te 
and its application to Fire Alarms. 71 
The transmitting apparatus, connected with the Alarm Circuit, 
consists of acommon Signal or rather Alarm Key, and of the 
District Keyboard. The purpose of both these instruments is to 
complete at suitable times the circuit, by which the machinery 
at the Alarm Stations is thrown into action. 
To obviate the difficulty of completing the circuit by the 
Alarm Key, with the absolute regularity necessary to strike the 
District Signals upon the bells, the District Key-board is intro- 
duced. This instrument, which in its simple form was early em- 
ployed in the Telegraph, is represented in fig.6. The Keys, with 
aes 
on 2S ee Ge ee a 
a ae. 
ed 
—S 
one exception, are seen marked with the numbers of the Districts. 
Below the Key-board is a cylinder, which is moved by clock- 
Work at a given rate when the instrument is in operation. The 
cylinder is of wood having a metallic core, e. Strips of metal, 
I J, connected through with the core, are set into the wood of the 
cylinder, so as to form groups under the several Keys, equal in 
humber to the District Signals, marked upon them. Thus, 
under the Key of District One, single strips widely separated 
are seen, under the Key of District Two, two strips, and after a 
considerable interval, two more, and so on. It is obvious, with 
this arrangement, that if each key should bring a conductor to 
at Upon the surface of the cylinder beneath, it would complete 
electric communication through to the core, at regular intervals 
Corresponding to the District Signals. : 
.* ot the sake of economy of battery power and the security of 
distinct Cirenits, it is desirable to throw the force of the Battery 
“pon the three Alarm Circuits separately and in succession. This 
S effected by attaching three metallic springs a’, b’, c’, to the under 
Surface of each key. These springs when the key is depressed, 
‘ Upon the cylinder in an oblique line, that is, a’ in advance of 
Y, and 6’ in advance of c’. The other extremities of these 
Springs make an ample bend behind the Key-board, to alloy Bee 7 
“om of motion to the keys, and are then fastened respectively 10. 
- Metallic bars a, b, c, the corresponding spring of each ke: 
being attached to the ‘same NOW, on, Geprentins: 
bar. 
