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Page:Popular Science Monthly Volume 4.djvu/556

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the positions of the wings of the gull in the course of a complete oscillation:

Fig. 11.
PSM V04 D556 Gulls flying.jpg
Showing the more or less Perpendicular Direction of the Stroke of the Wing in the Flight of the Gull.

When the down-stroke is completed the bird has been raised, but is lowered again when the wings have attained their maximum elevation. Thus it is seen how directly gravity aids in flight. The body is the weight; the wings are long levers attached to it at one end; the air is the fulcrum. Fig. 12 shows the undulatory track of a flying-bird:

Fig. 12.
PSM V04 D556 Bird wing strokes.jpg
Bird at a; down-stroke of the wing, b, lifts the hird to e—the track of the bird being in direction of the arrows.

The instant the descent of the wing begins, the body moves upward and forward; but it is shown by the author that some forward motion results also from the up-stroke. Certain it is that the upward movement must not counteract the other. There is no provision for

Fig. 13.
PSM V04 D556 Left wing of the albatross.jpg
Left Wing of the Albatross.d, e, f, Anterior or thick margin of pinion; b, a, c, Posterior or thin margin, composed of the primary (b), secondary (a), and tertiary (c) feathers.

waste of energy. The form of the upper surface of the wing is convex, the under surface being concave. The value of this will be apparent, as the Duke of Argyle suggests, if we attempt to move the