Voyage to Barbados on HMS Tartar
In 1764, William Harrison took his father's fourth time-keeper, known as H4  , on a trial to Barbados on board HMS Tartar . This was ordered by the Board of Longitude as a definitive test of the watch's ability to keep time accurately at sea, after a disputed first trial to Jamaica in 1761-2. By going to the West Indies, both trials aimed to fulfil the terms of the 1714 Act [RGO 14/1:10r] which had founded the Board and the reward money that the Harrisons hoped to win. The Act laid down that the full reward would be paid for a solution, '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.'
The trial to Barbados was used as an opportunity to test a number of proposals for finding longitude. Nevil Maskelyne and Charles Green set off earlier in 1764 on HMS Princess Louisa with Christopher Irwin's marine chair, in order to test its use in observing eclipses of Jupiter's satellites. On board, they also made observations for the lunar distance method proposed by Tobias Meyer. On arriving at Barbados, they set up an observatory in order to establish the longitude of the island by observations of the sun's equal altitudes. This would allow the longitude found by H4 to be compared. All of these arrangements were drawn up and agreed in previous meetings of the Board of Longitude [RGO 14/5:49]. Yet, Maskelyne's support of the lunar distance method was distrusted by the Harrisons, such that on William's arrival in Barbados he threw doubt on Maskelyne's fitness to take the comparative observations, as recorded in the 'Harrison Journal.'
Nonetheless, the trial proved successfully that H4 could find the longitude at sea within the narrowest limit of the 1714 Act. On the voyage to Barbados, William Harrison was able accurately to locate the ship in relation to the island of Porto Santo, which was recorded in a certificate by the Captain Sir John Lindsay. This event is also recorded in the logbooks of the voyage kept by Lieutenants Murray [ADM/L/T/22:29] and Barton [ADM/L/T/23:19], which are now in the National Maritime Museum [link] . On Harrison's return, Captain John Campbell, Doctor John Bevis, and Mr George Witchell were appointed by the Board to reduce the observations [RGO 14/5:64]. Their calculations showed that the watch had performed exceptionally.
It remained, however, to prove that the watch could be replicated, and made by makers other than Harrison. This was what the Commissioners sought to establish with the new parliamentary act in 1765 [RGO 14/1:29r], as discussed by Viscount Barrington in his archive of the proceedings [BGN].
History and Philosophy of Science
University of Cambridge