The title chosen for this paper is a somewhat ambitious one, as prophecies in any sphere of engineering are always dangerous. Especially is this so in such a young and progressive branch as aeronautics, where changes are rapid, and developments considered modern and up-to-date to-day, may, in a few years, be thrown aside as being obsolete. However, when I received the honour of an invitation to read a paper before the Royal Aeronautical Society, I chose this title with “malice aforethought,” if only with the idea of inviting criticism and discussion.