The Lapérouse Expedition and Geomagnetism: The Unexpected Discovery of Lamanon’s ‘Lost’ Letter and Ledru’s Instructions

Abstract

In January 1787, on board Lapérouse's Boussole anchored off Macao, the chevalier de Lamanon wrote a letter to the marquis de Condorcet, the then permanent secretary of the Académie Royale des Sciences in Paris. Lamanon's letter contained a summary of his magnetic observations made up to that point on Lapérouse's famous but ill-fated expedition. The letter, amongst other detail, included evidence that the Earth's magnetic field increased in intensity from the equator towards the poles. Sent to Condorcet via the then minister for the French Navy (the maréchal de Castries), the letter was subsequently lost, but not before it was copied. The copy, with early nineteenth-century ownership identified first to Nicolas Philippe Ledru and subsequently to Louis Isidore Duperrey, was itself then lost for over 150 years, but recently rediscovered bound-in with other manuscripts related to the Lapérouse expedition and terrestrial magnetism, including instructions by Ledru and remarks written in the 1830s and 1840s by Duperrey on Lamanon's letter and observations. The significance of Lamanon's letter and the Ledru and Duperrey manuscripts to the history of geomagnetism is discussed here. Duperrey's notes are transcribed in French for the first time and the Lamanon, Ledru and Duperrey manuscripts are translated into English, also for the first time.