In the stability investigations which the late Captain Ferber published in the Revue d'Artillerie, the sustaining and other surfaces of an aeroplane were in certain cases taken to be represented, for dynamical purposes, by a system of three plane resisting laminæ fixed mutually at right angles. Unfortunately, however, such a system cannot in general be made equivalent to a collection of surfaces, such as those of an aeroplane, with the result that Captain Ferber's investigation failed to give the correct conditions of lateral stability. At the same time, Ferber's system of three orthogonal planes is so convenient, especially for forming a general idea of the effects of wind gusts on an aeroplane, that it is desirable to investigate conditions and limitations under which such a representation is valid. The desirability of a further investigation of the forces and couples acting on a system of resisting surfaces of a general character was foreshadowed in “Stability in Aviation,” and a more detailed discussion of the problem has now become necessary in order to prepare the way for further studies in the rigid dynamics of the motions of aeroplanes or of systems resembling them.