This clock was used as The Sidereal Standard at the Royal Observatory, Greenwich for over 50 years until it was superseded in 1922.
This clock has a highly sophisticated detached detent escapement designed by the seventh Astronomer Royal, Sir George Airy. One of the first of its type, the pendulum compensates for temperature and barometric pressure changes.
The escapement is a form of George Biddell Airy’s detent escapement. The crutch is fitted with a fine regulation device (also designed by The Astronomer Royal) in the form of a brass block, suspended on a threaded rod that is moved up or down by turning a hand screw. This device enabled the clock’s timekeeping to be fine tuned without stopping the pendulum or adjusting the length of the suspension.
In fact this regulator was so reliable in its rate, it was noted that a rise in barometric pressure equal to one inch of mercury led to a daily loss of 0.3 seconds. In 1873 Airy wrote to Dent proposing the barometric compensation that can be seen on the clock today. Inside the weight chamber is a mercury barometer tube and the float connects to a pivoted ‘see-saw’ like arm carrying a permanent magnet. Barometric changes will change the magnets position relative to the permanent magnet on the pendulum and by doing so will increase or lessen its influence on the clock’s rate.
Due to the action of the escapement, the clock only beats every two seconds.