"It is evident that in order to produce this kind of deformation it is sufficient to apply one positive and one negative moment increment, each of the same magnitude dM, simultaneously about each of the rectangular axes x and y, respectively. These moments produce elongations and shortenings in the exterior and interior fibers of the plate or shell independently in two directions at right angles to each other in the same manner as in a beam but none whatever are produced in the neutral surface, so that the resistance that is offered to this bending arises from the resistance which the fibers offer to such elongation and shortening. The principal curvatures of the surface after bending will be 1/R + d(1/R) and 1/R - d(1/R) respectively."