POINSOT, LOUIS (1777-1859), French mathematician, was born at Paris on the 3rd of January 1777. In 1794 he became a scholar at the Ecole Polytechnique, which he left in 1796 to act as a civil engineer. In 1804 he was appointed professor of mathematics at the Lycee, in 1809 professor of analysis and mechanics, and in 1816 examiner at the Ecole Polytechnique. On the death of J. L. Lagrange, in 1813, Poinsot was elected to his place in the Academic des Sciences; and in 1840 he became a member of the superior council of public instruction. In 1846 he was made an officer of the Legion of Honour; and on the formation of the senate in 1852 he was chosen a member of that body. He died at Paris on the sth of December 1859.
Poinsot's earliest work was his Eltmens de statique (1803; 9th edition, 1848), in which he introduces the idea of statical couples and investigates their properties. In the Thtorie nouvelle de la rotation des corps (1834) he treats the motion of a rigid body geometrically, and shows that the most general motion of such a body can be represented at any instant by a rotation about an axis combined with a translation parallel to this axis, and that any motion of a body of which one point is fixed may be produced by the rolling of a cone fixed in the body on a cone fixed in space. The previous treatment of the motion of a rigid body had in every case been purely analytical, and so gave no aid to the formation of a mental picture of the body's motion; and the great value of this work lies in the fact that, as Poinsot himself says in the introduction, it enables us to represent to ourselves the motion of a rigid body as clearly as that of a moving point. In addition to publishing a number of works on geometrical and mechanical subjects, Poinsot also contributed a number of papers on pure and applied mathematics to Lionmile's Journal and other scientific periodicals.
See J. L. F. Bertrand, Discours aux funerailles de Poinsot (Paris, 1860).
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)