Poisson, Siméon Denis (sēmāôNˈ dənēˈ pwäsôNˈ) [key], 1781–1840, French mathematician and physicist. From 1802 he taught at the École polytechnique, Paris, and was also on the faculty of sciences at the Univ. of Paris from 1809. His chief interest lay in the application of mathematics to physics, especially in electrostatics and magnetism. He developed a two-fluid theory of electricity and provided theoretical support for the experimental results of others, notably C. A. de Coulomb. Poisson also made important contributions to mechanics, especially the theory of elasticity; to optics; to the calculus, especially definite integrals; to differential geometry; and to probability theory. Other studies were concerned with heat and the motions of the moon. In all he wrote more than 300 papers on mathematics, physics, and astronomy, and his *Traité de mécanique* (1811) was long a standard work.

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