Most historiography on the determination of longitude at sea gives a primary role to the British Board of Longitude. Yet the study of documents from the second half of the 18th century in French institutions shows that the question took on a distinct national character in France.

Many projects were sent to the Ministry of Marine (i.e. the King), the Academy of Marine, and above all to the Academy of Science during the period after the Seven Years War (1756-1763), this last institution putting the quest of longitude at sea to competition between 1767 and 1773.

The context was favourable after the success of the sea trials of John Harrison’s watch (H4). On one hand Lalande (1759) and on the other Maskelyne diffused the use of lunar distances, the latter after he returned from Saint Helena in 1762, in particular through the Nautical Almanac. Maskelyne’s works promoted the method that Lacaille had first published in 1755.

Four voyages at sea took place between 1767 and 1772, led independently or jointly by the Academy of Science and the Ministry of Marine. Pierre Leroy’s chronometers were tested on board of the Aurore (1767) and the Enjouée (1769), with Berthoud’s clocks tested on board the Isis in the same period (1768-1769).

The King, the Academy of Science, and the Academy of Marine jointly set up a fourth voyage. On the Flore, both Berthoud’s and Leroy’s watches were tested in the same conditions and places. Furthermore, octants and sextants revealed their qualities as the best instruments adapted to the lunar-distance method. They were also used to check the working of the marine clocks. This expedition would mark an important step in the life of Jean-Charles de Borda, one of the three commissioners appointed to it. Leroy won prizes in 1769 and 1773, Berthoud not being eligible.

With the testing on the Flore and Academy’s prizes, the quest of longitude at sea was effectively closed in France. The four expeditions were representative of the period, and each of them showed progress regarding methods of control. From the amateurism exhibited on the Aurore (‘marine en dentelles’, scattered observations, sometimes isolated, no possible comparison) to the rigorous method practised with a team spirit on the Flore (groups of savants, results gathered and compared, averages and calculation of errors), we can see that elements of scientific expeditions and a learned marine was coming into place in France at the end of the 18th century.