Harrison's H4 Tested Again William Harrison departed for Barbados aboard the Tartar on 28 March 1764, thus beginning the second trial at sea of the H4 timekeeper his father, John Harrison, had designed to solve the problem of determining longitude at sea. When the average was calculated, H4 was in error by just 39.2 seconds after 47 days. This level of accuracy was three times better than that necessary to win the Longitude Prize. Harrison had done it! However, the Board of Longitude remained reluctant to reward Harrison for his outstanding achievement. He would not receive the prize or receive proper recognition for his contribution to navigation until 1773. In her book, Longitude, science writer Dava Sobel provides a full history of Harrison's dramatic quest to solve the problem of longitude. You can also read an account Harrison’s trials and tribulations on Clocktime. Reference Sobel, D. 2011. Longitude: The true story of a lone genius who solved the greatest scientific problem of his time. London: Harper Perennial. Image Credit Harrison's Marine Timekeeper H4, 1759. Object: ZAA0037. © National Maritime Museum, Greenwich, London, Ministry of Defence Art Collection, CC BY-NC-ND 3.0, https://www.rmg.co.uk/collections/objects/rmgc-object-79142.