1736 - A Prototype Marine Chronometer

John Harrison, 46 years old, trials his first marine chronometer H1, built last year, on a round trip between London and Lisbon.

A total failure on the way outward - it lost time -, the way back brings an encouraging success in the form of a precise landing against a 60-mile error in longitude for the traditional method of navigation by dead reckoning and by position shift through sun observation.

This unexpected half-success rouses the Board of Longitude which was quietly and passively waiting for lunar tables.

It launches a debate between accurate time keeping versus celestial measurement. Before Harrison’s demonstration, it has seemed that the lunar distance method was the only practicable path, provided that an astronomer would supply precise data from the whimsical moves of our satellite. On his own, Isaac Newton (†1727) expected the appearance of accurate lunar tables prior to any accurate watch. It may be the watch the winner as the lunar tables are still to be expected.

Harrison started to work on a marine chronometer after reading Horologist Henry Sully's work Description abrégée d'une horloge d'une nouvelle invention pour la juste mesure du temps sur mer, avec le jugement de l'Académie Royale des Sciences published in 1728 when the author was already dead.

Sully had tried three variants of sea watch, the first in 1716, the second and the third in 1723, without achieving to deal with the shocks induced by ship rolling and pitching. Astronomer Jacques Cassini trialed the sea watch on a carriage. He observed that the watch did not keep its accuracy after a few weeks of operation - it lost about two minutes a week. Even if outstanding in the absolute in its days, it did not meet the reference requirements of the Board of Longitude, about ten times stricter.

During the five years of manufacturing that H1 required, Harrison has been supported, and likely sponsored a bit, by an old acquaintance of ours, Edmund Halley, 80, Astronomer Royal since 1720, and by Horologist George Graham, 63. They have been convinced by Harrison's enthusiasm and by his methodic approach of the different issues he has been facing.

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LINK WITH PREVIOUS CHRONICLE

1714 - The Longitude Act

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SUGGESTED BIBLIOGRAPHY

Henry Sully - Description abrégée d'une horloge d'une nouvelle invention pour la juste mesure du temps sur mer, avec le jugement de l'Académie Royale des Sciences plus the complement of the second part - Paris, 1728 - available on the internet

Dava Sobel – Longitude: The True Story of a Lone Genius Who Solved the greatest Scientific Question of His Time – New York, 1995

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CREDIT

James Tassie - John Harrison's Portrait - glass paste medallion, circa 1775 - © National Portrait Gallery