Gilles Personne Or Personier De Roberval, a French mathematician, born at Roberval, near Beauvais, Aug. 8, 1602, died in Paris, Oct. 27, 1675. He went to Paris in 1627, became professor of philosophy in the college of Maî-tre Gervais and of mathematics in the royal college, and was one of the members of the academy of sciences at its foundation in 1665. He early discovered a method of investigating problems similar to the "method of indivisibles," but kept it to himself in order to surpass his contemporaries in the solution of problems, and thus lost the honor of originating it. A method of determining the direction of a tangent at any point of a curve, which Torri-celli claimed to have made in 1644, Roberval, in a letter to Torricelli, declared was known to himself in 1636. Torricelli gave the name of Robervallian lines to curves with infinite branches which admit of an expression for the area between them. Roberval discovered rules for finding the volume of solids formed by the revolution of a cycloid about its base and about its axis.

His principal works were published after his death in the old Mémoires de l'académie, vol. vi. (1693).