A ship at sea can read its latitude off the sun. Longitude is a question about time: how far local noon has drifted from noon at a port whose position is already known.
The pendulum clocks of the seventeenth century kept excellent time on land and none at all on a deck that pitched, rolled, and changed temperature twice a day.
John Harrison spent thirty-one years on it. The first three machines were large, ingenious, and impractical. The fourth was the size of a pocket watch.
H4 lost five seconds on the passage to Jamaica in 1761 — about a minute and a quarter of longitude, comfortably inside what the prize demanded.
The Board of Longitude paid him in instalments, over fourteen years, and never in full.
Within a generation the chronometer was ordinary. Ships carried three, because two that disagree tell you nothing at all.