_The Wing acts upon yielding Fulcra._--The chief peculiarity of the wing, as has been stated, consists in its being a twisted flexible lever specially constructed to act upon yielding fulcra (the air). The points of contact of the wing with the air are represented at _a b c d e f g h i j k l_ respectively of figs. 84 and 85, p. 160; and the imaginary points of rotation of the wing upon its long and short axes at 1, 2, 3, and 4 of the same figures. The assumed points of rotation advance from 1 to 3 and from 2 to 4 (_vide_ arrows marked _r_ and _s_, fig. 85); these constituting the steps or pulsations of the wing. The actual points of rotation correspond to the little loops _a b c d f g h i j l_ of fig. 85. The wing descends at _A_ and _C_, and ascends at _B_ and _D_.
_The Wing acts as a true Kite both during the Down and Up Strokes._--If, as I have endeavoured to explain, the wing, even when elevated and depressed in a strictly vertical direction, inevitably and invariably darts forward, it follows as a consequence that the wing, as already partly explained, flies forward as a true kite, both during the down and up strokes, as shown at _c d e f g h i j k l m_ of fig. 88; and that its under concave or biting surface, in virtue of the forward travel communicated to it by the body in motion, is closely applied to the air, both during its ascent and descent--a fact hitherto overlooked, but one of considerable importance, as showing how the wing furnishes a persistent buoyancy, alike when it rises and falls.
In fig. 88 the greater impulse communicated during the down stroke is indicated by the double dotted lines. The angle made by the wing with the horizon (_a b_) is constantly varying, as a comparison of _c_ with _d_, _d_ with _e_, _e_ with _f_, _f_ with _g_, _g_ with _h_, and _h_ with _i_ will show; these letters having reference to supposed transverse sections of the wing. This figure also shows that the _convex_ or non-biting surface of the wing is always directed upwards, so as to avoid unnecessary resistance on the part of the air to the wing during its ascent; whereas the _concave_ or biting surface is always directed downwards, so as to enable the wing to contend successfully with gravity.