And such a different direction it was.
Rather than the large, boxlike constructions of H1—H3, H4 was what we recognize instantly today as a large pocket watch, about 12 centimeters in diameter, with a jeweled mechanism into which Harrison had put “fifty years of self-denial, unremitting toil, and ceaseless concentration.” He wrote:
“I think I may make bold to say that there is neither any other Mechanism or Mathematical thing in the World that is more beautiful or curious in texture than this my watch or Time-keeper for the Longitude.”
Now age 68, Harrison wasn’t up to accompanying H4 on a sea trial to the West Indies, so he entrusted it to his son William. It was secured on board the Deptford bound for Jamaica in a case with four locks, one key to each being given to William, Governor Lyttleton of Jamaica (taking passage on the ship), Captain Dudley Digges, and his lieutenant. All four needed to be simultaneously present with their keys to unlock the case for each winding. The ship sailed on 18 November 1761.
Nine days out, the Deptford‘s longitude, by dead reckoning, was 13 50′ W, but according to Harrison’s calculations, based on the timepiece, it was 15 19’ W – a difference of nearly 160 kilometers. Harrison told a skeptical Captain Digges that if Madeira was properly placed on the chart they should sight land the next day. Despite his reservations, Digges held to Harrison’s course. At 6 a.m. the next day, the lookout reported seeing land dead ahead. It was Porto Santo, the northeastern island of the Madeira group.
Upon landing in Jamaica on 19 January 1762, H4 was found to be only 5.1 seconds slow, the equivalent of an error in longitude of 1.25’—1.25 nautical miles, or 2.3 kilometers. H4’s performance resoundingly qualified Harrison to win the £20,000 prize. (One degree of longitude varies in size depending on how close to or far from the equator one is, but at the equator it’s approximately 69 miles [the same size as the invariant size of one degree of latitude]; a minute is approximately 1.15 miles and a second 0.02 miles, or about 100 feet.)
Here, alas, the story descends into the petty disputatiousness the Board of Longitude adopted, requiring additional condition upon additional condition (another sea trial, full disclosure of H4’s workings to a special committee, and Harrison’s making four copies of H4, all to be handed over to the Board), and even then they withheld the full prize. The final insult came when they demanded Harrison make two successor time-keepers to H4 (which became known as H5 and K1—the “K” referring to Larcum Kendall, a leading watchmaker whom Harrison recruited for assistance), but meanwhile the Board took the original away to the Royal Observatory “for testing.”
