Harrison’s H4

H4 is a watch-sized marine timekeeper, about 13 centimetres across in silver pair cases; the museum’s record calls it Harrison’s prize-winning longitude watch.^1, 2

It began with a pocket watch made to Harrison’s design by John Jefferys in 1753. The museum’s catalogue dates the start of work on H4 to 1755; another of its accounts says around 1751–52.^1, 2

Its dial plate is inscribed John Harrison and Son A.D 1759. It beats five times a second, and its escapement is a modified verge with diamond pallets.^1, 2, 5

William Harrison carried H4 on HMS Deptford from November 1761. At Kingston, Jamaica it was about five seconds slow, roughly one nautical mile of longitude, but the Commissioners judged the trial insufficient.^{2, 5, 6, 7}

On the trial to Barbados in 1764 its error was well inside the Act’s limit: 39 seconds by Wikipedia’s account, 34.4 seconds over seven weeks by the Captain Cook Society’s.^5, 7

In February 1765 the Board found it had performed within the strictest limit of the 1714 Act.^2, 5

In August 1765 Harrison explained H4 to six experts chosen by the Board, among them the watchmaker Larcum Kendall.^3, 8

On 5 May 1766 it was handed, sealed in a box, to the Astronomer Royal Nevil Maskelyne and tested at the Royal Observatory for ten months. It did not perform well, and how Maskelyne treated its rate was disputed.^2, 5

The Board published a description of its principles in 1767, and Kendall’s copy, K1, finished in 1769, let others test it at sea, above all on Cook’s second voyage.^3, 9

Rupert Gould cleaned H4 a second time in 1935, as part of his restoration of the Harrison timekeepers.¹¹

Wikipedia’s account says that, unlike H1 to H3, it needs oil to run, so it is kept stopped.⁵

Where it can be seen changes: in November 2026 the museum’s record listed it as not on display, while the Royal Observatory, where Harrison’s timekeepers are usually shown, was closed for refurbishment.^1, 12