The air flow passing the front edge will be deflected gradually from its horizontal direction and led downward. The flow gains a horizontal velocity component without any sudden impact. The explanation based on the relation between aerodynamic lift and the acceleration of a downward air flow prevailed in textbooks in this simple form until 1920 without having been elaborated further. By approximately the year 1920, when aviation gained much interest in science and public, the explanation based on Bernoulli's law appeared and displaced the explanation based on reaction forces. In any case it was necessary that the explanation of lift using Bernoulli's law had to be complemented by giving a cause for the higher streaming velocity of the wing's upper surface.
Dotted lines connect air volumes originally adjacent.
With this diagram Prandtl tried to show that air at the inner layers stick to the surface. In respect to one point this diagram is not correct. It indicates that the volumes remain at the same vertical position and it indicates that originally adjacent air meets again at the end of the airfoil. This diagram miss the phase shift shown in figure 2, which seems not to have been observed by Prandtl at that time. Diagrams of this type might have misled scolars to the hypothesis that adjacent air has to meet again after passing the airfoil.