John Harrison with his marine chronometer, c 1767.
© Science Museum / Science & Society Picture Library
Detail of an oil painting by Thomas King of John Harrison (1693-1776) with the marine chronometer which he invented. Harrison 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.