Before beginning my lecture, I should like to say that it gives me particular pleasure to be able to take part, as a guest, in this meeting of the Royal Aeronautical Society. So much important progress in the development of aeronautics has had its origin within the circle of the Royal Aeronautical Society, the oldest society of its kind in the world, and so many successes in the scientific and practical spheres are coupled with the names of its members that all the staff of the D.V.L. regard as a particular honour the friendly interest in our work, expressed by your invitation.