D'Alembert , Jean Le Rond

(1717–1783) French mathematician, encyclopedist, and philosopher

D'Alembert was the illegitimate son of a Parisian society hostess, Mme de Tenzin, and was abandoned on the steps of a Paris church, from which he was named. He was brought up by a glazier and his wife, and his father, the chevalier Destouches, made sufficient money available to ensure that d'Alembert received a good education although he never acknowledged that d'Alembert was his son. He graduated from Mazarin College in 1735 and was admitted to the Academy of Sciences in 1741.

D'Alembert's mathematical work was chiefly in various fields of applied mathematics, in particular dynamics. In 1743 he published his Traité de dynamique (Treatise on Dynamics), in which the famous d'Alembert principle is enunciated. This principle is a generalization of Newton's third law of motion, and it states that Newton's law holds not only for fixed bodies but also for those that are free to move. D'Alembert wrote numerous other mathematical works on such subjects as fluid dynamics, the theory of winds, and the properties of vibrating strings. His most significant purely mathematical innovation was his invention and development of the theory of partial differential equations. Between 1761 and 1780 he published eight volumes of mathematical studies.

Apart from his mathematical work he is perhaps more widely known for his work on Denis Diderot's Encyclopédie as editor of the mathematical and scientific articles, and his association with the philosophes. D'Alembert was a friend of Voltaire's and he had a lively interest in theater and music, which led him to conduct experiments on the properties of sound and to write a number of theoretical treatises on such matters as harmony. He was elected to the French Academy in 1754 and became its permanent secretary in 1772 but he refused the presidency of the Berlin Academy.