# Simeon Denis Poisson

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## Poisson, Siméon Denis

(sēmāôN` dənē` pwäsôN`), 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.

## Poisson, Siméon Denis

Born June 21, 1781, in Pithiviers, department of Loiret; died Apr. 25, 1840, in Paris. French scientist. Member of the Académie des Sciences (1812). Honorary member of the Imperial Academy of Sciences in St. Petersburg (1826).

Poisson joined the staff of the Ecole Polytechnique in Paris after graduating from that institution in 1800. He became a professor there in 1806. In 1809 he was made a professor at the University of Paris. His research dealt with theoretical mechanics, celestial mechanics, mathematics, and mathematical physics.

Poisson was the first to write equations in analytic mechanics in terms of momentum components. In hydrodynamics, Poisson generalized the Navier-Stokes equation to the case of the motion of a viscous compressible fluid with heat transfer being taken into account. He solved a number of problems in elasticity theory, introduced what is now known as Poisson’s ratio, and generalized the equations of elasticity theory to anisotropic bodies. In celestial mechanics, Poisson investigated the stability of the motion of the planets in the solar system; he also attempted to solve the problem of the perturbations of the planets’ orbits and the problem of the motion of the earth about its center of gravity. He introduced what is now referred to as the Poisson equation into potential theory and used it to solve problems in gravitation and electrostatics.

Poisson was the author of studies on the integral calculus, the calculus of finite differences, the theory of partial differential equations, and probability theory. In the field of probability theory, he proved a special case of the law of large numbers and a limit theorem.

Poisson also investigated problems in thermal conductivity, magnetism, capillarity, the propagation of sound waves, and ballistics. He was a staunch supporter of P. S. Laplace’s atom-

### WORKS

*Traité de mécanique*, 2nd ed., vols. 1–2. Paris, 1833.

*Théorie nouvelle de l’action capillaire*. Paris, 1831.

*Théorie mathématique de la chaleur*…. Paris, 1835.

*Recherches sur la probabilité…*. Paris, 1837.

### REFERENCES

Arago, F.*Biografii znamenitykh astronomov, fizikov i geometrov*, vol. 3. St. Petersburg, 1861. (Translated from French.)

Klein, F.

*Lektsii o razvitii matematiki*v

*XIX stolelii*, part 1. Moscow-Leningrad, 1937. (Translated from German.)

I. D. ROZHANSKII