72 On the Municipal Electric Telegraph, 
the strips of the revolving cylinder come in contact with the 
three springs in succession, and if the Alarm Circuits are each 
connected at one extremity with the three screw cups a, }, ¢, and 
at the other, through the Alarm Battery with the screw cupd 
and core e, the current is thrown on to each of these circuits if 
quick succession, and at intervals corresponding to the number 
of the District marked upon the key. 
The figure is represented with seven District Keys, and at 
eighth, intended for the Signal “ All Out,” the use of which will 
be described hereafter. The number of the keys may be it 
creased at pleasure. A key for fast striking, that is once in about 
two seconds, will be introduced in the Boston System as a meals 
of general alarm, before striking the District Signals. Three 
spare keys will also be assigned for the direction of engines, be 
longing to different sections of the City, or for other purpo 
ured intervals over the wires. 
It may often be desirable to confine an alarm to only one % 
two of the Circuits. This is easily effected by a Switch place 
upon each Circuit, so as to shut it off at will from connectio 
with the cylinder. 
The Battery connected with the District Key-board and 
Alarm Circuits consists of about twenty-five pairs of Grove’s & 
rangement of large size. Very considerable power is required © 
produce the necessary electro-magnetic effects at the Alarm Ste 
tions, even through circuits not exceeding four and a quart 
miles in length. A great economy of battery power is therefore 
