§ 124. Modified Systems.—In the examples given in Figs. 70, 71, 72, 73, 74, there are in all cases abrupt motions of the fluid at certain points, such as could not occur in practice where a real viscous fluid such as air is concerned; the stream lines that most nearly fulfil the necessary conditions are those belonging to the elliptical cylinder (Fig. 74).
If we select from Fig. 74 a pair of stream lines possessing the requisite smoothness of curvature as the boundary of a supposed aerofoil, and, having truncated the fore and aft extremities,
Fig. 78.
proceed to whittle away the abruptness of the ends so formed (Fig. 76), we obtain a possible wing section whose form, derived entirely from theoretical considerations, bears an unmistakable resemblance to an actual section taken through the thick of the wing of one of the larger soaring birds. The whittling process is supposed carried out just as would be done in the case of a plank, originally sawn with square edges, to which it is desired to give a stream line form (Fig. 77).
A different and perhaps not quite so legitimate subterfuge is employed in Fig. 78, in which the space enclosed within the dotted line is supposed to contain uniform rotation. This
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