We herewith illustrate an exceedingly simple form of detecter, to show
if the night watchmen perform their visits regularly and punctually.
In the case, C, is a clockwork apparatus driving the axle, S, at the
end of which is a worm which gears into the wheel of the drum, D. The
rotation of D, thus obtained unrolls a strip of paper from the other
drum, D. This paper passes over the poles of as many electro-magnets
as there are points to be visited, and underneath the armatures of
these electro-magnets. Each armature has a sharp point fixed on its
under side, and when a current passing through the coils causes the
attraction of the armature, this point perforates the paper. The
places to be visited are connected electrically with the binding
screws shown, and the watchman has merely to press a button to make
the electric circuit complete. It has been found in practice that
plain paper answers every purpose, as the clock giving an almost
uniform motion enables the reader, after having seen the perforated
slips once or twice, to determine fairly well the time which elapses
between each pressure of the button.--_The Engineer._
[Illustration: WATCHMAN'S DETECTER]
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