What is "watchman's detector"

Watchman \Watch"man\, n.; pl. Watchmen.

1. One set to watch; a person who keeps guard; a guard; a sentinel.

2. Specifically, one who guards a building, or the streets of a city, by night.

   Watchman beetle (Zo["o]l.), the European dor.

   Watchman's clock, a watchman's detector in which the apparatus for recording the times of visiting several stations is contained within a single clock.

   Watchman's detector, or Watchman's time detector, an apparatus for recording the time when a watchman visits a station on his rounds.

   Watchman's rattle, an instrument having at the end of a handle a revolving arm, which, by the action of a strong spring upon cogs, produces, when in motion, a loud, harsh, rattling sound.