When your Secretary, shortly before Easter, wrote to ask me to give the Wilbur Wright Lecture he made me give him a title before writing the lecture. I therefore had to think of one which would fit any lecture I might conceivably want to give, so that when I came back after Easter I should be able to pick out one of those subjects connected with aeronautics with which I have been specially concerned, and discuss the results arrived at. When, however, I came to sit down to write the lecture itself it struck me that it might be more interesting to a general audience if I followed the line which is perhaps most obviously indicated by my title, and directed my remarks towards tracing and comparing some of the principal lines of argument which have been used in aeronautical research, without going particularly deeply into the results.