Every 12 hours, the stepper motor moves a lever and slowly stops the pendulum over the course of a dozen or so seconds, silencing the clock movement. Twelve hours later, the motor turns again ...

This ensures the pendulum doesn’t change length with temperature, but it still only brings the clock into the 0.1 second/day range. Clearly that’s not good enough for a clock at CERN ...

A watch differs from a clock In its having a vibrating wheel instead of a vibrating pendulum ; and ... during a few seconds, of the hand which winds up, gives motion for twenty-four or thirty ...