MERCIER, Louis Charles Antoine, French engineer, born in Melun in 1744: died in Rouen in 1812. He was the son of a director of the mint, entered the marine guards when scarcely fifteen years old, and was employed for several years in Canada and Martinique. When the war of American independence began he asked permission to serve as a volunteer, and, coming in 1776 to this country, was employed as an artillery officer, directing" the batteries during the siege of Savannah by Count d'Estaing in 1779. He was reinstated as major in the French army, and, after being employed for several months to rebuild the fortifications of the island of St. Eustache, returned again to the American continent in 1780 and took part under Lafayette in the Virginian campaign, being wounded at the head of his regiment at Yorktown. After the conclusion of peace in 1783 he was appointed director of the fortifications of Santo Domingo. He afterward became assistant commander of the artillery in the navy-yard of Brest, but during the reign of terror he came to Louisiana and was a professor of mathematics in New Orleans till 1803, when he was commissioned by the first consul to study the water-front of the colony, and forward plans for the protection of the coast. The cession of Louisiana to the United States brought the mission to an end, but Mercier had meanwhile become interested in it, and, having been left a small fortune by a relative, continued it at his own expense. He devoted five years to the exploration of the country as far north as Oregon, west to California, and east to Texas, sailed for 900 miles on Mississippi and Missouri rivers, made a thorough study of the hydrography of the country that is watered by the Lafourche, Atchafalaya, Black, and Washita rivers, and also took barometric levels along Perdicco river, the former boundary of Louisiana. He presented to the United States authorities in 1807 plans for the drainage of flooded lands in the delta of the Mississippi. Returning to France in 1808, he settled in Rouen. He published "Memoire sur les vapeurs de l'atmosphere le long du cours du Mississilsi" (Paris, 1808); "Carte du bassin du Mississipi" (1808) ; "Systame hydrographique de la Louisiane" (Rouen, 1809); " Carte du delta du Mississipi" (1810) ; "Etudes topographiques, geo-graphiques, hydrographiques, geologiques et geo-desiques sur la Louisiane" (1811) ; and "Tableau du climat de la Louisiane, et de son influence sur les Europoens et les Creoles" (1812).