POISSON, SIMON-DENIS

born June 21, 1781, Pithiviers, Fr. died April 25, 1840, Sceaux French mathematician known for his work on definite integrals, electromagnetic theory, and probability. His family coerced him into studying medicine, which he abandoned in 1798 in favour of mathematics, studying at the cole Polytechnique, Paris, under the mathematicians Pierre-Simon Laplace and Joseph-Louis Lagrange, who became his lifelong friends. His life was almost entirely engaged in mathematical research and in teaching. He became a deputy professor at the cole Polytechnique in 1802 and a full professor in 1806. In 1808 he was made astronomer at the Bureau des Longitudes, and, when the Facult des Sciences was instituted in 1809, he was appointed professor of pure mathematics. Poisson's most important work concerned the application of mathematics to electricity and magnetism, mechanics, and other parts of physics. His Trait de mcanique (1811 and 1833; Treatise on Mechanics) was the standard work in mechanics for many years. One of his publications (1812) contained many of the most useful laws of electrostatics and his theory that electricity is made up of two fluids, in which like elements repel and unlike attract. Poisson contributed to celestial mechanics by extending the work of Lagrange and Laplace on the stability of planetary orbits and by calculating the gravitational attraction exerted by spheroidal and ellipsoidal bodies. His expression for the force of gravity in terms of the distribution of mass within a planet has been used in the late 20th century for deducing details of the shape of the Earth from accurate measurements of the paths of orbiting satellites. Poisson's other works include Thorie nouvelle de l'action capillaire (1831; A New Theory of Capillary Action) and Thorie mathmatique de la chaleur (1835; Mathematical Theory of Heat). In Recherches sur la probabilit des jugements . . . (1837; Researches on the Probability of Opinions . . .), an important work on probability, the Poisson distribution, or Poisson law of large numbers, first appeared. Although originally derived as merely an approximation to Bernoulli's binomial law, it is now fundamental in the analysis of problems concerning radioactivity, traffic, and the random occurrence of events in time or space. In pure mathematics his most important works were a series of papers on definite integrals and his advances in Fourier's series, which paved the way for the researches of Peter Dirichlet and Bernhard Riemann on the same subject.