Jean Le Rond D Alembert, a French mathematician and man of letters, born in Paris, Nov. 10, 1717, died there, Oct, 29, 1783. He was the illegitimate child of the poet Des-touches, commissary of artillery, and Madame de Tencin, a court lady, more celebrated for wit and beauty than for virtue. The infant, exposed on the steps of the church of St, Jean le Rond, was picked up by the police, and given to a glazier's wife, to whose affection the great philosopher responded throughout his life. He lived with her for 40 years, and when in the days of his tame Madame de Tencin came forward and avowed her relationship, he repudiated her, alleging that she was but a stepmother, and the glazier's wife his real parent. Soon after his discovery his father acknowledged him and settled upon him a pension of 1,200 francs, which was sufficient to provide for his education. In 1721 he was sent to a boarding school. At the age of 12 he was transferred to the Mazarin college of Paris. His philosophical studies here were eminently successful. He was for some time restrained from the study of mathematics, and applied himself to law, which he soon abandoned for medicine; but the irrepressible bent of his mind overcame all obstacles, and he at last betook himself with renewed ardor to his favorite employments.

A memoir and some remarks on the Analyse demontree of Reyneau procured him the membership of the academy of sciences in 1741. His celebrated Traite de dynamique appeared in 1743, and created a new branch of science. In 1744 he published his Traite de l'equilibre et du mouvement des fluides. In 1746, the Berlin academy of sciences having proposed the general causes of the winds as the subject for the prize essay, D'Alembert's treatise gained him the prize and the membership of the academy; in this he attributed the currents to the combined influence of the sun and moon in creating an action resembling the flux and reflux of the tides. In 1754 he became a member of the French academy, and in 1772 its perpetual secretary, and within the next three years wrote historical eulogies upon 70 members deceased since 1700, which were published in 0 vols. 12mo. He was early connected with the freethinkers of his age in the preparation of the Encyclopedic, and his Dis-cours preliminaire was designated by his associate editor Condorcet as a production of which only one or two men in a century could be found capable.

The progress of the work was interrupted by the government at the end of the second volume, at which time D'Alembert finally withdrew from the editorship, but continued to write the mathematical articles. He was a member of most of the learned societies of Europe, and was in intimate personal communication with Frederick the Great, who invited him to reside at the court of Berlin. This, however, he declined. The empress Catharine offered him the post of tutor or governor to the cezarevitch, with an income of 100,000 livres, but this he also declined. He was a man of singularly independent mind and manners, without degenerating into discourtesy or indifference to the feelings or necessities of others. His connection with the Encyclopedic involved him unjustly in the general censure which attached itself to the impiety and intolerance of many of its contributors. D'Alembert's means were limited and insufficient to keep pace with his benevolence; for when only in the enjoyment of two pensions of about $000 a year, one from Frederick, and the other from his own country, he gave away more than half their amount in charity.

His grief on the death of Mlle, de l'Espinasse, for whom he entertained a strong attachment, which she requited with less ardent sentiments, is believed to have hastened his death. Among his works not already mentioned are: Recherches sur differents points importants du systeme du mondc, (3 vols. 4to, 1754 - '6); Opuscules mathematiques (8 vols. 4to, 1761-'80), a rich mine of original materials; Elements de musique, upon the system of Rameau; Melanges de litterature et de philosophic (5 vols. 12mo); Sur la destruction des Jesuites en France; and a life of Queen Christina of Sweden. No single collection of his mathematical works has been published, but Bastien collected his literary and philosophical writings in 18 vols., with a full life of the author prefixed (Paris, 1805). A more complete edition was published by Bossange in 5 vols. 8vo (1821), containing several pieces not before published, and the correspondence of D'Alembert with Voltaire and Frederick II.