Louis was born in Paris on 3 January 1777. He attended the school of Lycée Louis-le-Grand for secondary preparatory education for entrance to the famous École Polytechnique. In October 1794, at age 17, he took the École Polytechnique entrance exam and failed the algebra section but was still accepted. A student there for two years, he left in 1797 to study at École des Ponts et Chaussées to become a civil engineer. Although now on course for the practical and secure professional study of civil engineering, he discovered his true passion, abstract mathematics. Poinsot thus left the École des Ponts et Chaussées and civil engineering to become a mathematics teacher at the secondary schoolLycée Bonaparte in Paris, from 1804 to 1809. From there he became inspector general of the Imperial University of France. He shared the post with another famous mathematician, Delambre. On 1 November 1809, Poinsot became assistant professor of analysis and mechanics at his old school the École Polytechnique. During this period of transitions between schools and work, Poinsot had remained active in research and published a number of works on geometry, mechanics and statics so that by 1809 he had an excellent reputation. By 1812 Poinsot was no longer directly teaching at École Polytechnique using substitute teacherReynaud, and later Cauchy, and lost his post in 1816 when they re-organized, but he did become admissions examiner and held that for another 10 years. He also worked at the famous Bureau des Longitudes from 1839 until his death. On the death of Joseph-Louis Lagrange in 1813, Poinsot was elected to fill his place at the Académie des Sciences. In 1840 he became a member of the superior council of public instruction. In 1846 he was awarded an Officer of the Legion of Honor, and on the formation of the Senate in 1852 he was chosen a member of that body. Poinsot was elected Fellow of the Royal Society of London in 1858. He died in Paris on 5 December 1859. From the diary of Thomas Hirst, 20 December 1857:
Poinsot was the inventor of geometrical mechanics, which showed how a system of forces acting on a rigid body could be resolved into a single force and a couple. Previous work done on the motion of a rigid body had been purely analytical with no visualization of the motion, and the great value of the work, as Poinsot says, it enables us to represent to ourselves the motion of a rigid body as clearly that as a moving point. In particular he devised, what is now known as, Poinsot's construction. This construction describes the motion of the angular velocity vector of a rigid body with one point fixed. He proved that the endpoint of the vector moves in a plane perpendicular to the angular momentum of the rigid body. He discovered the four Kepler-Poinsot polyhedra in 1809. Two of these had already appeared in Kepler's work of 1619, although Poinsot was unaware of this. The other two are the great icosahedron and great dodecahedron, which some people call these two the Poinsot solids. In 1810 Cauchy proved, using Poinsot's definition of regular, that the enumeration of regular star polyhedra is complete. Poinsot worked on number theory studying Diophantine equations. However he is best known for his work in geometry and, together with Monge, regained geometry's leading role in mathematical research in France in the 19th century. Poinsot also contributed to the importance of geometry by creating a chair of advanced geometry at the Sorbonne in 1846. Poinsot created the chair for Chasles which he occupied until his death in 1880.
