has long divided elasticians into two opposing factions. The uni-constant isotropy of Navier and Poisson had been questioned
by Cauchy, and was now severely criticised by Green
and Stokes.
Barré de Saint-Venant (1797–1886), ingénieur des ponts et
chaussées, made it his life-work to render the theory of elasticity of practical value. The charge brought by practical engineers, like Vicat, against the theorists led Saint-Venant to
place the theory in its true place as a guide to the practical man. Numerous errors committed by his predecessors were removed. He corrected the theory of flexure by the consideration of slide, the theory of elastic rods of double curvature by the introduction of the third moment, and the theory of torsion by the discovery of the distortion of the primitively plane section. His results on torsion abound in beautiful graphic illustrations. In case of a rod, upon the side surfaces of which no forces act, he showed that the problems of flexure and torsion can be solved, if the end-forces are distributed over the end-surfaces by a definite law. Clebsch, in his
Lehrbuch der Elasticität, 1862, showed that this problem is
reversible to the case of side-forces without end-forces. Clebsch[]68 extended the research to very thin rods and to very thin plates. Saint-Venant considered problems arising in the scientific design of built-up artillery, and his solution of them differs considerably from Lamé's solution, which was popularised by Rankine, and much used by gun-designers. In Saint-Venant's