 



We supposed that the operator at the central station received the 

 signal of fire from district 3, station 5. He now places his finger on 

 the key of district 3, in the keyboard. Instantly all the alarm bells in 

 the city begin to strike s3aichronously the district number three^ and 

 continue, no matter what their number or what the weight of their 

 hammers, so long as that single finger rests on that key. 



But the operator has a finger key before him connected with the 

 signal circuits, by which he can answerback and strike a little electro- 

 magnet, armature, and bell, enclosed in each signal box. He has re- 

 ceived a signal of fire from district 3, station 5. While his hand rests 

 on the key of district 3 he taps occasionally j^yfi times on the return key 

 of the signal circuits, which I have just described. The little bell in 

 each signal box, at the corner of every square, strikes jive. The fire- 



