Those who search the book in the h0pe of finding a strictly lo l and mathematical theory of the behaviour of a screw pro ller If be disappointed. Those, however, who read with the object of earning how to design a propeller for actual use, will, it is hoped, he more satisfied. It is for such readers that the book is intended. For the former task, I have neither the inclination nor the mathematical knowledge. As regards the latter, it was my fortune during the war to be in charge, first at the Admiralty and later at the Air Ministry, of technical work in connection with propellers for aircraft, and it therefore happened that many of the prepellers used on British aeroplanes, seaplanes and ships were designed either by In self or under my direct supervision. The experience gained is my just' cation for attempting this work. Referring to my work with the British Government Departments, this opportunity is taken of recognizing the help given by all the members of my staff, but in particular I would mention that of Major F. E. Pollard, arcs, who ably seconded me throughout practically the whole of the time during which I held the post.