Thomas Earnshaw and the Marine Chronometer

July 29, 2019

The story of the quest to find a way of calculating longitude at sea is well known, thanks largely to Dava Sobel’s excellent book Longitude. John Harrison was responsible for the eventual solution, by constructing a clock of unprecedented accuracy. He devoted forty-three years of his life to marine chronometers, battling against both the numerous engineering problems to be overcome, and the resistance of the scientific establishment of his day. His first marine chronometer (H1), produced in 1737, was a mass of whirling rods and springs that weighed seventy five pounds, and required a four foot square case to house it. His final chronometer (H5) resembled a large pocket watch, weighed three pounds, and with a five inch diameter, could fit in a sea captain’s hand. Having perfected his solution, the exhausted Harrison, now well into his seventies, never made another clock.

 

The proof of Harrison’s success came in 1770, when a copy of his chronometer made by another watchmaker, Larcum Kendall, was given to James Cook to test on his second voyage of discovery. The great explorer reported that the “...watch exceeded the expectations of its most zealous advocate and...has been our faithful guide through all vicissitudes of climates.” Splendid news then - the marine chronometer had proved to be a triumph, and the problem of calculating longitude was solved. But there was a catch.

 

Cook, a skilled astronomer and mathematician, had been able to regularly check that the timepiece was performing correctly, using the alternative lunar method of calculating longitude. Less talented captains (which meant almost everyone else), would need to carry a minimum of three marine chronometers, in order to spot if one was malfunctioning. It had taken Kendall two years of skilled work to make the single copy given to Cook, at a cost of £450. Unless the price and production time of the new technology could be substantially reduced, equipping the nation’s shipping with chronometers was going to take centuries and cost an astonishing amount of money.

 

The challenge of solving this problem fell to the next generation of watchmakers. Pocket watches were highly sought after fashion accessories in late 18th century Britain, and had spawned a considerable industry. Amongst their ranks were several men of business, ready to take on the challenge of mass producing marine chronometers, such as John Arnold, Josiah Emery and above all Thomas Earnshaw.

 

Born in the rapidly industrialising English Midlands, Earnshaw started life as an apprentice to Thomas Wright, where he learnt his watch making skills. He was only in his twenties when he first began work on the mass production of marine chronometers. His first challenge was to simplify Harrison’s design, without compromising on the level of its time-keeping ability. He was responsible for numerous changes, most of which were minor, but some were substantial. He devised a different design of escapement for his chronometers, using springs instead of pivots. He also modified the bimetallic balance in the original clock, which Harrison had engineered to compensate for changes in temperature encountered on long voyages. When he had completed work on the design, he was ready to go into production.

 

The second challenge Earnshaw faced was to streamline the manufacturing process. He did this by farming out production of the simpler components in his chronometers to lesser craftsmen, allowing him and his team of watchmakers to concentrate on making the most critical parts, and the final assembly. He set up a small factory to produce them in substantial numbers. Soon accurate marine chronometers were becoming available by the hundred, at a much reduced cost. In the 1780s a new boxed Earnshaw chronometer cost as little as £65. A further boost to take up came when the Admiralty introduced a scheme where if a captain bought the first one, they would pay for a second.

 

Earnshaw chronometers proved to be very popular. They were magnificent pieces of engineering, still highly accurate, yet affordable in reasonable quantities. Captain William Bligh, of mutiny on the Bounty fame, owned one. Mathew Flinders took two with him onboard the Investigator, when he made the first circumnavigation of the coast of Australia, along with two made by Earnshaw’s biggest rival, John Arnold. Flinders landed regularly to check the accuracy of his chronometers against the stars, and concluded that the Earnshaw’s were much superior, describing them as "...excellent timekeepers" in his account of the voyage. When HMS Beagle was sent around the world in 1831 with the young Charles Darwin onboard, it carried no fewer than twenty-two marine chronometers, including a number of Earnshaw’s.

 

In 1737 there was one marine chronometer in the world. By 1815, thanks in no small part to Thomas Earnshaw, there were approximately five thousand.

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