Jean Le Rond D'alembert
1717-1783
French Mathematician and Physicist
The name of Jean Le Rond d'Alembert belongs among the most honored of the philosophes, French thinkers whose ideas exemplified the Enlightenment. D'Alembert is most readily associated with Denis Diderot (1713-1784), to whose Encyclopédie he was a significant contributor. As a mathematician and physicist, his contributions include d'Alembert's principle, an extension of Newton's third law of motion.
D'Alembert's early years were not happy ones. His mother, Claudine-Alexandrine Guérin, marquise de Tencin, was a former nun who lived with a number of men—including, for a short time at least, Louis-Camus, chevalier Destouches-Canon, a military officer. From this union, a son was born in Paris on November 17, 1717, but the mother regarded her pregnancy as an unpleasant interruption in her affairs, and abandoned the infant on the steps of the church at Saint-Jean-le-Rond.
Thus the boy was baptized as Jean Le Rond, and afterward was sent to live in a foster home at Picardy. (Later, in college, he began calling himselfJean-Baptiste Daremberg, and this was eventually shortened to d'Alembert.) Unlike d'Alembert's mother, his father continued to care for him, and later arranged for him to be raised by a Madame Rousseau, a working-class woman who d'Alembert came to regard as his true mother. He lived in her home until he was nearly 50 years old. The father died when d'Alembert was just nine, leaving him with an income of 1,200 livres a year. These funds permitted him the independence he needed to engage in his later scholarly pursuits.
D'Alembert attended Mazarin College, or the Collège des Quatre-Nations, from which he received his bachelor's degree in 1735. Three years later, he earned his license to practice law, then went on to study medicine before rejecting both careers in favor of mathematics. He submitted his first paper to the Académie Royale des Sciences in 1739, and over the next two years bombarded the Académie with papers until in 1741 he was admitted to membership. In the years that followed, he became heavily involved in the world of the salons, social gatherings in which a number of philosophes came to prominence. He became particularly close to Julie de Lespinasse, a popular hostess, and though they never married, they were intimate for many years.
During the early 1740s, d'Alembert studied questions of dynamics, or the effects of force on moving bodies. In Traité de dynamique (1743), he introduced d'Alembert's principle, which maintains that for an object in motion to resist acceleration, the force of this resistance must be equal and opposite to the force producing the acceleration. This extended to moving bodies, the application of Newton's third law of motion, which holds that for every action, there is an equal and opposite reaction. In 1747, Réflexions sur la cause générales des vents marked the first use of partial differential equations in mathematical physics, and won d'Alembert a prize from the Prussian Academy. Also in 1747, an article on the motion of vibrating strings contained the first use of a wave equation in physics.
D'Alembert followed these writings with works on astronomy, but his attention was turning from mathematics and science to other areas. During the 1750s, he devoted himself to work on Diderot's Encyclopédie, producing first the Discours préliminaire (1751), an introduction that showed the links between various disciplines. D'Alembert later contributed some 1,500 articles to the Encyclopédie, but later resigned in the face of a scandal surrounding a 1757 article on Geneva, Switzerland, that criticized both Catholics and Calvinists.
During the two decades from 1761 to 1780, d'Alembert produced scientific and mathematical works on a wide array of subjects, but his work suffered due to physical and personal problems. Deathly ill in 1765, he moved in with Julie de Lespinasse, who nursed him back to health. The two lived together until her death in 1776, after which he discovered that she had long maintained affairs with other men. Lonely and bitter, he lived his remaining eight years in a Paris apartment provided by the French Académie (he had been accepted for membership in 1754), and died on October 29, 1783.
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