The clockmaker and the longitude problem
By the beginning of the eighteenth century, a ship's captain could work out his latitude, his distance north or south of the equator, without much difficulty, by measuring the height of the sun or of certain stars above the horizon. Longitude, his position to the east or west, was a different matter. There was nothing in the sky that would give it directly, and navigators could do little more than estimate it from their speed and direction. The consequences were often fatal. In 1707 four warships returning to England under Admiral Sir Cloudesley Shovell struck rocks off the Scilly Isles in fog, their officers having misjudged how far west they were, and more than 1,400 men were drowned.
The principle on which a solution might be based was well understood. The Earth turns through 360 degrees every twenty-four hours, or fifteen degrees an hour. If a navigator knew what time it was at his home port at the moment when the sun was highest in the sky where he was, the difference between the two times would give his longitude: every four minutes corresponded to one degree. What he needed, therefore, was a clock that would go on showing the time at home throughout a voyage of many weeks. No such clock existed. The best timekeepers of the day depended on a pendulum, which was useless on a rolling deck, and all of them were upset by changes in temperature and by damp.
In 1714 the British Parliament offered a reward of up to £20,000, an immense fortune, for a method of finding longitude to within half a degree, and set up a committee, the Board of Longitude, to judge the proposals. Few scientists expected the answer to be a clock. Isaac Newton himself doubted that one could ever be made accurate enough. Astronomers put their faith instead in the moon, whose position against the background of the stars changes from hour to hour and could in theory be used as a clock in the sky, though the method required elaborate tables and hours of calculation.
The man who proved them wrong had no scientific education at all. John Harrison, born in 1693, was the son of a carpenter and taught himself to make clocks, at first almost entirely out of wood. For certain parts he chose a tropical timber that releases its own natural oil, so that his clocks never had to be lubricated, and by the 1720s they were losing no more than a second a month, a performance unmatched anywhere in the country.
Harrison arrived in London in 1730 with drawings for a clock that would work at sea. He was encouraged by Edmond Halley, the Astronomer Royal, and by the leading clockmaker George Graham, who lent him money to begin. The result, completed in 1735 and now known as H1, was a machine of brass weighing thirty-four kilograms. In place of a pendulum it had two balances linked together in such a way that any movement of the ship affected them equally and in opposite directions. Tested the following year on a voyage to Lisbon, it performed well. Harrison, however, was a perfectionist. Instead of claiming the reward, he asked only for funds to build something better. His second clock, H2, was never tried at sea, because he discovered a fault in its design. His third, H3, occupied him for nineteen years. In the course of that work he devised the bimetallic strip, in which two metals are joined so as to cancel out the effects of heat and cold, and a new kind of bearing. Both are still in everyday use.
Then, in his sixties, Harrison changed direction completely. He had come to believe that a small timekeeper beating rapidly would be more stable than a large one. H4, finished in 1759, was only thirteen centimetres across and looked like an oversized pocket watch. In 1761 his son William took it on a trial voyage to Jamaica. After eighty-one days at sea, it was found to have lost just five seconds.
The Board of Longitude, on which astronomers were strongly represented, was reluctant to accept the result and suggested that it might have been luck. A second trial, to Barbados in 1764, was equally successful. Even then the Board paid Harrison only half the money and insisted that he hand over the watch and reveal exactly how it worked. Now in his seventies, he appealed to King George III, who tested one of his watches personally and took his side. In 1773 Parliament finally voted him the remainder of the money. He died three years later.
By then his work had been tested in the hardest possible way. Captain James Cook carried a copy of H4 on his second voyage of exploration, between 1772 and 1775, and came to rely on it completely. Marine timekeepers, or chronometers, were at first too expensive for most ships, but by the early nineteenth century they had become standard equipment. Satellite navigation has since replaced them, yet it rests on the same idea: that to know exactly where you are, you must know exactly what time it is.