The whole subject of cyclic motion in the case of a viscid fluid has not been thoroughly investigated. It is evident that to a certain extent the restrictions proper to the inviscid fluid must apply, but since we can generate rotation we are able to induce vortices with a freedom not possible when viscosity is absent.
Basing our argument on the facts as already ascertained, it is evident that if we continuously generate vortices at the right and left hand extremities of the aerofoil, as in Fig. 79, we can regard these vortices as forming in effect, taken in conjunction with the
Fig. 83.
aerofoil itself, an obstacle to connectivity, so that, although the vortex dies away after a while, it persists as long as is necessary to permit of a cyclic system being established and maintained.
It is probable that these terminal vortices do not each actually consist of a single vortex but rather of a multiple system of smaller vortices; especially should this be the case with the larger birds, and similarly for mechanical models of any size.
We can conceive that these vortices are formed after the manner indicated in Fig. 83, in which an aerofoil is represented in end elevation with the flow indicated diagrammatically. We
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