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One clock face drawn three times, tilted the way a deck tilts Three identical round dials in a row, each with two hands. The first stands upright. The second is tipped to the left and the third is tipped further to the right, as a clock on a moving deck would be. Beneath all three runs one long wavy line standing for the sea, which is the thing tipping them.

The Clock That Had to Survive a Ship

About 15 minutes

Part 2 boiled the whole of longitude down to one question: what time is it at home? And the obvious answer is to take a clock.

The obvious answer was right. It just took about fifty years of one man's life to make it work, and understanding why is more interesting than the clock.

A clock is a machine that repeats

Every clock does the same trick. It has something that goes back and forth at a rate that does not change, and it counts the swings.

A pendulum is the classic one. A weight on a rod swings, and — this is the useful part — how long each swing takes depends on the length of the rod and almost nothing else. Push it harder and it swings wider, but not faster. Wind the clock and it keeps counting.

By the 1600s pendulum clocks in observatories were good to a few seconds a week. That is far better than the longitude problem needs. If you could put one on a ship you were finished.

You cannot put one on a ship, and it is worth going through exactly why, because every reason is about the sea rather than about clocks.

Four separate tiles, one for each thing a ship does to a clock Four square tiles stand side by side with clear gaps between them. The first holds two wavy lines for a moving deck. The second holds two solid bars, the lower one longer than the upper, for metal growing in the heat. The third holds four scattered drops for salt air. The fourth holds a clock dial with a dashed empty circle below it, standing for the second clock that is not there to check it against. the deck the tropics salt air no second clock

Four things a ship does to a clock. Choose one to see what it does and what it took to beat it.

  • The deck never stops moving — a pendulum needs to hang straight down and swing freely. On a ship it is tipped, lifted and dropped every few seconds, and its rate changes with every wave. John Harrison's answer was to throw the pendulum away: he used two weighted bars linked together by springs, arranged so that whatever the sea did to one, it did the opposite to the other, and the two cancelled out.
  • The tropics are hot and the North Atlantic is not — metal grows when it is warmed. A longer pendulum rod swings slower; a warmer spring is weaker and lets the wheel swing wider. Either way the clock loses time in hot weather and gains it in cold, and a voyage to the West Indies goes through both. Harrison's answer was to fasten two different metals together in one strip. They grow by different amounts, so as the strip warms it bends, and the bend is used to shorten the spring by exactly as much as the heat had weakened it.
  • Salt air gets into everything — it rusts steel, and it thickens and fouls the oil that every set of moving parts depends on. A clock running dry wears out; a clock running on gummed oil runs slow. Harrison's answer to that was strange and completely his own: he made the moving parts out of a tropical hardwood called lignum vitae, which is naturally oily, so those parts needed no oil at all and could not rust.
  • Nobody can check it — this is the one that has nothing to do with engineering. A clock in a cathedral is compared with the Sun every clear day and nudged back into line. A clock at sea is the only timekeeper there is. If it drifts, there is nothing on board to notice it against, and it will be believed all the way to the rocks.

The prize

Ships were being lost, and not by ones and twos. In 1707 a British fleet returning from the Mediterranean came up the Channel in thick weather, misjudged where it was, and struck the rocks of the Isles of Scilly. Four ships went down in a night and somewhere between about fifteen hundred and two thousand men were lost. The navigators were not careless. They were working with dead reckoning, and dead reckoning had failed them.

In 1714 Parliament passed the Longitude Act. It set up a board of experts and offered £20,000 — a colossal sum — to anyone who could find longitude at sea to within half a degree at the end of a voyage to the West Indies.

Read that as a clock and it is a fearsome specification. Half a degree of longitude is two minutes of time. The voyage takes about six weeks. So the clock may gain or lose no more than three seconds a day, for forty days, on a wooden ship crossing from a cold winter into the tropics, with nobody able to check it and nobody able to open it.

John Harrison

Harrison was a carpenter's son, brought up in Lincolnshire, who built clocks in his spare time and had no training in any of this. He started with wooden clocks so accurate that people came to look at them.

Then he spent the rest of his life on the sea problem. His first machine, finished in 1735, is a metre-tall brass contraption with the two linked balances described above and no pendulum anywhere in it. It went to Lisbon and back and did well. He was not satisfied with it, so he built a second, then a third, spending nearly twenty years on the third alone.

Then he did something that must have looked like giving up. He abandoned the big machines altogether and made a watch — thirteen centimetres across, the sort of thing you could hold in two hands. Everything he had learned went into it: the two metals, the oil-free wood, a balance that ran fast and did not care what the ship did.

In 1761 that watch, now called H4, sailed for Jamaica. When it was checked at the far end, eighty-one days later, it was about five seconds slow.

What the Longitude Act allowed a clock, against what H4 used Two bars of exactly the same length, one above the other. The upper bar is filled all the way across and marked as the allowance the prize gave: a hundred and twenty seconds. The lower bar is filled only by a narrow sliver at its left end and marked as what the watch actually used on the voyage to Jamaica: about five seconds. what the prize allowed what the watch used three seconds a day, across a forty-day voyage: 120 seconds about five seconds, across eighty-one days
What the Act asked for, against what H4 did. The allowance was three seconds a day for forty days. The watch used five seconds in eighty-one.

The Board did not simply hand over the money. Harrison was made to repeat the trial, hand over his drawings, and let other people take the watch apart. He got the last of what he was owed in 1773, when he was eighty, after the King himself became involved. He died three years later.

Why is a pendulum clock, which keeps excellent time on land, useless on a ship?

One watch is one ship

So the clock method worked. It had been proved at sea, in public, against a specification written by people who wanted it to fail.

And still, for the next fifty years, most navigators finding their longitude were not using a clock.

The reason is not mysterious. H4 was one object, made by hand by a genius over years. The first copy of it, made by another master craftsman, took two and a half years and cost £450, at a time when that was a fortune. There were thousands of ships. You could not simply order chronometers, and for a long while only the wealthiest voyages carried one.

Which raises an obvious question, and it has a beautiful answer. If a clock is that hard to build and that hard to buy, what were all the other ships doing?

They were reading the time off something nobody had to build, nobody could break, and nobody could take away from them.