Note 69 — PROGRESSION IN OR THROUGH THE AIR.

In this diagram I have purposely represented the right wing by a straight rigid rod. The natural wing, however, is curved, flexible, and elastic. It likewise moves in curves, the curves being most marked towards the end of the up and down strokes, as shown at m n, o p. The curves, which are double figure-of-8 curves, are obliterated towards the middle of the strokes (a r). This remark holds true of all natural wings, and of all artificial wings properly constructed. The curves and the reversal thereof are necessary to give continuity of motion to the wing during its vibrations, and what is not less important, to enable the wing alternately to seize and dismiss the air.