Let me commence with an explanation and an apology. About a year ago we were discussing the Annual Report of the Advisory Committee for Aeronautics for 1918é19, and I suggested that in that report was ample material for an interesting Paper dealing in great measure with advances during the war; material which up to then had been strictly confidential in character. Some few months ago an invitation came to me from your Council to read such a Paper, and I hastily accepted.

Six weeks ago I sat down to put my ideas in form and wrote some few pages when it suddenly occurred to me that Prof. Bairstow, in his Wilbur Wright lecture last year, had dealt with the subject in a most able and complete way and with very full knowledge, and that I had but little to add to his admirable exposition.