**POISSON, SIMEON DENIS** (1781-.1840) a celebrated French mathematician, was born at Pithiviers in the department of Loiret, on the 21st June 1781. His father, Simeon Poisson, served as a common soldier in the Hanoverian wars ; but, disgusted by the ill treatment he received from his patrician officers, be deserted. About the time of the birth of his son Simeon Denis he occupied a small administrative post at Pithiviers, and seems to have been at the head of the local government of the place during the revolutionary period. The infant Poisson was put out to nurse, and concerning his nursing Arago relates the following story, which he had from its hero himself. One day the anxious father went to visit his son, but found that the nurse had gone to the fields. Impatient, he broke into the cottage, and there saw, with painful astonishinent, the object of all his hopes suspended by a small cord to a nail fixed in the wall. This was a precaution on the part of the peasant nurse to prevent her charge from perishing under the teeth of the carnivorous and unclean animals that circulated in the house. Poisson, in telling the story, added: " A gymnastic effort carried me incessantly from one side of the vertical to the other ; and it was thus, in my tenderest infancy, that I made my prelude to those studies on the pendulum that were to occupy so much of my maturer age."

Having survived the perils of infancy, and received the elements of his education (reading and writing) from his father, the question arose what calling be was to follow. It was at first suggested that he should be made a notary ; but the family council, with amusing irony, decided that this profession made too great demands upon the intellect, and surgery was preferred. He was sent to an uncle who exercised this art at Fontainebleau, and forthwith began to take lessons in bleeding and blistering, then the leading branches of a surgeon's practice. To train him in the former, be was set to prick the veins of cabbage leaves with a. lancet, but made little progress; how he sped in the latter he himself relates as follows : - "Once my uncle sent me, with one of my comrades, M. Yanneau, now established in the colonies, to put a blister on the arm of a child ; the next day, when I presented myself to remove the apparatus, I found the child dead ; this event, very common they say, made the most profound impression upon me ; and I declared at once that I would never be either physician or surgeon. Nothing could shake my resolution, and they sent me back to Pithiviers." Here accident and the bent of nature solved the problem that had passed the wisdom of the family council. The elder Poisson, being .a Government official, received a copy of the Journal Polytechnique; the son read it, and soon began unaided to solve the problems propounded there from time to time; and thus his mathematical talent was discovered. He was sent to the Ecole Contrale of Fontainebleau, and was fortunate in having a kind and sympathetic teacher, M. Billy, who, when he speedily found that his pupil was becoming his master, devoted himself to the study of higher mathematics in order to follow and appreciate him, and predicted his future fame by the punning quotation from Lafontaine - " Petit Poisson deviendra grand Pourvu Tie then lui Fete vie."

At the age of seventeen the young provincial, less remarkable for the elegance of his attire than for the profundity of his scientific knowledge, came up to Paris to undergo the entrance examination for the Polytechnic School. He passed first in his year, and immediately began to attract the notice of the professors of the school, who, seeing his obvious genius, excused him from the ordinary drudgery of the curriculum, and left him free to follow the studies of his predilection. The wisdom of this course was soon proved ; for, in 1800, less than two years after his entry, he published two memoirs, one Bezout's method of elimination, the other on the number of integrals of an equation of finite differences. The latter of these meinoirs was examined by Lacroix and Legendre, who recommended that it should be published in the Recueil des Savants Etrangers, an unparalleled honour for a youth of eighteen. This success at once procured for Poison an entry into the Parisian scientific society of the day, the like of which for brilliancy has never elsewhere been seen. Its two kings both patronized him. Lagrange, whose lectures on the theory of functions he attended at the Polytechnic School, early recognized his talent, and became his friend ; while Laplace, in whose footsteps Poisson followed, regarded him almost as his son. The rest of his career, till his death on the 25th of April 1840, was almost entirely occupied in the composition and publication of his many works, and in discharging the duties of the numerous educational offices to which lie was successively appointed. Immediately after finishing his course at the Polytechnic School he was appointed repetiteur there, an office which be had discharged as an amateur while still ft 1/111)11 in the school ; for it had been the custom of his comrades often to resort to his room after an unusually' difficult lecture to hear him repeat and explain it. He was made professeur suppleant in 1802, and full professor in succession to Fourier in 1806. In 1808 he became astronomer to the Bureau des Lok"itudes ; and, when the Faculte des Sciences was instituted in 1809, he was appointed Professeur de la Mecanique Itationelle. He further became member of the Institute in 1812, examiner at the military school at St Cyr in 1815, leaving examiner at the Polytechnic in 1816, councillor of the university in 1820, and geometer to the Board of Longitude in succession to Laplace in 1827.

In 1817 lie married Mademoiselle Nancy de Bardi, daughter of a French family which had emigrated to England, and by ber he had two sons and two daughters.

Poisson was a simple-minded affectionate man. This is seen in the close relations which he kept up with his old teacher M. Billy, who ardently loved and admired his former pupil, and whose presence at the Institute was a well-known sign that Poisson was to read a paper there. Although he never returned to Pithiviers after his entry into the Polytechnic School, he corresponded constantly with his parents, more especially with his mother ; and he regularly sent copies of his memoirs to his father, who read and re-read with unwearied patience the parts of them within his comprehension. His tastes seem to have been of the simplest description ; he took- little exercise, and he had more than a Frenchman's horror of travelling. Arago says that he only travelled once, and that by medical prescription, disguised under the form of some mission connected with the Polytechnic School, and that, after , devoting his savings to the purchase of a beautiful farm in the department of Seine-et-Marne, he never so much as visited it.

