Mathematician, b. 21 June 1781 (Pithiviers, France), d. 25 April 1840 (Sceaux).
Poisson's father served as a soldier in the French army, but bad and contemptuous treatment of the ordinary ranks by the aristocratic officers caused him to take his leave and accept a low level employment in the state administration. Siméon-Denis was eight years old when the revolution broke out in 1789 and his father became president of the district of Pithiviers, about 80 km south of Paris.
The young Siméon-Denis was sent to an uncle's surgery in Fontainebleau to become an apprentice surgeon. But he proved to be inept with manual work and failed to make the grade.
France had become a republic in the meantime, and institutions of higher education had become accessible to commoners. In 1796 Poisson was sent to the École Centrale in Fontainebleau, where he showed a great gift in mathematics. His teachers encouraged him to take the entrance exam to the prestigious new École Polytechnique in Paris. He passed the exam with the highest grade and entered the École Polytechnique in 1798.
Poisson's lecturers - Laplace and Lagrange among them - quickly .recognized Poisson's abilities. His paper on the theory of equations written in his third year was of such quality that Poisson could graduate without taking the final examination. He was employed as tutor and two years later in 1802 appointed deputy professor. In 1806 he became full professor.
Poisson was one of the few scientists of the time who remained aloof to science; all he was interested in was mathematics applied to practical problems of science. His interest shifted to physics, although his clumsiness kept him away from experimental science. His many contributions to the solution of physical problems were instrumental to the establishment of theoretical physics as an independent and important branch of physics.
In 1808 Poisson was also appointed an astronomer at the Bureau des Longitudes. In the following year, in recognition of his interest in physics, he was also appointed to the chair of mechanics in the new Faculty of Sciences.
Poisson kept a collection of unsolved problems of mathematics and physics in his notebook and worked through them in a systematic way, guided by the responsibilities of his employment and the suggestions of the Académie des Sciences. His Traité de mécanique ("Treatise on Mechanics") of 1811 and 1813 remained a classic textbook for decades. He worked on problems in so many areas of physics that his colleague Fourier wrote: "Poisson has too much talent to apply it to the work of others. To use it to discover what is already known is to waste it."
Poisson's arguably most important contribution to physics came as his response to the annual competition of the Academy of 1811, specifically formulated to allow him to prove himself worthy of admission in the Academy's physics section. The problem posed was
Poisson never completed the experiment, but his mathematical solution was enough to see him elected. During the following years Poisson developed the theory of potentials; the relevant differential equation is now known as Poisson's equation.
Poisson continued to produce important results during his lifetime, among them the work Recherches sur la probabilité des jugements en matière criminelle et matière civile ("Researches on the Probability of Opinions in Criminal and Civil Matters") of 1837 which introduced the "Poisson distribution" into statistics. It describes the probability that a random event will occur in a time or space interval under the conditions that the probability of the event occurring is very small but the number of trials is very large, so that the event actually occurs a few times.
O'Connor, J. J. and E. F. Robertson (2002) Siméon-Denis Poisson, School of Mathematics and Statistics, University of St Andrews, Scotland,
http://www-history.mcs.st-andrews.ac.uk/Mathematicians/Poisson.html (accessed 26 May 2004)