In surveying the mass of aerodynamic data presented to the designer of aeroplanes, one is struck by the fact that so much attention has been devoted to quantitative measurements upon bodies in a flow of air, so little to the equally important qualitative measurements. It is known that any body subjected to a flow of air is the means of creating eddies, yet relatively little attention has been given to the study of these eddies. Possibly the assumption has been that such eddies are of little importance since in the case of the aeroplane thev are left behind the body that is causing them and so cannot exert a secondary effect upon it. In fact, an aeroplane in flight is the means of creating a vast number of whirls and eddies in the air, the majority of which are left harmlessly astern, some of which react upon its own structure. Of these whirls and eddies practically nothing is known. Indeed, they are so little considered that it comes almost as a surprise to a practical designer to see the vast disturbances that become evident behind an aeroplane “ skywriting.”