d'Alembert, Jean le Rond

Jean le Rond d'Alembert was born on November 16, 1717. He was a French mathematician, mechanician, physicist and philosopher who believed that all truth could be derived from a single, ultimate, yet-to-be-discovered mathematical principle. He considered mathematics the ideal form of knowledge, and the laws of physics to be the fundamental principles of the world.

He made several contributions to mathematics, including a suggestion for a theory of limits. He was one of the first to appreciate the importance of functions, and defined the derivative of a function as the limit of a quotient of increments. From these ideas he developed a test for convergence, found in Volume 5 of Opuscules mathématiques, known today as d'Alembert's ratio test. In France, the fundamental theorem of algebra is known as the d'Alembert/Gauss theorem.

In 1740, in his second scientific work on fluid mechanics, Memoire sur le refraction des corps solides, d'Alembert theoretically explained refraction and wrote about what is now called d'Alembert's paradox: that the drag on a body immersed in an inviscid, incompressible fluid is zero.

In 1742 d’Alembert began to read Traité de dynamique before the Academy. He improved Isaac Newton’s definition of force and helped resolve a controversy over the conservation of kinetic energy. He also clearly stated his belief that mechanics was a field of mathematics and should be made into a completely rationalistic mathematical system. He regarded Newton’s laws of motion as logical necessities, rather than the outcome of empirical research.

In 1747 d’Alembert published an article on vibrating strings which contained the first appearance in print of the wave equation, but his preference for mathematical deduction over actual observation led him to over-simplify certain boundary conditions, so that his conclusions were inaccurate. He pioneered the use of partial differential equations in physics, and won the 1747 prize of the Prussian Academy for Réflexions sur la cause générale des vents. Leonhard Euler (1707-1783), the Swiss mathematician, recognized the strength of d’Alembert’s methods and further developed them.

When the Encyclopédie was organised in the late 1740s, d'Alembert was engaged as co-editor (for mathematics and science) with Diderot, and served until a series of crises temporarily interrupted the publication in 1757. He authored over a thousand articles for it, including the famous Preliminary Discourse.

In 1754, d'Alembert was elected a member of the Académie des sciences, of which he became Permanent Secretary on 9 April 1772.

In 1757, an article by d'Alembert in the seventh volume of the Encyclopedia suggested that the Geneva clergymen had moved from Calvinism to pure Socinianism, basing this on information provided by Voltaire. The Pastors of Geneva were indignant, and appointed a committee to answer these charges. Under pressure from Jacob Vernes, Jean-Jacques Rousseau and others, d'Alembert eventually made the excuse that he considered anyone who did not accept the Church of Rome to be a Socinianist, and that was all he meant, and he abstained from further work on the encyclopaedia following his response to the critique.

d'Alembert was elected a Foreign Honorary Member of the American Academy of Arts and Sciences in 1781.

Jean le Rond d'Alembert died on October 29, 1783.

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