# 1911 Encyclopædia Britannica/Alembert, Jean le Rond d'

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​ALEMBERT, JEAN LE ROND D’ (1717–1783), French mathematician and philosopher, was born at Paris in November 1717. He was a foundling, having been exposed near the church of St Jean le Rond, Paris, where he was discovered on the 17th of November. It afterwards became known that he was the illegitimate son of the Chevalier Destouches and Madame de Tencin. The infant was entrusted to the wife of a glazier named Rousseau who lived close by. He was called Jean le Rond from the church near which he was found; the surname Alembert was added by himself at a later period. His father, without disclosing himself, having settled an annuity on him, he was sent at four years of age to a boarding-school. In 1730 he entered the Mazarin College under the Jansenists, who soon perceived his exceptional talent, and, prompted perhaps by a commentary on the Epistle to the Romans which he produced in the first year of his philosophical course, sought to direct it to theology. His knowledge of the higher mathematics was acquired by his own unaided efforts after he had left the college. This fact naturally led to his crediting himself with many discoveries which he afterwards found had been already established, often by more direct and elegant processes than his own.
On leaving college he returned to the house of his foster-mother, where he continued to live for thirty years. Having studied law, he was admitted as an advocate in 1738, but did not enter upon practice. He next devoted himself to medicine, but his natural inclination proved too strong for him, and within a year he resolved to give his whole time to mathematics. In 1741 he received his first public distinction in being admitted a member of the Academy of Sciences, to which he had previously presented several papers, including a Mémoire sur le calcul intégral (1739). In his Mémoire sur le réfraction des corps solides (1741) he was the first to give a theoretical explanation of the phenomenon which is witnessed when a body passes from one fluid to another more dense in a direction not perpendicular to the surface which separates the two fluids. In 1743 he published his Traité de dynamique, a work famous as developing the mechanical principle, known as “Alembert’s Principle,” first enunciated in 1742 (see Mechanics). In 1744 Alembert applied this principle to the theory of the equilibrium and the motion of fluids (Traité de l’équilibre et du mouvement des fluides), and all the problems before solved by geometricians became in some measure its corollaries. This discovery was followed by that of the calculus of partial differences, the first trials of which were published in his Réflexion sur la cause générale des vents (1747). This work was crowned by the Academy of Berlin, and was dedicated to Frederick the Great, who made several unsuccessful attempts to induce him to settle in Berlin. In 1763 he visited Berlin, and on that occasion finally refused the office of president of the Academy of Berlin, which had been already offered to him more than once. In 1747 he applied his new calculus to the problem of vibrating chords, the solution of which, as well as the theory of the oscillation of the air and the propagation of sound, had been given but incompletely by the geometricians who preceded him. In 1749 he furnished a method of applying his principles to the motion of any body of a given figure; and in 1754 he solved the problem of the precession of the equinoxes, determined its quantity and explained the phenomenon of the nutation of the earth’s axis. In 1752 he published an Essai d’une nouvelle théorie sur la résistance des fluides, which contains a large number of original ideas and new observations; In 1746 and 1748 he published in the Memoirs of the Academy of Berlin “Recherches sur le calcul intégral,” a branch of mathematical science which is greatly indebted to him. In his Recherches sur différents points importants du systéme du monde (1754–1756) he perfected the solution of the problem of the perturbations of the planets, which he had presented to the academy some years before.
