French astronomer, born at St. Lo; distinguished in chemistry before he devoted himself to astronomy; rose to eminence in the latter science by a paper on the variations in the orbits of the planets, and was led to the discovery of the planet Neptune from perturbations in the orbit of the planet Uranus; he indicated the spot where the planet would be found, and it was actually discovered a few days after by Galle at Berlin (1811-1877).