or Clairvaux, Alexis-Claude, was born May 7, 1713, at Paris, where his father was a teacher of mathematics. Under his father's tuition he made so rapid progress in mathematical studies, that in his thirteenth year he read before the French Academy a memoir of the properties of four curves which he had then discovered. When only sixteen, he finished his treatise on Curves of Double Curvature, which, at its publication, two years later, procured his admission into the Academy of Sciences, although even then he was below the legal age. Having formed an acquaintance with Maupertuis, Lemonnier, and others, he began his researches on the figure of the earth, the results of which were published in 1743. In his work on this subject he promulgated his famous theorem in regard to the variation of gravity, which has been corrected by Mr Airy. In 1750 he gained the prize of the St Petersburg Academy, for his treatise on the Lunar Theory; and in 1759 calculated the perihelion of Halley's Comet. Towards the close of his life he was regarded as the great rival of D'Alembert. Clairault died at Paris, May 17, 1765. See Astronomy.