WALKING, SWIMMING, AND FLYING. 



539 



concave or convex side of an umbrella against the wind ; one side 

 holds the air, the other discharges it. The wing of the albatross shows 

 how completely the feathers are adjusted, on the upper side, to avoid 

 any hold upon the ai . 



This arrangement, with the flexibility and screw-like motion of the 

 wings of the gull, shown in Fig. 14, explains the exceedingly small re- 

 sistance experienced in the upward movement, and also the forward 

 impetus which it communicates. 



Fig. 14. 



Shows the Twisted Levers or Screws formed by the Wings op the Guxl. 



It is in the down-stroke, or, as Dr. Pettigrew insists, in the begin- 

 ning of the down-stroke, that force is chiefly expended. This move- 

 ment is Essentially a muscular act, and by this force alone no bird 

 could sustain long-continued flight. The lark, whose flight is upward, 

 soon descends to the earth. It lifts itself against gravity, simply by 

 expenditure of vital force. But, the moment forward motion is attained, 

 other forces relieve the strain upon the pinions, and their inclined sur- 

 faces convert gravity into a propelling power. It is obvious, however, 

 that flight is attended with considerable muscular exertion. Migrat- 

 ing birds alight in unsuitable positions for rest, but the swallow will 

 fly 1,000 miles in a single journey, and the condor attains an altitude 

 of six miles. 



The heron will strike the air 60 times in a minute, which, with 60 

 up-strokes, gives 120 movements, and this is continued through long 



Fig. 15. 



The Gray Heron in Full Flight. 



flights ; and the same is true of many ducks and land-birds which 

 strike the air with extreme and apparently exhausting rapidity. So 

 swift are the motions of the wings of the humming-bird that they 

 produce only a blurred spot before the eye. 



That wings act as true kites, when in motion, is a familiar obser- 



