POINSOT, Louis (1777-1859), mathematician, was born at Paris January 3, 1777. In 1794 he became a scholar at the Polytechnic School, which he left in 1796 to act as a civil engineer. In 1804 he was appointed professor of mathematics at the Lyceum, in 1809 professor of applied mathematics and in 1816 examiner at the Polytechnic School. On the death of Lagrange in 1813, Poinsot was elected to his place in the French Academy; 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, December 5, 1859. Poinsot’s earliest work was his Dlêmens de Statique, in which he introduces the idea of statical couples and investigates their properties. In the Théorie Nouvelle de la Notation des Corps 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. Poinsot also contributed a number of papers on pure and applied mathematics to Liouville ’ s Journal and to the Journal of the Polytechnic School.