John Harrison. Historical portrait of the British inventor John Harrison (1693-1776) with one of his watches. In 1713 the British Government put up a prize of 20,000 pounds for the first person to accurately calculate longitude aboard a ship, and so allow precise navigation. Harrison decided to build a chronometer that could work on a moving ship and allow measurement of time differences, and hence longitude, around the world. His fifth and last chronometer, no bigger than a large watch, was more accurate at sea than any other clock was on land. However, the Government only paid the prize in full after the intervention of King George III. Engraving published in 1768.