Profession: mathematician

Hachette had been taught by Monge in Mézières. As a young man, he sent some papers on applied geometry to Monge, leading to an appointment alongside him, teaching descriptive geometry to students who included Poisson, Arago and Fresnel. Hachette had previously fought against the Austrian army with Guyton de Morveau (in the first battle to employ aerial surveillance). As Monge’s one-time protégé, his notable advancement of the subject included the continuation of Monge’s work. Faraday and Davy visited him during their two-year European journey, Faraday maintaining the connection back in London.

He was recommended by d’Alembert for his post at the École Militaire, where Laplace was a colleague. Condorcet encouraged him to write what became the standard textbook on geometry for the next century, succeeding Euclid’s ‘Elements’. Germain wrote to him about number theory, their ensuing correspondence amounting to a collaboration. He encouraged Cauchy’s geometrical work and praised Jacobi’s advances with elliptical functions. Abel described him as “extremely amiable, but unfortunately as old as the stones”, while Poisson said that his very modest colleague only wanted to be spoken about in respect of his work.

Cauchy received from Galois, still in his teens, a memoir on equation theory, and was impressed enough to suggest he send it to Fourier so it could be considered for a prize. However Fourier died and the moment was lost. Poisson later invited Galois to send a copy to him, but found it unclear, and by the time Galois received the response, he was in prison for revolutionary activities (and died very soon after, after a duel and in unclear circumstances).