POISSON, SIMEON DENIS (1781-184o), French mathe matician, was born at Pithiviers in the department of Loiret on June 21, 1781. He studied medicine but gave it up in favour of mathematics, for which he showed an aptitude. In 1798 he entered the Ecole Polytechnique at Paris, where he attracted the notice of Lagrange and Laplace, who remained his friends as long as they lived. In 1800 he published two memoirs, one on R. Bezout's method of elimination, the other on the number of integrals of an equation of finite differences. The latter of these memoirs was examined by Legendre, who recommended that it should be pub lished in the Recueil des savants strangers, an unparalleled honour for a youth of 18. The rest of his career, till his death on April 25, 1840, was almost entirely occupied in mathematical research and in teaching. He was made professeur suppleant at the Ecole Polytechnique in 1802, and full professor in succession to Fourier in 1806. In 1808 he became astronomer to the Bureau des Longi tudes; and when the Faculte des Sciences was instituted in 1809 he was appointed prof esseur de la micanique rationelle.
Poisson's work in mathematics was very wide; he was the author of more than 30o memoirs and papers. His most important
work was on the application of mathematics to physics, and in particular to electrostatics and magnetism. In the field of pure mathematics, his most important works were his series of memoirs on definite integrals, and his discussion of Fourier's series, which paved the way for the classical researches of Dirichlet and Riemann on the same subject. In addition we may also mention his essay on the calculus of variations and his memoirs on the probability of the mean results of observations.
Besides his many memoirs Poisson published a number of treatises, most of which were intended to form part of a great work on mathematical physics, which he did not live to complete.
Among these may be mentioned his Traits de mecanique (2 vols., 1811 and 1833), which was long a standard work; Theorie nouvelle de l'action cappillaire (1831) ; Theorie mathematique de la chaleur (1835); Supplement to the same (1837) ; Recherches sur la probabilite des jugements, etc. (1837).
See F. Arago, Biographie de Poisson, read before the Academie des Sciences on Dec. 16, 185o. This contains a list of Poisson's works, drawn up by himself.