Smithsonian Institute - On Time - History of the clock and how Americans have measured time and been affected by it. How Stuff Works: Atomic Clock - Concise history, background, definitions and operational information regarding atomic clocks. Doug's Atomic Clocks - Details on Doug Hogarth's experiments with his atomic clock. How Stuff Works: Pendulum Clock - Concise description using drawings and animation to show the operation of a pendulum clock. Vintage Watch Resources - Assistance for collectors, owners and enthusiasts of vintage wrist watches, identification and authentication of watches, catalog. Mechanical Watchmaking - Reviews, concise history, interesting facts and news about mechanical watches.