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A pendulum clock designed 300 years ago has been certified as the world&’s most accurate in its class by time-keeping experts from Guinness World Records. The design was was created in the 18th century — but never built — and a book describing the technique was dismissed as the “ramblings” of an old man. Its creator, a famous clock designer named John Harrison, had boasted that it would be not gain or lose a second in 100 days — a preposterous idea at the time. But when horology experts at Greenwich&’s Royal Observatory built the clock from Harrison&’s design within a perspex case, and monitored it against the speaking clock, he was right.

Harrison is world-famous as the inventor of the first clock accurate enough to permit British ships to navigate past the Equator, chronicled in the bestselling book Longitude. “It was a claim that Harrison made and a claim nobody believed because the best clocks of the day couldn’t do better than about a second a week, if they were lucky,” said Jonathan Betts of the Royal Observatory. “So the idea that somebody was going to keep time to an accuracy of a second in a 100 days was preposterous. It was only in the 20th century that people thought Harrison may have been right.

“As soon as we set the clock running, it was clear that it was performing incredibly well, so then we got the case sealed because nobody was going to believe how well the clock was running.”

The priceless Late Regulator clock took John Harrison, the pioneer of longitude, 36 years to build and he was still calibrating it when he died at his home in London on 24 March 1776, his 83rd birthday. Harrison believed that it would vary by only a second every 100 days. The timepiece&’s blend of lignum vitae wood, brass, bronze and steel components was designed to compensate for changes in temperature and atmospheric pressure over the seasons. Said Betts, “Originally Harrison used the movement of stars past a neighbour&’s chimney and his window frame to measure the effects of calibration, to see if the Late Regulator ticked with the same regularity in the summer as the winter. It was a huge task and very time-consuming.”

To calibrate the clock, albeit with the help of modern technology, Betts had to decipher Harrison&’s last known manuscript, A Description Concerning Such Mechanism as Will Afford a Nice or True Mensuration of Time. “On first reading, it sounds like gibberish but some of the concepts were new, so he had to invent names for them, such as dominion,” said Betts.

Harrison went against the grain of contemporary thinking by using large pendulum swings, enlarging the pendulum&’s “dominion” to reduce errors. He used his clocks as time standards for the marine chronometers he had pioneered to deliver accuracy great enough to allow the determination of longitude at sea. It was his fourth timekeeper, known today as the H4, that finally won him the £20,000 Longitude Prize, a fortune for those days because the project was the greatest scientific, economic and political problem of the age. The Late Regulator would have been used as the time standard for the H6 pocket timekeeper – “the lesser watch”, as he called it.

Harrison&’s notes and drawings suggest that the H6 was built but it has never been found. It looked like an overgrown pocket watch and Harrison scholars still dream of finding it in an attic. Among his many remarkable innovations was the gridiron mechanism, consisting of alternating brass and iron rods assembled so that expansion and contraction rates cancelled each other out as the chronometer moved from the tropics to colder climes. He was also the inventor of the first caged roller bearing, the father of the ball bearing, in his last clock.

Being almost frictionless, the Late Regulator required no oil for lubrication and therefore no cleaning. “No other mechanical clock had ever been made to work without oil,” said Betts, explaining how Harrison decided to do without by using rolling bearings instead of sliding ones. “It just goes on and on and on and never has to be taken apart.”

David Rooney, curator of timekeeping, said the Late Regulator “was Harrison&’s last word in precision pendulum clock design. It is fantastically well designed and built — an extraordinary thing.”