water and inverted (Fig. 80), the pressure region being on its upper surface, and the vortices being evidenced by the dimples in the surface of the water. This experiment shows that in practice the vortices are continually breaking up and being left behind as fragmentary eddies. If the experiment is tried in a comparatively narrow vessel the eddies are actually found to have retrograde motion, owing to the influence of their own "reflexion" in the sides of the vessel. If the experiment were tried in an open expanse of water on a large scale it would probably give more perfect results.
Fig. 80.
It would appear probable that in a fluid of very small viscosity vortices springing from the extremities of the aerofoil and terminating on the boundary surface may be permanent; in fact, we might regard the whole system as a single-vortex filament, with both its extremities situated on the boundary, and enclosing the aerofoil as an incident. Following out this idea, we should obtain, for an inviscid atmosphere, a system consisting primarily of a vortex hoop or halfring, loaded in the centre by the aerofoil (Fig. 81), and whose energy will be perfectly conserved, the aerofoil and its supporting vortex lying in a plane at right angles to the direction of flight. Such a system in a fluid that is truly inviscid would be uncreatable and indestructible, just as in such a fluid a vortex ring is uncreatable and indestructible. The system of static forces called into play is represented diagrammatically in Fig. 82, in which the tension due to the vortex motion is represented by an irregular polygon following the vortex core, the forces at right angles being those due, on the one hand to the load on the aerofoil, and on the other to the cyclic motion round the vortex core in translation,
174
