These points are illustrated at fig. 80, where _a b_ represents the short axis (root of wing) with a radius _e f_; _c d_ representing the long axis (anterior margin of wing) with a radius _g p_.
Fig. 80 also shows that, in the wing of the bird, the individual, primary, secondary, and tertiary feathers have each what is equivalent to a long and a short axis. Thus the primary, secondary, and tertiary feathers marked _h_, _i_, _j_, _k_, _l_ are capable of rotating on their long axes (_r s_), and upon their short axes (_m n_). The feathers rotate upon their long axes in a direction from below upwards during the down stroke, to make the wing impervious to air; and from above downwards during the up stroke, to enable the air to pass through it. The primary, secondary, and tertiary feathers have thus a distinctly valvular action.[76] The feathers rotate upon their short axes (_m n_) during the descent and ascent of the wing, the tip of the feathers rising slightly during the descent of the pinion, and falling during its ascent. The same movement virtually takes place in the posterior margin of the wing of the insect and bat.
[76] The degree of valvular action varies according to circumstances.
_The Wing vibrates unequally with reference to a given Line._--The wing, during its vibration, descends further below the body than it rises above it. This is necessary for _elevating purposes_. In like manner the posterior margin of the wing (whatever the position of the organ) descends further below the anterior margin than it ascends above it. This is requisite for _elevating and propelling purposes_; the under surface of the wing being always presented at a certain upward angle to the horizon, and acting as a true kite (figs. 82 and 83, p. 158. Compare with fig. 116, p. 231). If the wing oscillated equally above and beneath the body, and if the posterior margin of the wing vibrated equally above and below the line formed by the anterior margin, much of its elevating and propelling power would be sacrificed. The tail of the fish oscillates on either side of a given line, but it is otherwise with the wing of a flying animal. The fish is of nearly the same specific gravity as the water, so that the tail may be said only to propel. The flying animal, on the other hand, is very much heavier than the air, so that the wing requires both to propel and _elevate_. The wing, to be effective as an _elevating organ_, must consequently be vibrated rather below than above the centre of gravity; at all events, the intensity of the vibration should occur rather below that point. In making this statement, it is necessary to bear in mind that the centre of gravity is _ever varying_, the body rising and falling in a series of curves as the wings ascend and descend.