John Harrison, English inventor and horologist, 18th century.
© Science Museum / Science & Society Picture Library
Mezzotint engraved by J R Smith from a painting by J Wright. Harrison (1693-1776) became famous for resolving one of the most problematic isues of his day - how to determine longitude while at sea. This was to take him over forty years, during which time he devised many other ways to improve the accuracy of time-keeping.