Diagram of a pendulum clock, 1673.

Illustrated plate from 'Horologium Oscillatorium' (1673) by Christiaan Huygens, showing the mechanism and pendulum of a pendulum clock. Huygens (1629-1693), a Dutch physicist, designed the first clock controlled by the motion of a pendulum. He based his clock on the observations made by Galileo Galilei (1564-1642) in 1583, when he first noted the constancy of a pendulum's period by comparing the movement of a swinging lamp in Pisa Cathedral with his pulse rate.