D'Alembert, Jean le Rond, mathematician and encyclopædist, was born in Paris, November 16, 1717, and was found the day after his birth near the church of St Jean-le-Rond, from which he derived his name—the surname he himself added long after. He was the illegitimate son of Madame de Tencin and the Chevalier Destouches, and was brought up by the wife of a poor glazier; but his father, without publicly acknowledging the paternity, secured to him an allowance of 1200 francs a year. At twelve the boy entered the Collège Mazarin, where he soon showed his lifelong passion for mathematical studies. On leaving college, he returned to the humble home of his kind foster-mother, where he continued to live and pursue his favourite studies for thirty years, broken only by two ineffectual attempts to earn a living by law and medicine. 'You will never,' said his foster-mother, 'be anything but a philosopher; and what is a philosopher, but a fool who torments himself during his life that people may talk about him when he is dead?' His first distinction was admission at twenty-three to the Academy of Sciences. Two years later appeared his Traité de Dynamique, which reduces all the laws of motion to the consideration of Equilibrium, thereby making an epoch in mechanical philosophy. Later works were Réflexions sur le Cause générale des Vents, which gained the prize of the Academy of Berlin, 1746, and which contains the first conception and use of the Calculus of Partial Differences; Traité de l'Équilibre et du Mouvement des Fluides (1744); Recherches sur la Précession des Équinoxes et sur la Mutation de l'Axe de la Terre (1749); and Recherches sur Différents Points Importants du Système du Monde (1754). His Opuscules Mathématiques (8 vols. 1761–80) contain an immense number of memoirs, some on new subjects, some containing developments of his previous works.
But D'Alembert did not confine himself to physical science. For the great Encyclopédie planned by Diderot he wrote the famous Discours Préliminaire, a noble tribute to literature and philosophy, a model of lucid and eloquent exposition, although its classification of the sciences is open to question. Besides numerous articles in the Encyclopédie (the mathematical portion of which he edited), he published books on philosophy, literary criticism, the theory of music, and a treatise, Sur la Destruction des Jésuites (1765), which involved him in controversy. He became secretary to the Academy in 1772, and thereafter he wrote the lives of all the members deceased between 1700 and that year—one of the most pleasing of his works. His literary works have been published in a collected form, new edition, by Bossange (Paris, 5 vols. 1821). This edition contains his correspondence with Voltaire and the king of Prussia. His scientific works have never been collected.
So genuine was D'Alembert's love of independence that wealth and rank had no fascination for him. Frederick II. of Prussia offered him the presidency of the Academy of Berlin in 1752, but he declined to leave France, and only accepted a subsequent offer of a pension of 1200 francs. The king of France granted him a similar sum. In 1762 Catharine II. of Russia invited him, through her ambassador, to undertake the education of her son, with a salary of 100,000 francs; and when he declined, she wrote him an autograph letter, urging that to refuse to contribute to the education of a whole nation was inconsistent with his own principles; and inviting him, if he could not reconcile himself to the breaking-off of his pursuits and friendships, to bring all his friends with him, and she would provide both for them and for him everything they could desire. But he remained steadfast. D'Alembert never married. He was tenderly attached for many years to Mademoiselle Lespinasse (q.v.), with whom he lived in the same house in Platonic affection for nearly a dozen years, but who was scarce worthy of his devotion. Her death in 1776 was a crushing blow to the philosopher. He died October 29, 1783. See his Œuvres et Correspondances inédites, edited by Charles Henry (Abbeville, 1887), and the Life by J. Bertrand (Par. 1889).