POISSON, Traité de Mécanique, 1811

First edition.
Siméon Denis Poisson (1781-1840), French physicist and mathematician, it is in this Treatise on Mechanics that he introduces the coefficient which bears his name and which is one of the fundamental elements of the theory of elasticity. He demonstrated using molecular theory that this coefficient should have a value equal to 0.25. Value confirmed by measurements since most metals have a Poisson's ratio close to 0.3.