English: One of the first pendulum clocks designed by Christiaan Huygens, and his treatise on the pendulum, Horologium Oscillatorium published in 1673, on display in Museum Boerhaave in Leiden, Netherlands. This may be Huygens's second pendulum clock, which was also finished in 1673 and is shown on the open page of the book. In it, Huygens demonstrated how to make the swing of the pendulum isochronous using "cycloidal" cheeks of metal (shown on page) to confine the suspension cord and give the pendulum a cycloidal trajectory. Huygens claimed an accuracy of 10 seconds per day.