Harrison's fourth marine chronometer, 1759.
© Science Museum / Science & Society Picture Library
John Harrison (1693-1776), English inventor and horologist, discovered the means by which longitude could be determined accurately. He developed a marine chronometer which, in a voyage to Jamaica (1761- 1762) determined the longitude within 18 geographical miles. He also invented the gridiron pendulum (1726), the going fusee and the remontoir escapement.