the seat of a lively circulation as indicated by the arrows, the motion of the fluid in the vicinity of the plane being in the direction of flight, and that in the vicinity of the free surface being in the opposite direction. Now the result of this will be to produce a tangential drag in a forward direction; in fact, any skin friction experienced on the upper face or "back" of the plane will be of negative sign; we are thus unable to attribute the “retention” of the dead water to the direct influence of the plane.
On following the matter further it is evident that it is the partial vacuum in and about the region a a a that supplies the necessary 
Fig. 114. reaction to prevent the dead water from being washed away, the lines of flow being at this point in close proximity to one another, as indicated in the figure. We thus find that the back of the plane is not only apparently, but is really, relieved of the frictional drag, which is actually borne in some way dynamically by the fluid itself.
§ 183. Aeroplane Skin Friction. Further Investigation.—The present stage of our explanation cannot be regarded as entirely satisfactory. It would appear to be essential, if we suppose the aeroplane to be maintained in steady motion by an applied force, that all reactions experienced by the fluid must be eventually traceable to the applied force. In the case under consideration
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