Since this paper is prepared at the request of the Council of the Royal Aeronautical Society–a body which represents in the main the scientific aspect of aviation–I have endeavoured in preparing it to remember that I must infuse into its composition something of worth to the technician as well as to the pilot. If I manage to convey to the former some of the problems which confront the latter and which are not quite the same as those with which he is himself faced, I may have rendered some small service in response to the honour which the Council of the Society has done me in asking me to undertake this duty.