The Pendulum Has Stopped 1.
Mantel clocks have either a swinging pendulum or a balance wheel to regulate time keeping.
Find out by taking a peek through the back door of your mantel. If you don't see a pendulum attached to the back of your clock movement, then your clock is regulated by a balance wheel. First, be sure that you have wound up the pendulum spring. Wind it up all the way. Next, reach into the lock and gently swing the pendulum to one side with your hand.