106 



ANIMAL LOCOMOTION. 



the horizon,^ rotates upon its anterior margin as an axis during 

 its descent and causes its under surface to make a gradually 

 increasing angle with the horizon, the posterior margin (fig. 

 53, 6*) in this movement descending beneath the anterior 

 one. A similar but opposite rotation takes place during the 

 up stroke. The rotation referred to causes the wing to twist 

 on its long axis screw-fashion, and to describe a figure-of-8 

 track in space, one-half of the figure being described during 

 the ascent of the wing, the other half during its descent. 

 The twisting of the wing and the figure-of-8 track described 

 by it Avhen made to vibrate, are represented at fig. 53. 

 The rotation of the wing on its long axis as it ascends and 

 descends causes the under surface of the wing to act as a 

 kite, both during the up and down strokes, provided always 

 the body bearing the wing is in forward motion. But the 

 upper surface of the wing, as has been explained, acts when 

 the wing is being elevated, so that both the upper and under 

 surfaces of the wing are efficient during the up stroke. When 

 the wing ascends, the upper surface impinges against the air; 

 the under surface impinging at the same time from its being 

 carried obliquely forward, after the manner of a kite, by the 

 body, which is in motion. During the down stroke, the 

 under surface only acts. * The wing is consequently effective 

 both during its ascent and descent, its slip being nominal in 

 amount. The wing acts as a kite, both when it ascends and 

 descends. It acts more as a propeller than an elevator during 

 its ascent ; and more as an elevator than a propeller during 

 its descent. It is, however, effective both in an upward and 

 downward direction. The efficiency of the wing is greatly in- 

 creased by the fact that when it ascends it draws a current of 

 air up after it, which current being met by the wing^ during 

 its descent, greatly augments the power of the down stroke. 

 In like manner, when the wing descends it draws a current 

 pf air down after it, which being met by the wing during its 

 ascent, greatly augments the power of the up stroke. These 

 induced currents are to the wing what a stiff autumn breeze is 

 to the boy's kite. The wing is endowed with this very re- 



^ In some cases the posterior margin is slightly elevated above the horizon 

 (fig. 53, g). 



