A precision pendulum clock is a highly accurate type of clock, which was used as a time standard for public and scientific timekeeping.
Since the oscillations of the pendulum are affected by temperature and air pressure, the clocks are equipped with a compensating pendulum (invented by John Harrison (1693-1776)). Sometimes the pendulum also swings in a tank, minimizing the possible interference caused by air pressure.
In very good precision clocks the time deviation does not exceed more than 1 second per month.