H4 chronometer graphic

Classic projects: H4 chronometer

Date: Complete 1759Designer: John HarrisonUnit Cost: £400 (£48,100 today)

As best-selling author of the modern non-fiction classic ‘Longitude’, Dava Sobel, once said: “To know one’s longitude at sea, one needs to know the time it is aboard ship and also the time at the home port… at the very same moment.” For every hour of rotation of the Earth a fixed point will move longitudinally 15 degrees. The problem for 18th Century seafarers in the Age of Sail was that it was impossible to know the precise time simultaneously in two places, due to the somewhat obvious reason that contemporary technology pendulum clocks could not be relied on at sea. The problem of calculating longitude was so pressing that Charles II “resolved to build a small observatory within Our Park at Greenwich”, while by 1714 the British government was offering £20,000 to the person that could provide a solution to providing a longitude reading ‘Practicable and Useful at Sea’ to within half of a degree. It was widely supposed in the media that, as with the challenge of finding perpetual motion, the task was impossible.

But it was taken up by many horologists, with engineer John Harrison of Yorkshire emerging as the protagonist in the design of a timepiece that would seize the day. After several abortive and long-winded attempts with large scale ‘sea clocks’ (H1-3) the problem was eventually solved with a marine chronometer - the H4 - that, although more than five inches in diameter, bore more resemblance to a modern watch than the models it derived from.

After decades of development and promising sea trials, Harrison reached the conclusion that the earlier models were flawed in that their bar balances did not oscillate rapidly enough to work on a moving ship. And so the designer shifted his emphasis to a balance that was smaller and could oscillate quickly enough to cope with a ship’s motion.

Despite Harrison’s manifest successes in later trial voyages supervised by his son William, the then Astronomer Royal, Nevil Maskelyne was convinced that the best solution for computing longitude was by lunar and star tables. Maskelyne sat on the Board of Longitude responsible for awarding the prize. Though the H4 proved its accuracy to well within the competition specifications on two separate voyages from Britain to the Caribbean, Maskelyne refused to accept the mechanical device. The Astronomer Royal, being better educated than Harrison, was able to convince the board of his opinion that the timepiece’s inaccuracies cancelled each other out to create the appearance of being on spec.

But the prize was tempting for Harrison (the equivalent of nearly £3m in today’s money - a sure fire indicator of how seriously the government took the problem), and the reluctance of the board to disburse it caused widespread acrimony, that eventually led to the issue being taken to the highest authority in the land: King George III.

The monarch tested the H4 and concluded that the watchmaker had been hard done by. Harrison was duly awarded £8,750 to go with payments that the board had grudgingly paid out in partial rewards. Despite Harrison ending up with more than the original prize, the board said the money was a bounty extracted from the government.

Harrison’s efforts led to the rules being tightened up (including a year’s post-voyage observation at Greenwich) and by the time the Board of Longitude had disbanded in 1828, the problem had been solved and Harrison had been, according to the Royal Naval Museum Library at least, ‘the main winner’.

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