It was 250 years ago today that Christopher Irwin's marine chairs were loaded onto the Princess Louisa to head off to Barbados. Marking the anniversary of Nevil Maskelyne's departure from London, to act as observer on the trial, I I outlined the purpose of the voyage over on The H Word blog. Irwin's chair was being tested alongside Tobias Mayer's lunar tables and John Harrison's sea watch.

On 13 September 1763, the log of Lieutenant Patrick Fotheringham records how the ship "Came alongside a Hoy with two Marine Chairs and apparatus for observing the Planet Jupiter in order to finding yet Longde. at Sea the Commissioners for ye Discovery to examine these Machines under ye Direction of Adml. Tyrrell in ye course of his Voyage; Do. came on Bd Mr. Christopher Erwin the Inventor of ye Marine Machine".

After some to-ing and fro-ing the Princess Louise finally sailed for Barbados on 23 September. During the voyage Maskelyne and Charles Green took many lunar-distance observations (with Maskelyne later claiming that his final observation was within half of degree of the truth) and struggled a couple of times with the marine chair. Maskelyne's conclusion was that the Jupiter's satellites method of finding longitude would simply never work at sea because the telescope magnification required was far too high for use in a moving ship.

Our evidence for Maskelyne's experience on the voyage and at Barbados comes from the evidence  he presented at a meeting of the Board when he returned, but also some letters to his brother Edmund. These are held in the NMM's Caird Library and have been digitised.

On 8 September 1763, he writes to his "Dearest Brother" to say that "Tomorrow morning at one o'clock I set out for Portsmouth" and asking him to deal with financial matters until his return. On 29 December 1763 he wrote again, reporting his safe arrival on 7 November after "an agreeable passage of 6 weeks". He noted that he had been "very sufficiently employed in making the observations recommended to me by the Commissioners of Longitude" and that it was at times "rather too fatiguing".

Barbados suited him better than the ship: "Since my arrival here I have passed my time much for agreably, to which the great civilities I have received from the gentlemen of this place have not a little contributed. I have just got my observatory finished, a room of 18 feet long & 12 wide; situate in an open airy place a little out of the town & at a moderate elevation...". He commended the "purer & serener" air, that was much better for observation than that of England, and the green countryside, although he noted that it could get a bit too hot.

It was in this letter that Maskelyne said that Irwin's chair "proves a mere bauble, nor proves in the least useful for the purpose intended". He felt "so clear in my opinion about its inutility, in which Mr Green my associate agrees also with me, that I have no apprehensions my opinion will be censured or contradicted". Meanwhile, his lunar observations had been as frequent as on his St Helena voyage and he "found them equally certain".

There is not another letter from Barbados, so the only account of what happened when H4 and William Harrison arrived there comes from the Harrison Journal, probably compiled somewhat after events, and when relationships had soured considerably. Despite the complaint there that Maskelyne, closely associated with the lunar-distance method, should be the one determining the longitude of Barbados, the Harrisons were aware long before that Maskelyne and Green were appointed to this role.

We know that Maskelyne and Green took the relevant observations on different days of the week, and they were all witnessed by four others. Whether this was, as the Journal suggested, enforced by Harrison's suspicion of Maskelyne's partiality or if such precautions were already planned, we can't be sure.

The next letter from Maskelyne to his brother in the archive is 15 May 1766. Whatever disputes had or had not occurred in Barbados, and whatever hostilities were simmering as a result of the passing of the 1765 Longitude Act, were largely irrelevant. Maskelyne was now writing from Greenwich, as Astronomer Royal, and had "lately received Mr Harrison's watch here to try its going". The other three timekeepers were soon to arrive, he was shortly to publish the drawings of H4, someone (Kendall) had been commissioned to make a copy, and "persons" had been appointed to compute "a very complete nautical and astronomical ephemeris".

Maskelyne ended by asking if his brother might bring a little callico with him when he returned from India: "a little that would suit a lady may possibly be useful some time or other". Maskelyne's professional life at Greenwich was up and running but, if that little bit of callico had been intended as a courtship present, things didn't go quite so smoothly in his domestic arrangements - it was nearly another twenty years before he married.