LAGNY, THOMAS FANTET DE, a French mathematician, was born at Lyon in 1660, and died at Paris 12th of April 1734. At an early period his scientific attainments led to his being appointed hydrographer royal at Rochefort. Subsequently he became sub director of the general bank of Paris, and lost the principal part of his fortune by the failure of that establishment. His mathematical labours appear to have been in a great measure directed to objects of mere curiosity ; as an instance of which be occupied himself with the quadrature of the circle, and computed the ratio of the circum ference to the diameter, as far as 120 decimal places, a degree of approximation which could never be of any practical utility. He however has called forth the eulogium of Fontenelle, who, speaking of hie treatise on the • Cubature of the Sphere,' says, "it is a choice and singular production which only a great mathematician could have written." His methods of facilitating the solution of indeterminate
problems are ingenious, and the theorems which he added to the arithmetic of sines are important. He was elected member of the Royal Academy of Paris in 1696; assoclate-geometrician in 1699; veteran pensioner in 1723; and fellow of the Royal Society of London in 1718. The following is a list of his published works : ' New Method of Extracting and Approximating to the Roots of Quadratic and Cubic Equations,' l'aris, 1691, of which an enlarged edition was published in the following year ;`Elements of Arithmetic) and Algebra,' Paris, 1697; • Cubature of the Sphere,' La Rochelle, 1702; Binary System of Arithmetic,' Rochefort, 1703; ' Analysis of the New Methods of Resolving Problems,' Paris, 1733; besides numerous memoirs in the Transactions of the Royal Academy.