The purpose of the knob on the top of the clock was to record the nightwatchman's wakefulness. He had to be sufficiently alert to depress the knob on the top of the clock as it struck and thus drive the pin which at that moment was immediately below the plunger. A clock of this pattern is illustrated in A. and R. Shenton, Collectable Clocks 1840-1940, 3rd. ed., Woodbridge, 1994, p. 224, col. pl. 25