LAGNY (Thomas Fantet de)

, an eminent French mathematician, was born at Lyons. Fournier's Euclid, and Pelletier's Algebra, by chance falling in his way, developed his genius for the mathematics. It was in vain that his father designed him for the law; he went to Paris to deliver himsels wholly up to the study of his favourite science. In 1697, the Abbé Bignon, protector-general of letters, got him appointed professor-royal of Hydrography at Rochfort. Soon after, the duke of Orleans, then regent of France, fixed him at Paris, and made him sub-director of the General Bank, in which he lost the greatest part of his fortune in the failure of the Bank. He had been received into the ancient academy in 1696; upon the renewal of which he was named Associate-geometrician in 1699, and pensioner in 1723. After a life spent in close application, he died, April 12, 1734.

In the last moments of his life, and when he had lost all knowledge of the persons who surrounded his| bed, one of them, through curiosity, asked him, what is the square of 12? To which he immediately replied, and without seeming to know that he gave any answer, 144.

De Lagny particularly excelled in arithmetic, algebra, and geometry, in which he made many improvements and discoveries. He, as well as Leibnitz, invented a binary arithmetic, in which only two figures are concerned. He rendered much easier the resolution of algebraic equations, especially the irreducible case in cubic equations; and the numeral resolution of the higher powers, by means of short approximating theorems.—He delivered the measures of angles in a new science, called Goniometry; in which he measured angles by a pair of compasses, without scales, or tables, to great exactness; and thus gave a new appearance to trigonometry.—Cyclometry, or the measure of the circle, was also an object of his attention; and he calculated, by means of infinite series, the ratio of the circumference of a circle to its diameter, to 120 places of figures.—He gave a general theorem for the tangents of multiple arcs. With many other curious or useful improvements, which are found in the great multitude of his papers, that are printed in the different volumes of the Memoirs of the Academy of Sciences, viz, in almost every volume, from the year 1699, to 1729.

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Entry taken from A Mathematical and Philosophical Dictionary, by Charles Hutton, 1796.

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* LAGNY (Thomas Fantet de)
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