The 776th Lecture was read before the Society on the 31st March 1949 at the Institution of Civil Engineers, Great George Street, London, S.W.I. Dr. H. Roxbee Cox, B.Sc, D.I.C., F.R.Ae.S., F.I.Ae.S., President of the Society, introduced the lecturer, Dr. P. B. Walker, M.A., F.R.Ae.S., Head of the Structures Department at the Royal Aircraft Establishment.