By the 17th century, European nations including France were expanding their trade networks and overseas territories, and appreciated the need for better navigational techniques. Day-to-day, sailors navigated by dead reckoning, estimating position from direction and distance sailed. It was mostly successful – and is still standard – but seamen knew it could be unreliable. What would help would be ways of separately determining latitude (north-south position) and longitude (east-west position).
Latitude was relatively easy to find from the Sun or pole star. Longitude was also understood – in theory. As a measure in the direction of Earth’s rotation, the longitude between two places is effectively the difference between their local times, i.e. time from the Sun, which is at its highest at local noon. Since Earth rotates 360° in twenty-four hours, one hour of time is equivalent to 15° of longitude. If it’s noon for an observer and they know it’s 3pm at their home port, they are 45° of longitude from that port.
Two ways of finding that reference time also seemed clear: through astronomical observation; or with a timekeeper. But managing either at sea seemed impossible. Ships’ movements made accurate observations difficult and affected the reliability of timekeepers, which could also be impacted by changes in temperature and the effects of sea air.
Histories of the quest for longitude have mostly focused on 18th-century Britain, particularly John Harrison’s invention of a reliable timekeeper, from which the marine chronometer developed. His invention revolutionised navigation, allowing sailors to accurately calculate longitude at sea.
But as the Science Museum’s next exhibition, Versailles: Science and Splendour will show from 12 December, clockmakers in France played an important part too. Around 1670, for example, sea clocks by Dutch mathematician Christiaan Huygens were tested on French ships, not long after he had been invited to join the French Royal Academy of Sciences.
The invitation to join an institution specifically intended to promote science in the service of the state recognised Huygens’ expertise in an area of strategic significance. As at the Royal Society, founded in Britain around the same time, finding longitude was one of the topics the Academy’s members understood as requiring a solution. In the decades that followed, there was both rivalry and exchange between the two nations and their learned academies over this and other topics of potential practical benefit.
French interest in longitude matters continued into the 18th century, encouraged from 1720 by the Academy of Sciences’ biennial Meslay prize for astronomical and navigational problems. But it was only three decades later that work on marine timekeepers intensified in France. This was led by Ferdinand Berthoud (1727-1807), a Swiss-born clockmaker working in Paris. Appointed clockmaker to the French king and navy by Louis XV in 1770, Berthoud made 75 sea clocks over his lifetime, of which his No. 4 is on display in Versailles.
Berthoud made this sea clock in 1765 at an important moment in his career. He had been elected to the Royal Society a year earlier, having travelled to London in the hope of learning from John Harrison’s successes. On this occasion and again in 1766, Harrison refused to divulge the secrets of his famous timekeeper (now called H4) – the first truly reliable timekeeper for finding longitude at sea. So, Berthoud talked to English watchmaker Thomas Mudge, who had seen H4 and described it to him. And when the British Board of Longitude published a description of H4 in 1767, a French translation appeared within weeks.
Commentators had a low opinion about how useful this publication was, but it does seem to have changed how makers in France and elsewhere constructed timekeepers. Berthoud’s rival Pierre Le Roy, for one, openly acknowledged its influence.
By this time, Harrison’s success had also spurred the Academy of Sciences to offer its 1765 Meslay prize for timekeeping at sea. Le Roy and Berthoud each submitted instruments, with Le Roy eventually winning for his montre marine (sea watch) after sea trials in 1767-68. This timekeeper incorporated innovations such as the detached escapement that proved as crucial as other innovations in Harrison’s watch (H4) for the eventual development of the marine chronometer.
By contrast, Berthoud favoured simple mechanisms that were easy to repair. It was an approach that won state support and until the 1790s the French navy mostly acquired his timekeepers, many of them used on voyages of exploration as France sought new territories overseas. One such voyage is a point of focus in the Versailles exhibition.
In the mid-1780s, the naval officer Jean François de Galaup, comte de La Pérouse (1741–c.1788) was chosen to lead a major expedition to the Pacific. Commissioned by Louis XVI, the expedition had a dual aim. It sought to establish trade connections around the Pacific as well as further scientific knowledge: mapping coastlines as yet uncharted by Europeans and collecting scientific data. La Pérouse’s ships, the Boussole and Astrolabe, took the best instruments available, including five timekeepers by Berthoud. They performed excellently.
On reaching California, La Pérouse wrote that ‘Mr Berthoud has exceeded himself, since, after 18 months on board, his No. 19 and No. 18 [timekeepers] gave results that were as satisfactory as at the time we sailed, and enabled us to determine, several times a day, our precise longitude’.
While La Pérouse’s ships never returned, having sunk with their crews on the reefs of Vanikoro, Solomon Islands, some of their results had been sent to France and were used to improve charts of the Pacific coast of North America in particular.
And like Harrison’s and Le Roy’s technical ideas, data crossed borders: English charts incorporated French figures – including data from Berthoud’s clocks. Even in periods of international rivalry and conflict, new ideas and information often flowed freely.
