warning-piece

from The Century Dictionary.

• noun Something that warns.
• noun A warning-gun; a signal-gun; the discharge of a cannon intended as a notification. Compare piece, 4 .
• noun In horology, a part of the striking-mechanism of a clock that, by the movement of the lower wheel, throws the striking-system periodically into action. It is also operated by the strike-or-silent mechanism, so that the striking-mechanism may be thrown out of gear at will. When in position to work, it causes a slight noise at the instant of starting the striking-parts, and thus gives warn ing that the clock is about to strike.