He invented the gridiron pendulum, consisting of alternating brass and iron rods assembled in such a way that the thermal expansions and contractions essentially cancel each other out.

In his earlier work on sea clocks, Harrison was continually assisted, both financially and in many other ways, by the watchmaker and instrument maker George Graham.

Over long voyages, cumulative errors in estimates of position by dead reckoning frequently led to shipwrecks and a great loss of life.

Avoiding such disasters became vital in Harrison's lifetime, in an era when trade and the need for accurate navigation were increasing dramatically around the world.

Harrison set out to solve the problem directly, by producing a reliable clock that could keep the time of the reference place accurately over long intervals without having to constantly adjust it.

The difficulty was in producing a clock that was not affected by variations in temperature, pressure, or humidity, resisted corrosion in salt air, and was able to function on board a constantly moving ship.

Huygens ran trials using both a pendulum and a spiral balance spring clock as methods of determining longitude, with both types producing inconsistent results.

[7] In 1716, Sully presented his first Montre de la Mer to the French Académie des Sciences[8] and in 1726 he published Une Horloge inventée et executée par M.

He presented his ideas to Edmond Halley, the Astronomer Royal, who in turn referred him to George Graham, the country's foremost clockmaker.

The master noted that his own calculations had placed the ship sixty miles east of its true landfall which had been correctly predicted by Harrison using H1.

In any event, Harrison suddenly abandoned all work on this second machine when he discovered a serious design flaw in the concept of the bar balances.

After steadfastly pursuing various methods during thirty years of experimentation, Harrison found to his surprise that some of the watches made by Graham's successor Thomas Mudge kept time just as accurately as his huge sea clocks.

Aided by some of London's finest workmen, he proceeded to design and make the world's first successful marine timekeeper that allowed a navigator to accurately assess his ship's position in longitude.

But in this, as will be seen from the points of the teeth rest, for a considerable portion of the supplementary arc—from 90° to 145° (limit of banking) past the dead point—upon the backs of the pallets, and tend to assist the balance towards the extreme of its swing and to retard its return.

[18] For technical reasons the balance was made much larger than in a conventional watch of the period, 2.2 inches (56 mm) in diameter weighing 28+5⁄8 Troy grains (1.85 g) and the vibrations controlled by a flat spiral steel spring of three turns with a long straight tail.

[16]: 56  William Harrison returned aboard the 14-gun HMS Merlin, reaching England on 26 March 1762 to report the successful outcome of the experiment.

During Harrison's second trial of his 'sea watch' (H4), Nevil Maskelyne was asked to accompany HMS Tartar and test the Lunar Distances system.

He returned a report of the watch that was negative, claiming that its "going rate" (the amount of time it gained or lost per day) was due to inaccuracies cancelling themselves out, and refused to allow it to be factored out when measuring longitude.

After three years he had had enough; Harrison felt "extremely ill used by the gentlemen who I might have expected better treatment from"[22] and decided to enlist the aid of King George III.

[23] This gave him a reasonable income for most of his life (equivalent to roughly £450,000 per year in 2007, though all his costs, such as materials and subcontracting work to other horologists, had to come out of this).

It was not recovered from Pitcairn Island until 1808, when it was given to Captain Mayhew Folger, and then passed through several hands before reaching the National Maritime Museum in London.

Using a chronometer to aid navigation simply saved lives and ships – the insurance industry, self-interest, and common sense did the rest in making the device a universal tool of maritime trade.

[2] He was buried in the graveyard of St John's Church, Hampstead, in north London, along with his second wife Elizabeth and later their son William.

[29] There is a blue plaque dedicated to Harrison on the wall of Summit House, a 1925 modernist office block, on the south side of the square.

[33] After World War I, Harrison's timepieces were rediscovered at the Royal Greenwich Observatory by retired naval officer Lieutenant Commander Rupert T. Gould.

[35] The system challenged the traditional view that harmonics occur at integer frequency ratios and in consequence all music using this tuning produces low-frequency beating.

In 2002, Harrison's last manuscript, A true and short, but full Account of the Foundation of Musick, or, as principally therein, of the Existence of the Natural Notes of Melody, was rediscovered in the US Library of Congress.

Finally, over a 100-day period from 6 January to 17 April 2015, Clock B was secured in a transparent case in the Royal Observatory and left to run untouched, apart from regular winding.

"[37] In 1995, inspired by a Harvard University symposium on the longitude problem organized by the National Association of Watch and Clock Collectors, Dava Sobel wrote a book about Harrison's work.

Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time became the first popular bestseller on the subject of horology.