It is probable that his simplicity of character had much to do with his passing apparently quite undisturbed through the stormy time in which he lived, a period in which many men of mark lost their heads, and few such escaped without loss of office and fortune. His father, whose early experiences led him to hate aristocrats, bred him in the stern creed of the first republic. Throughout the empire Poisson faithfully adhered to the family principles, and refused to worship Napoleon. Napoleon, however, never interfered with Poisson's promotion. He said once himself that be never did anything uselessly, certainly never committed a useless crime; and he was wise enough to sec that nothing was to be gained by persecut-ing the harmless academician, whose fame he doubtless regarded like that of the other savants of France as an apanage of his own glory. When the Bourbons were restored, his hatred against Napoleon led him to beeoine a Legitimist - a conclusion which says more for the simplicity of his character than for the strength or logic of his political creed.

He was faithful to the Bourbons during the Hundred Days, in fact was with difficulty dissuaded from volunteer-ing to fight in their cause. After the second restoration his fidelity was recognized by his elevation to the dignity of baron in 1825; but he never either took out his diploma or used. the title. The revolution of July 1830 threatened hitn with the loss of all his honours ; but this disgrace to the Government of Louis Philippe was adroitly averted by Arago, who, while his " revocation " was being plotted by the council of ministers, procured him an invitation to dine at the Palais Royale, where he was openly and effusively received by the citizen king, who " remembered " him. After this, of course, his degradation was impossible; he was left in undisturbed possession of all his well-earned appointments ; 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 more than ordinarily successful, as might have been expected from his early promise as a repetiteur at the Polytechnic School. As a scientific worker his activity 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 and applied mathematics. There are two remarks of his, or perhaps two versions of the same remark, that explain how he accomplished so much : one, " La vie West bonne qu' a, deux choses - it faire des mathematiques et a les professer ;" the other, " La vie c'est le travail."

A list of Poisson's works, drawn up by himself, is given at the end of Arago's biography. A lengthened analysis of them would be out of place here, and all that is possible is a brief mention of the more important. There are few branches of mathematics to which he did not contribute something, but it was in the applica-tion of mathematics to physical subjects 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. They have been already repeatedly referred to in the articles ELatutterrv and MAGNETISM (q.v.). Next (perhaps in the opinion of some first) in importance stand the memoirs on celestial mechanics, in which he proved him-self a worthy successor to Laplace. The most important of these are his memoirs " Sur les in6galWs skulaires des moyens motive-mcnts (les planete,' " Sur la variation des constantes arbitraires dans les questions de mkaniquc," both published in the Jourttai of the Polytechnic School, 1809 ; " Sur la libration (le lune," in Conitaiss. (7.. Maps, 1821, &c.; and "Sur la mouvement (le la terve antour de son centre de gravit6," i» Heni. l' A earl., 1827, &c. In the first of these memoirs Poisson discusses the famous question of the stability of the planetary orbits, 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 au important advance in the 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 Ids memoirs, viz., that "Sur la th6orie des variations (Wments des planetes, et en particulier des variations des grands axes de lours orbites." So highly (lid he think of Poisson's memoir that he made a copy of it with his own hand, which VMS found among his papers after his death. Poisson made important contributions to the theory- of attraction. llis well-known eorrection of Laplace's partial differential equation for the. potential was first pnblished in the Bulletin de la SoeieM Philoinalique, 1813. His two most important memoirs on the subject are "Sur l'attraetion des spUroides "(Connaiss. d. Temps, 1829), and "Sur l'attraetion (run ellipsoide hornogene (Meta. d.

183:5). In concluding our selection from his physical memoirs we may mention his memoir on the theory of' waves (Mom. l' Acad., 1825).

In. pnre mathematics, his most importa»t 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; those are to be found in the Journal of the Polytechnic School from 1813 to 1823, and in the Memoirs of the Academy for 1823. lit addition we may also mention his essay on the calculus of variations (Hem.

1833), and his memoirs on the probability of the mean results of observations (Connaiss. d. Temps, 1827, &c.).

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 elitl not live to complete. Among these may be mentioned his T mile. .1keanique, 2 vols. 8vo, 1811 and 1833, which was long a standard work ; Tlic;orie Nouvelle de l' Action Capillaire, 4to, 1831 ; Theorie Malhe'inalique de Chaleur, 4to, 1835; Appleme.nl to the same, 4to, 1837; Ileeherehes sur la probabilile des jugemenls en maliercs &c. 4to, 1837, all published at Paris.

Enough has been said to establish Poisson's fertility as a writer on mathematical subjects, and the question naturally suggests itself, What is his rank among the mathematicians of all ages? Sinee his own age was more productive of great mathematicians than any other the world has yet seen, it is natural to compare him with his contemporaries, chief among whom were Lagrange and Laplace. In so doing we see at once that, although we cannot seat him alongside of these mighty sovereigns, yet it is impossible to deny him the nearest rank to them in the temple of mathematical fame. In confirmation of this judgment, we cannot do better than quote one of them - " I am old," said Lagrange to Poisson one day ; "during my long intervals of sleeplessness I divert myself by making numerical approximations. Keep this one ; it may interest you. Huygens was thirteen years older than Newton, I am thirteen years older than Laplace ; D'Alembert was thirty-two years older than Laplace, Laplace is thirty-two years older than you." Arago, who gives this story, justly remarks that no more delicate way conld be conceived of intimating to Poisson his admission into the inner circle of the fraternity of mathematical (Tains. (G. en.)