No one who has the honour to serve on the Council of this Society, and can therefore be tackled in person once in every month, can long avoid the additional but more alarming honour of being called upon to lecture. When I in my turn was brought to bay, I cast about for a suitable subject, but could not satisfy myself that any of the work in which I happen personally to be engaged had a sufficiently general appeal. I therefore decided to take advantage of my association with the Aerodynamics Department of the National Physical Laboratory, and to base my lecture on the work of other men. I want to make this clear at the outset, because I may omit in some instances to attribute credit where credit is due, and at all events it must be understood that the credit does not belong to me.