John Harrison (1693-1776) was the English clockmaker who designed and built the first successful chronometers, clocks accurate enough to take to sea to measure longitude. He began his work in 1730 with the first sea trial in 1736. After 5 versions of his chronometer, he satisfied Britain's Board of Longitude government, earning some 23,000 pounds which he had recieved in increments as partial payments for various versions. The Board never did award him their full 20,000 pound prize.
This image is a version of P.L. Tassaert's half-tone print of Thomas King's original 1767 portrait of John Harrison, located at the Science and Society Picture Library, London.