Thanks to the pioneer work of Lanchester and Prandtl everyone seriously interested in aeronautics has nowadays sufficiently clear ideas about the power and the associated drag which are essential for the support of an aeroplane in flight. This induced drag, as it is now called, can be calculated with an accuracy which is ample for most practical purposes, and even with the very clean aeroplanes of the present time it contributes, at and above cruising speed, a minor part only to the total drag. We are equally familiar with the idea that the remainder of the drag—the drag involved in getting the aeroplane through the air as opposed to holding it up—can be reduced, by careful design, nearly to the mere skin-friction of the air rubbing over exposed surfaces; the reason being, of course, that the pressures on the surface of a well streamlined aeroplane are so distributed that their horizontal components form an almost completely balanced system.