The document discusses the history and parts of the pendulum clock. It was invented in 1656 by Dutch scientist Christiaan Huygens and had an error of 15 minutes per day. The key parts include the escapement, mainspring, gear train, pendulum, and pointers. The pendulum's diameter and material are important for keeping accurate time. The escapement uses copper gears for low corrosion. A gear train with smaller to larger gears slows the movement. The mainspring provides force to power the clock for up to 400 days.