Voyage to Jamaica on HMS Deptford
Jamaica was the location chosen for the trial of John Harrison's watch, 'H4  ,' by his son William on board HMS Deptford in 1761-2. The 1714 Act [RGO 14/1:10r], which established the Board of Longitude and the reward money which they administrated, laid down that the reward could only be won, 'when a Ship by the Appointment of the said Commissioners … shall thereby actually sail over the Ocean, from Great Britain to any such Port in the West Indies.' This was the first attempt at such a trial.
Given the terms of the Act, it was possible that the outcome of the Jamaica trial might lead Harrison's watch to find longitude within the limits required for the reward, so the trial was planned carefully. The Royal Society were asked to recommend how the trial should take place and what instruments should be used [RGO 14/5:30]. The sources available for the early history of the Board increase greatly in 1761. The planning process is documented in the meetings of the Board of Longitude [RGO 14/5:26], which increased in frequency from this point, and were properly documented by a newly-appointed secretary [RGO 14/5:42]. It is also at this point that the 'Journal' record of the Harrisons' experiences takes up the tale. Many of these materials were used by both the Harrisons and the Board later in the decade, as their relationship became increasingly acrimonious.
The watch showed its usefulness early on the voyage, when William Harrison was able to predict the ship's arrival at Madeira more accurately than the ship's dead reckoning. A certificate from the Captain, Dudley Digges, confirms this in the 'Journal.' However, on arrival at Jamaica, Harrison and John Robison (who was sent to help with the observations to compare with the watch) had trouble establishing an observatory and taking observations of the sun's equal altitudes before re-embarking for England. On the return voyage, HMS Merlin met with extremely bad weather, which caused Harrison trouble in keeping the watch dry, and caused him to become ill.
On William Harrison's return, the Board agreed that the watch showed great promise and voted the Harrisons £2,500 [RGO 14/5:37]. But, problems were found with the manner and small number of William's observations at Jamaica, meaning that the longitude of the island could not be properly established. A second trial was required, which would eventually take place on HMS Tartar on route to Barbados in 1764. The Harrisons' ongoing disagreement with the Commissioners over the outcome of these trials, would lead to further parliamentary acts [RGO 14/1:29r] and publications.
History and Philosophy of Science
University of Cambridge