In April, 1919, the lecturer had the honour to address the Society on the subject of Air Navigation, the Chairman being Air Commodore Brooke Popham, who had just been appointed as the first “ Director of Research ” at the Air Ministry. During the introductory remarks made by the Chairman at that lecture, an aircraft was described as being an “ encumbrance ” unless “ one could get it where it was wanted.” Now since it happens that one of the main purposes for which aircraft instruments exist is to enable such craft to be got “ where they are wanted,” the importance of our subject to-night will not be denied. Moreover, according to a recent most able paper in our Journal, Leonardo da Vinci himself stated his conviction that “ Instrumental or mechanical science is very noble, and useful beyond all others.“