"Whenever very thin rods or plates are employed in constructions it becomes necessary to consider the possibility of , and thus there arises the general problem of elastic stability. [T]he first investigations... of this kind were made by Euler and Lagrange. ...In all [isolated problems] two modes of equilibrium with the same type of external forces are possible, and the ordinary proof of the determinacy of the solution of the equations of Elasticity is defective."