"[C]onsider the bending moments that must be applied to produce such a deformation. It is evident from the fundamental equation EI/R = M of the ordinary theory of thatEI\;d(\frac{1}{R}) = dMis the equation which expresses the relation between the increment (or decrement) of curvature d(1/R) and the increment (or decrement) dM of the applied moment which produces this change of curvature about x (or y) in which the value of I may be calculated in case of only slight curvature just as in a flat plate."