Harrison · 1761
H4 and the Longitude Prize
On the eighteenth of November 1761, in the cabin of HMS *Deptford* lying at anchor at Spithead, John Harrison's son William, thirty-three years old, climbed aboard with the silver pocket-watch H4, the fourth and final marine chronometer designed by John Harrison (1693–1776), the Yorkshire carpenter who had spent the previous forty-eight years on the problem of finding longitude at sea. The Board of Longitude, by the Longitude Act of 1714, had offered a reward of £20,000 to anyone who could devise a method for determining a ship's longitude to within half a degree (about thirty miles at the equator) on a voyage from Britain to the West Indies. H4 was, by the test of the *Deptford*'s eighty-one-day voyage to Port Royal in Jamaica, accurate to within five seconds over the voyage (the equivalent of about one and a half miles of longitude). The Board took thirteen further years and three more trials to formally award the prize; Harrison received £8,750 in 1773 and the remainder by direct Parliamentary grant of 1773 after the personal intervention of George III, on the principle that the Board had withheld the prize unjustly. Harrison was eighty by the time he received the payment. The marine chronometer is the foundational navigational instrument of the modern age.
It is twenty past ten on the morning of the eighteenth of November 1761, in the cabin of HMS Deptford, a fifty-gun fourth-rate ship of the line at anchor at Spithead in heavy autumn cold off Portsmouth. He is thirty-three years old. He is William Harrison, son of John Harrison the Yorkshire-Lincolnshire carpenter, in his eighth year of formal apprentice-and-collaborator's work with his father on the marine-chronometer project (the father is sixty-eight and at home at Red Lion Square in central London). William has, in a mahogany box on his knees, the H4: the fourth marine chronometer of the series, made by John Harrison in the workshops at Red Lion Square between 1755 and 1759, in the form of a silver pocket-watch about five inches in diameter, with the pair of bi-metallic temperature-compensation arms, the remontoire (the constant-force mechanism), and the diamond-and-ruby caged-bearings of John Harrison's late-period horological invention.
He has, in his pocket, the sealed instructions of the Board of Longitude for the test voyage to Port Royal in Jamaica: the H4 to be wound by William each morning, to be observed twice daily for variation against the astronomical noon-sight of the ship's navigator, and at landfall to be checked against the astronomical longitude of Port Royal harbour as established by the Board's observers in Jamaica.
He thinks: the Board has, since 1714, been certain that the longitude problem will be solved by astronomical observation of the Moon and the lunar-distance method. The Board has not been willing to accept the alternative method by clock until the clock has, on a test voyage, proved itself.
He thinks: the H4 is the test voyage's vehicle. The H4 has, in the workshop trials of the previous two years, kept time to less than a tenth of a second a day on a stationary mounting. The question of the voyage is whether the H4 can do the same on a ship.
The Deptford sailed from Spithead on the voyage on the twenty-fifth of November 1761 (delayed a fortnight by Atlantic weather) and arrived at Port Royal in Jamaica on the nineteenth of January 1762, eighty-one days later. The H4, by William's daily winding and the logbook of the ship's master, lost five seconds over the voyage. The computation of the longitude of Port Royal from the H4, by the method (compare the H4's time at noon at Port Royal to the H4's time at noon at Greenwich on the day of sailing, multiply the difference in hours by fifteen degrees per hour), produced a result one and a quarter miles east of the Board observers' astronomical longitude of Port Royal. The tolerance for the £20,000 award was thirty miles. The H4 had exceeded the requirement by a factor of twenty-four.
The Board of Longitude, when William Harrison returned to England in March 1762 with the H4 and the log, accepted the result but withheld the full prize money on the technical objection that the voyage had been a single trial and could have been a fluke. The Board required a second trial, conducted with the Tartar to Barbados in 1764, in which the H4 maintained an even better accuracy (forty seconds over a hundred-and-fifty-six-day voyage, the equivalent of about ten miles of longitude). The Board still withheld the full prize on the further technical objection that John Harrison had to produce a second working copy of the H4 (the H5) under inspection of the Board's observers before the payment could be made.
John Harrison received £8,750 of the prize in 1765 on the second-trial result; he received the remaining £11,250 by direct Parliamentary grant of 1773 after the personal intervention of King George III, who had taken Harrison's case up after a private interview with William Harrison in 1772 in which the king had said, by the memoir of William Harrison published in 1796, these people have been used very ill, by God, Harrison, I will see you righted. The Parliamentary grant of 1773 was, by the wording, to John Harrison, in recognition of his services in the discovery of the longitude. Harrison was eighty by the time of the payment.
John Harrison died at his house at Red Lion Square on the twenty-fourth of March 1776, his eighty-third birthday. He is buried at the Hampstead parish church of St John-at-Hampstead. The H1, H2, H3 and H4 chronometers are in the care of the Royal Observatory at Greenwich, where they are all kept running by the horological staff (the H4 is, since its Bicentennial restoration in 1962, not kept running because of the degradation risk to the original components). The marine chronometer became the standard navigational instrument of the Royal Navy from 1780 onward; by 1815 every British capital ship carried at least one chronometer. By 1875, the date of the first British longitude-by-chronometer-only voyages of the P&O service to Australia, the Harrison instrument had reorganised the geography of the world.