Cobra Effect · Measurement
The watch that found longitude
How a clockmaker solved longitude with a watch, and fought for years to be paid.
7 cards, read aloud in 2:46, with a test and sources.
At sea in the early 1700s, sailors could find their latitude, but not their longitude.
Latitude could be worked out from the height of the Sun or the stars. Longitude needed the time back at a home port, and no clock kept good time on a rolling ship. Ships that misjudged their position could be wrecked.
In 1714, the British Parliament offered a reward for finding longitude at sea.
The top prize, twenty thousand pounds, was for finding it to within half a degree. Many experts thought the answer would come from the stars, not from clocks.
John Harrison, a carpenter turned clockmaker, set out to build a clock for the sea.
His first sea clock was finished in 1735, and tried on a voyage to Lisbon the next year. On the way home, it showed that the ship’s own reckoning of its position was wrong. Over the next decades he built two more sea clocks, each an attempt to do better.
His breakthrough was not a bigger clock, but a large watch.
It was finished around 1759. On a trial voyage to Jamaica, begun in 1761, it arrived only a few seconds out after weeks at sea. A second trial, to Barbados in 1764, tested it again.
But the Board of Longitude did not simply hand over the prize.
In 1765, it paid Harrison ten thousand pounds, on condition that he explained how the watch worked. The rest would depend on other makers being able to build copies. The astronomer Nevil Maskelyne, who favoured a method based on the stars, then tested the watch at Greenwich, and it ran poorly. A long dispute followed.
In the end, Parliament paid him, not the Board.
Harrison’s son William appealed to King George the Third, who tested a later watch himself and backed their case. In 1773, Parliament awarded Harrison eight thousand seven hundred and fifty pounds. Altogether he received roughly the prize money, but the prize itself was never officially awarded. He died three years later.
So when a problem seems to need one kind of answer, ask whether another kind could work.
Many experts looked to the stars for longitude, and a clockmaker found it in a watch. Proving that the watch could be trusted, and copied, took years more. In the end, sailors used both methods for a long time.
Sources
- Harrison’s clocks and the longitude problem, Royal Museums Greenwich. The story of the timekeepers, from the museum where they are displayed.
- John Harrison, Linda Hall Library. A short profile, including how he was paid.
- John Harrison, Wikipedia. A detailed biography with the dates of each timekeeper.
Nearby ideas
- The metre and the lumpy Earth. How two astronomers measured the metre, and why it came out slightly short.
- Light takes time to arrive. How a moon of Jupiter and a spinning wheel showed that light takes time to arrive.
- The kilogram that drifted. How the kilogram stopped being a lump of metal, and became a constant of nature.
- Weighing the Earth in a shed. How Henry Cavendish measured the density of the whole Earth from inside a shed.
- A metre made of light. How the metre went from a metal bar to the speed of light.