Alembert’s association with Diderot in the preparation of the Dictionnaire Encyclopédique led him to take a somewhat wider range than that to which he had previously confined himself. He wrote for that work the Discours préliminaire on the rise, progress and affinities of the various sciences, which he read to the French Academy on the day of his admission as a member, the 18th of December 1754. He also wrote several literary articles for the first two volumes of the Encyclopaedia, and to the remaining volumes he contributed mathematical articles chiefly. One of the few exceptions was the article on “Geneva”, which involved him in a somewhat keen controversy in regard to Calvinism and the suppression of theatrical performances within the town. During the time he was engaged on the Encyclopaedia he wrote a number of literary and philosophical works which extended his reputation and also exposed him to criticism and controversy, as in the case of his Mélanges de Philosophies, d’Histoire, et de Littérature. His Essai sur la société des gens de lettres avec les grands was a worthy vindication of the independence of literary men, and a thorough exposure of the evils of the system of patronage. He broke new ground and showed great skill as a translator in his Traduction de quelques morceaux choisis de Tacite. One of his most important works was the Eléments de Philosophies published in 1759, in which he discussed the principles and methods of the different sciences. He maintained that the laws of motion were necessary, not contingent. A treatise, Sur la destruction des Jésuites (1765), involved him in a fresh controversy, his own share in which was ​rendered very easy by the violence and extravagance of his adversaries. The list of his more noteworthy literary works is completed by the mention of the Histoire des membres de l’Académie française, containing biographical notices of all the members of the Academy who died between 1700 and 1772, the year in which he himself became secretary. Alembert was much interested in music both as a science and as an art, and wrote Éléments de musique théorique et pratique (1779), which was based upon the system of J. P. Rameau with important modifications and differences.
Alembert’s fame spread rapidly throughout Europe and procured for him more than one opportunity of quitting the comparative retirement in which he lived in Paris for more lucrative and prominent positions. The offer of Frederick the Great has already been mentioned. In 1762 he was invited by Catherine of Russia to become tutor to her son at a yearly salary of 100,000 francs. On his refusal the offer was repeated with the additional inducement of accommodation for as many of his friends as he chose to bring with him to the Russian capital. Alembert persisted in his refusal, and the letter of Catherine was ordered to be engrossed in the minutes of the French Academy. In 1755, on the recommendation of Pope Benedict XIV., he was admitted a member of the Institute of Bologna. A legacy of £200 from David Hume showed the esteem in which he was held by that philosopher.
Alembert continued to the end to lead the quiet and frugal life dictated by his limited means as well as his simple tastes. His later years were saddened by circumstances connected with a romantic attachment he had formed for Mademoiselle de Lespinasse, whose acquaintance he made at the house of Madame du Deffand, a noted resort of literary men and savants. She nursed him assiduously during an illness he had in 1765, and from that period till her death in 1776 they lived in the same house without any scandal. On her part there seems to have been from first to last nothing more than warm friendship, but his feelings towards her were of a stronger kind and her death deeply affected him. He never recovered his elasticity of spirits, though he continued to occupy himself with his favourite pursuits, and to frequent the society of his brother philosophers. After the death of Voltaire (1778), whose friend and correspondent he had been for more than thirty years, he was regarded as the leader of the philosophical party in the Academy. He died at Paris on the 29th of October 1783.
The chief features of Alembert’s character were benevolence, simplicity and independence. Though his income was never large, and during the greater part of his life was very meagre, he contrived to find means to support his foster-mother in her old age, to educate the children of his first teacher, and to help various deserving students during their college career. His cheerful conversation, his smart and lively sallies, a singular mixture of malice of speech with goodness of heart, and of delicacy of wit with simplicity of manners, rendered him a pleasing and interesting companion; and if his manner was sometimes plain almost to the extent of rudeness, it probably set all the better an example of a much-needed reform to the class to which he belonged. The controversy as to the nature of his religious opinions, arising as it did chiefly out of his connexion with the Encyclopaedia, has no longer any living interest now that the Encyclopaedists generally have ceased to be regarded with unqualified suspicion by those who count themselves orthodox. It is to be observed, moreover, that as Alembert confined himself chiefly to mathematical articles, his work laid him less open to charges of heresy and infidelity than that of some of his associates. The fullest revelation of his religious convictions is given in his correspondence with Voltaire, which was published along with that with Frederick the Great in Bossange’s edition of his works.
The scientific works of Alembert have never been published in a collected form. The most important of them have been mentioned above, with the exception of the Opuscules mathématiques (1761–1780), 8 vols. 4to. His literary and philosophical works were collected and edited by Bastien (Paris, 1805, 18 vols. 8vo). A better edition by Bossange was published at Paris in 1821 (5 vols. 8vo). The best account of the life and writings of Alembert is contained in Condorcet’s Éloge, presented to the Academy and published in 1784.