A Congreve clock is a type of clock

Clock

A clock is an instrument used to indicate, keep, and co-ordinate time. The word clock is derived ultimately from the Celtic words clagan and clocca meaning "bell". A silent instrument missing such a mechanism has traditionally been known as a timepiece...

 that uses a ball rolling along a zig-zag

Zigzag

A zigzag is a pattern made up of small corners at variable angles, though constant within the zigzag, tracing a path between two parallel lines; it can be described as both jagged and fairly regular....

 track rather than a pendulum

Pendulum

A pendulum is a weight suspended from a pivot so that it can swing freely. When a pendulum is displaced from its resting equilibrium position, it is subject to a restoring force due to gravity that will accelerate it back toward the equilibrium position...

 to regulate the time. It was invented by Sir William Congreve

William Congreve (inventor)

Sir William Congreve, 2nd Baronet was an English inventor and rocket artillery pioneer distinguished for his development and deployment of Congreve rockets.-Biography:...

 in 1808. The ball takes between 15 seconds and one minute to run down the zig-zag track, where it trips the escapement

Escapement

In mechanical watches and clocks, an escapement is a device that transfers energy to the timekeeping element and enables counting the number of oscillations of the timekeeping element...

 which in turn reverses the tilt of the tray and at the same time causes the hands of the clock to move forward. Thus the angle of the plate reverses and the clock hands move forward between one and four times every minute. On versions of the clock with a dial to indicate seconds, the second hand jumps forward either 15 or 30 seconds on each oscillation depending on the length of the track.

Congreve appears to have been unaware that rolling ball clocks had been invented earlier by both Nicolas Grollier de Servière

Nicolas Grollier de Servière

Nicolas Grollier de Servière was a French inventor and ornamental turner who became well known for creating a series of fantastic machines....

 and Johann Sayller. Congreve's version differed from Grollier's rolling ball clock in that the ball travelled down a zig-zag rather than straight path. While Sayller's version had also used a zig-zag path, it had utilized a number of balls and a fixed table rather than the single ball and tilting table employed by Congreve. The "Extreme Detached Escapement" (as Congreve referred to his escapement design) was patented in 1808. Congreve, who was not a clock maker, hired Gravel and Tolkien to produce the first working version, which he then presented to the Prince of Wales

George IV of the United Kingdom

George IV was the King of the United Kingdom of Great Britain and Ireland and also of Hanover from the death of his father, George III, on 29 January 1820 until his own death ten years later...

 in 1808. This version was weight-driven, but the second design, which appears to have been constructed by John Moxon, was spring-driven. The second model is in the collection of Buckingham Palace

Buckingham Palace

Buckingham Palace, in London, is the principal residence and office of the British monarch. Located in the City of Westminster, the palace is a setting for state occasions and royal hospitality...

.

Congreve clocks are unreliable timekeepers — the time taken for the ball to travel along the track varies greatly depending on the cleanliness of the track and ball, and since the plate is aligned horizontally it is easy for dust to accumulate.