# 1911 Encyclopædia Britannica/Poisson, Siméon Denis

**POISSON, SIMÉON DENIS** (1731–1848), French mathematician,
was born at Pithiviers in the department of Loiret, on the
21st of June 1781. His father, Siméon Poisson, served as a
common soldier in the Hanoverian wars; but, disgusted by the
ill-treatment he received from his patrician officers, he deserted.
About the time of the birth of his son, Siméon 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. Poisson was first sent to an uncle, a
surgeon at Fontainebleau, and began to take lessons in bleeding
and blistering, but made little progress. Having given promise
of mathematical talent he was sent to the École Centrale 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^{[1]}:—

" Petit Poisson deviendra grand

Pourvu que Dieu lui préte vie."

In 1798 he entered the École Polytechnique at Paris as first
in his year, and immediately began to attract the notice of the
professors of the school, who left him free to follow the studies
of his predilection. In 1800, less than two years after his entry,
he published two memoirs, one on E. 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 S. F. Lacroix and A. M. Legendre, who recommended that
it should be published in the *Recueil des savants étrangers*, an
unparalleled honour for a youth of eighteen. This success at
once procured for Poisson an entry into scientific circles. J. L.
Lagrange, whose lectures on the theory of functions he attended
at the École Polytechnique, early recognized his talent, and
became his friend; while P. S. 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 he was successively appointed. Immediately
after finishing his course at the École Polytechnique he was
appointed *repetiteur* there, an office which he had discharged as
an amateur while still a pupil 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 suppléant* in 1802, and full professor in
succession to J. Fourier in 1806. In 1808 he became astronomer
to the Bureau des Longitudes; and when the Faculté des Sciences
was instituted in 1809 he was appointed *professeur de la mécanique*
*rationelle*. He further became member of the Institute
in 1812, examiner at the military school at St Cyr in 1815, leaving
examiner at the École Polytechnique in 1816, councillor of the
university in 1820, and geometer to the Bureau des Longitudes
in succession to P. S. Laplace in 1827. 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. When the Bourbons were restored, his
hatred against Napoleon led him to become 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 volunteering 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 him with the loss of all his honours;
but this disgrace to the government of Louis Philippe was
adroitly averted by F. 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, 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 École Polytechnique. 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 n'est bonne qu'a deux choses—a faire des mathématiques
et à les professeur; " 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 application
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. Next (perhaps in the
opinion of some first) in importance stand the memoirs on celestial
mechanics, in which he proved himself a worthy successor to
P. S. Laplace. The most important of these are his memoirs “ Sur
les inégalltés séculaires des moyens movements 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 *Connaiss. d. temps* (1821), &c.;
and “ Sur la movement de la terre autour de son centre de gravité,"
in *Mém. d. l'acad.* (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 an 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 his
memoirs, viz. that *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 well-known correction of Laplace's
partial differential equation for the potential was first published
in the *Bulletin de la société philomatique* (1813). His two most
important memoirs on the subject are “ Sur l'attraction des
sphéroides ” (*Connaiss. d. temps*, 1829), and “ Sur l'attraction d'un
ellipsoid homogene ” (*Mém. d. l'acad.*, 1835). In concluding our
selection from his physical memoirs we may mention his memoir
on the theory of waves (*Mém. d. l'acad.*, 1825).

In 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 L. Dirichlet
and B. Riemann on the same subject; these are to be found in the
*Journal* of the École Polytechnique from 1813 to 1823, and in the
*Memoirs de l'académie* for 1823. In addition we may also mention
his essay on the calculus of variations (*Mem. d. l'acad.*, 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 did not live to complete. Among these
may be mentioned his *Traité de mécanique* (2 vols. 8vo, 1811 and
1833), which was long a standard work; *Théorie nouvelle de l'action*
*cappillaire* (4to, 1831); *Théorie mathématrque de la chaleur* (4to, 1835);
*Supplément* to the same (4to, 1837); *Recherches sur la probabilité des*
*jugements en matières criminelles*, &c (4to, 1837), all published at Paris.

See F. Arago, *Biographie de Poisson*, read before the Académie des Sciences on the 16th of December 1850.

- ↑ This prediction is sometimes attributed to Laplace.