Page:Popular Science Monthly Volume 4.djvu/556

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



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:



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



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