On a foggy October night in 1707, a British fleet returning home from the Mediterranean smashed into the rocks of the Isles of Scilly. Four warships went down, and somewhere between 1,400 and 2,000 sailors drowned, including the fleet's commander, Admiral Sir Cloudesley Shovell. The navigators had miscalculated where they were — not their latitude, which any competent officer could read from the sun and stars, but their longitude, their east–west position. For longitude, the greatest maritime empire on Earth was still essentially guessing.
The problem was, at bottom, about time. The Earth turns 360 degrees in 24 hours — 15 degrees every hour. If a navigator knows the exact time at a reference point (say, his home port) at the same moment he measures local noon from the sun, the difference tells him precisely how far east or west he is. The catch: it requires carrying the home port's time across an ocean, accurate to within seconds, for months. Pendulum clocks were hopeless on a pitching ship, and no watch of the era came close. The alternative was to read time from the sky — using the Moon's movement against the stars as a celestial clock — but in 1707 the astronomy and the mathematics weren't ready either.
In 1714, prodded by the Scilly disaster and petitions from merchants and captains, the British Parliament passed the Longitude Act, offering a prize of up to £20,000 — a colossal fortune, worth millions today — for a practical method of finding longitude at sea to within half a degree. A Board of Longitude was created to judge claims. Europe's greatest minds, including Isaac Newton, expected the answer to come from astronomy. Nobody expected it to come from a carpenter in Lincolnshire.
The carpenter of Barrow
John Harrison, born in 1693, was a joiner by trade who taught himself clockmaking and built his early clocks largely of wood. He was already a quietly radical inventor: he devised the gridiron pendulum, which used the different expansion rates of brass and steel to cancel out temperature errors, and the nearly frictionless 'grasshopper' escapement. In the 1720s his precision clocks were arguably the most accurate in the world. Then he turned to the sea prize, and it consumed the rest of his life.
His first sea clock, H1 — a gleaming, alien-looking machine of springs and counterbalanced bar balances — was completed in 1735 and performed impressively on a trial voyage to Lisbon. But Harrison, a perfectionist, kept asking the Board for funds to do better rather than claiming the prize. H2 followed, then H3, which occupied him for some nineteen years and introduced inventions still in use, including the bimetallic strip (found in thermostats to this day) and the caged roller bearing. And then Harrison did something astonishing: he abandoned the big-machine approach entirely. The answer, he realised, was a watch.
H4, finished in 1759, looks like an oversized pocket watch, about 13 centimetres across, in a silver case. On its trans-Atlantic trial to Jamaica in 1761–62, it lost only about five seconds in 81 days at sea — accuracy that translated to a longitude error well within the prize's most demanding limit.
The Board fights back
Winning the trial was not the same as winning the prize. The Board of Longitude — increasingly influenced by the astronomer Nevil Maskelyne, who became Astronomer Royal in 1765 and was himself championing the rival lunar-distance method — demanded further trials, then full disclosure of H4's workings, then the surrender of all four timekeepers, then the construction of duplicate watches. Maskelyne's lunar method genuinely worked, and his Nautical Almanac, first published for 1767, made it practical; but it required long calculations, while Harrison's watch simply told you the time. Harrison received £10,000 in 1765 after handing over his secrets, but the second half of the reward was withheld year after year.
The endgame required a king. In 1772, Harrison's son William secured an audience with George III, who tested Harrison's fifth timekeeper, H5, at his private observatory and was won over — reportedly exclaiming, 'By God, Harrison, I will see you righted!' With royal backing, Parliament granted Harrison £8,750 in 1773. He was 80 years old. He died three years later, on 24 March 1776 — his 83rd birthday. The Board of Longitude never formally awarded its great prize to anyone.
Vindication came at sea. Larcum Kendall's K1, an exact copy of H4 commissioned by the Board, sailed with Captain James Cook on his second voyage (1772–75). Cook, initially a sceptic, came back calling it 'our trusty friend the watch.' Marine chronometers descended from Harrison's work became standard equipment, and with them ships could finally draw reliable maps of the world. Three of Harrison's four sea clocks — H1, H2 and H3 — lovingly restored, still run today at the Royal Observatory in Greenwich, the very place from which the world now measures longitude zero; H4 is kept stopped to spare its delicate movement from wear. Dava Sobel's 1995 bestseller Longitude turned the stubborn carpenter into a late, global celebrity.
Quiz nuggets
- The 1707 Scilly disaster, which killed some 1,400–2,000 sailors, helped prompt Britain's Longitude Act of 1714 and its £20,000 prize.
- Longitude is a time problem: the Earth rotates 15 degrees per hour, so comparing local sun time with home-port time reveals east–west position.
- John Harrison's H4 watch lost only about 5 seconds on its 81-day 1761–62 voyage to Jamaica.
- Harrison also invented the bimetallic strip and gridiron pendulum; he received his final reward in 1773 only after King George III intervened.
- Captain Cook carried K1, a copy of H4, on his second voyage and praised it as 'our trusty friend the watch.'