MECHAIN, PIERRE FRANÇOIS ANDRÉ, a practical astronomer and geographer, was born at Laon on the 16th April 1744. His father, who was an architect, educated him for his own profession, but he accidentally became acquainted with Lalande, under whose patronage he was subsequently brought forward as an observer, surveyor, and computer. He made two voyages with M. de la Bretonnière, and assisted him in surveying some parts of the coast of France. He was afterwards employed in various computations by the Marquis de Chabert and the Duc D'Ayen. Having obtained a prize from the Academy of Sciences in 1782, for a Memoir on Comets, he became a member of the Academy the same year. About the year 1785 he undertook the publication of the Connaissance des Temps, and continued it till he was employed in geodetical operations at a distance from Paris. He was appointed member of a committee, along with Cassini, De Thury, and Legendre, to meet the English astronomers for the determination of the relative situation of the observatories which had been proposed by Cassini. It was in these operations that he first brought Borda's circle into general use. In 1791 he was appointed, in conjunction with Delambre, to execute the intentions of the Constituent Assembly with regard to the determination of a basis of linear measures. He was made director of the observatory of Paris, and he entered with great zeal on a series of observations which were to rival those of Flamsteed, Bradley, and Maskelyne; but he afterwards solicited the appointment to assist in the measurements required for the still farther extension of the arc of the meridian to the S. of Barcelona. But the secret motive for his seeking this humbler employment appears to have been a desire to remove some doubts which he entertained respecting the latitude of Barcelona, as it appeared after his death from his papers that there had been a discordance of 3" in some previous observations. Shipwreck and disease awaited him, however, and he died of fever on the 20th September 1805.
Of his publications the most important are to be found in the Mémoires des Savans étrangers; also a Memoir on the Comets of 1532 and 1661, which gained the prize. In the Mémoires of the Academy, and in the Connaissance des Temps, from 1782 to 1785, there are several of his observations.