I saw a clock with a strange-looking pendulum recently, during a tour of historic Denver mansions. The pendulum had two columns, both partially filled with mercury. Ordinary clock pendulums will slow down or speed up a clock as they expand and contract, since the pendulum's period changes with length. But mercury-filled pendulums reduce this inaccuracy. As a mercury-filled pendulum expands or contracts, the mercury rises or falls, keeping the pendulum's center of mass more-or-less unchanged. What a clever idea!

According to Wikipedia, the mercury pendulum was invented by George Graham in 1721.

Clock with Mercury Pendulum	Mercury Pendulum Detail