John Harrison

Harrison, a carpenter who taught himself clockmaking, built a series of marine timekeepers to find longitude at sea: H1 between 1730 and 1735, then H2, H3 and, in 1759, the watch-sized H4. In the 1764 Barbados trial H4 kept time within the most stringent limits of the 1714 Longitude Act, and Royal Museums Greenwich says his timekeepers are arguably the most important ever made.

In the mid-1720s he designed precision longcase clocks that Royal Museums Greenwich says kept time to within a second a month, and his H3 introduced the bimetallic strip and the caged roller bearing, which the museum says are still in use in thermostats and in mechanical engineering. The Science Museum Group's timeline records that the Royal Society awarded him its Copley gold medal in 1749 for his experimental work.

After the 1764 trial the Board of Longitude paid only half of the top reward and, according to Cambridge University Library, on condition that he disclose how H4 was made and that copies be built; Harrison appealed to King George III, and most of the remainder was paid in 1773. The first copy of H4, made by Larcum Kendall, sailed with Captain Cook on his second voyage and, the Board's archive says, proved exceptionally accurate, with praise from Cook and his astronomer William Wales.

Harrison finished H2 within the two years he had promised after the Lisbon trial, but he discovered a fundamental flaw in it and never allowed it to be tested at sea. He then began H3 in 1740 and worked on it for nearly 19 years, making many changes and adjustments, because he could not persuade its two large balances to keep time well enough.

This is consistent with an inventor who held his own work to a high standard, though both episodes come from one museum and the difficulty of the problem may explain the length of the work. Perfectionism

In 1767 Harrison published 'The Case of Mr. John Harrison' against the Board and a pamphlet attacking Nevil Maskelyne, whom he suspected of jeopardising his reward to promote the lunar method; a Cambridge scholar calls the pamphlet vitriolic. Maskelyne had published the negative results of his Greenwich test of H4 that year, and his own letter to Lord Sandwich shows that the attacks affected him and that he dropped a reply, thinking such abuse needed no refutation.

This is consistent with a man willing to contest the commissioners and a rival in public over a long period, though the sources show both a grievance Harrison plainly felt and a Board and Astronomer Royal with reasons of their own. Conflict Tolerance

By the early 1750s Harrison had spent over a decade on H3 without getting its large balances to keep time well enough. A pocket watch made to his design by John Jefferys, which the museum dates to about 1751-53, went so well that he realised the solution lay in smaller watches; he began H4 in 1755 and completed it in 1759, and it went on trial to Jamaica in 1761.

This is consistent with an inventor who changed the form of his solution after years invested in another, though the account comes from one museum and the Harrisons later held firmly to their own reading of the Board's terms. Belief Updating