Before reading my paper I wish to express my thanks to the President and Council of the Society, and my great appreciation of the privilege accorded to me of speaking before this expert gathering.

My ambitions with regard to the subject of this paper were originally very farreaching, but I soon realised that it would be impossible to cover the enormous field falling under the heading of “ rotating wings,” and at the same time to advance sufficiently into detail to escape the criticism that this was merely a new collection of facts, already known and very general in terms, and therefore possibly only of interest to students of the history of aviation.