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Em 1798 entrou na [[École Polytechnique]] em [[Paris]], como primeiro colocado de sua turma, atraindo imediatamente a atenção dos professores da escola, deixando-o livre para escolher o que estudar. Em 1800, menos de dois anos depois de seu ingresso, publicou duas memórias, uma sobre o método da eliminação de [[Étienne Bézout]], e a outra sobre o número de integrais de uma equação em [[Diferença finita|diferenças finitas]]. Esta última foi examinada por [[Sylvestre François Lacroix]] e [[Adrien-Marie Legendre]], que recomendaram sua publicação no ''Recueil des savants étrangers,'' uma honra sem precedentes para um jovem de dezoito anos.
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This success at once procured entry for Poisson into scientific circles. [[Joseph Louis Lagrange]], whose lectures on the theory of functions he attended at the École Polytechnique, recognized his talent early on, and became his friend (the [[Mathematics Genealogy Project]] lists Lagrange as his advisor, but this may be an approximation); while [[Pierre-Simon Laplace]], in whose footsteps Poisson followed, regarded him almost as his son. The rest of his career, till his death in [[Sceaux, Hauts-de-Seine|Sceaux]] near Paris, was almost entirely occupied by the composition and publication of his many works and in fulfilling the duties of the numerous educational positions to which he was successively appointed.
Immediately after finishing his studies at the École Polytechnique, he was appointed ''[[répétiteur]]'' (teaching assistant) there, a position which he had occupied as an amateur while still a pupil in the school; for his schoolmates had made a custom of visiting him in his room after an unusually difficult lecture to hear him repeat and explain it. He was made deputy professor (''professeur suppléant'') in 1802, and, in 1806 full professor succeeding [[Jean Baptiste Joseph Fourier]], whom [[Napoleon]] had sent to [[Grenoble]]. In 1808 he became [[astronomer]] to the [[Bureau des Longitudes]]; and when the [[Faculte des Sciences|Faculté des Sciences]] was instituted in 1809 he was appointed professor of [[rational mechanics]] (''professeur de mécanique rationelle''). He went on to become a member of the Institute in 1812, examiner at the military school (''École Militaire'') at [[École Spéciale Militaire de Saint-Cyr|Saint-Cyr]] in 1815, graduation examiner at the École Polytechnique in 1816, councillor of the university in 1820, and geometer to the Bureau des Longitudes succeeding Pierre-Simon Laplace in 1827.
In 1817, he married Nancy de Bardi and with her he had four children. His father, whose early experiences had led him to hate aristocrats, bred him in the stern creed of the First Republic. Throughout the Revolution, the Empire, and the following restoration, Poisson was not interested in politics, concentrating on mathematics. He was appointed to the dignity of [[baron]] in 1821; but he neither took out the diploma or used the title. In March 1818, he was elected a [[Fellow of the Royal Society]]<ref>{{cite web|url=http://www2.royalsociety.org/DServe/dserve.exe?dsqIni=Dserve.ini&dsqApp=Archive&dsqCmd=Show.tcl&dsqDb=Persons&dsqPos=0&dsqSearch=%28Surname%3D%27poisson%27%29|title= Library and Archive Catalogue|publisher=The Royal Society|accessdate= 4 October 2010}}</ref> and in 1823 a foreign member of the [[Royal Swedish Academy of Sciences]]. The [[July Revolution|revolution of July 1830]] threatened him with the loss of all his honours; but this disgrace to the government of [[Louis-Philippe of France|Louis-Philippe]] was adroitly averted by [[François Jean Dominique Arago]], who, while his "revocation" was being plotted by the council of ministers, procured him an invitation to dine at the Palais Royal, where he was openly and effusively received by the citizen king, who "remembered" him. After this, of course, his degradation was impossible, and seven years later he was made a [[peer of France]], not for political reasons, but as a representative of French [[science]].
As a teacher of mathematics Poisson is said to have been extraordinarily successful, as might have been expected from his early promise as a ''répétiteur'' at the École Polytechnique. As a scientific worker, his productivity has rarely if ever been equalled. Notwithstanding his many official duties, he found time to publish more than three hundred works, several of them extensive treatises, and many of them memoirs dealing with the most abstruse branches of pure mathematics, [[applied mathematics]], [[mathematical physics]], and rational mechanics.
A list of Poisson's works, drawn up by himself, is given at the end of Arago's biography. All that is possible is a brief mention of the more important ones. It was in the application of mathematics to physics that his greatest services to science were performed. Perhaps the most original, and certainly the most permanent in their influence, were his memoirs on the theory of [[electricity]] and [[magnetism]], which virtually created a new branch of mathematical physics.
Next (or in the opinion of some, first) in importance stand the memoirs on [[celestial dynamics|celestial mechanics]], in which he proved himself a worthy successor to Pierre-Simon Laplace. The most important of these are his memoirs ''Sur les inégalités séculaires des moyens mouvements des planètes'', ''Sur la variation des constantes arbitraires dans les questions de mécanique'', both published in the ''Journal'' of the École Polytechnique (1809); ''Sur la libration de la lune'', in ''Connaissances des temps'' (1821), etc.; and ''Sur le mouvement de la terre autour de son centre de gravité'', in ''Mémoires de l'Académie'' (1827), etc. In the first of these memoirs, Poisson discusses the famous question of the stability of the planetary [[orbit]]s, which had already been settled by Lagrange to the first degree of approximation for the disturbing forces. Poisson showed that the result could be extended to a second approximation, and thus made an important advance in [[planetary theory]]. The memoir is remarkable inasmuch as it roused Lagrange, after an interval of inactivity, to compose in his old age one of the greatest of his memoirs, entitled ''Sur la théorie des variations des éléments des planètes, et en particulier des variations des grands axes de leurs orbites''. So highly did he think of Poisson's memoir that he made a copy of it with his own hand, which was found among his papers after his death. Poisson made important contributions to the theory of attraction.
His name is one of the [[List of the 72 names on the Eiffel Tower|72 names inscribed on the Eiffel Tower]].
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Poisson desenvolveu o expoente de Poisson, usado na [[transformação adiabática]] de um [[gás]]. Este expoente é a razão entre a capacidade térmica molar de um gás a [[pressão]] constante e a capacidade térmica [[mol]]ar de um gás a [[volume]] constante. A lei de transformação adiabática de um gás diz que o produto entre a pressão de um gás e o seu volume elevado ao expoente de Poisson é constante.
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