XH INTRODUCTORY. 



of the body are useful to press upon the air, which acts as on 

 a paper kite. The base of the wing also, in the bird, is broad, 

 and provided with feathers, which form a broad surface, on 

 which the air presses with a force and method very efficacious 

 in supporting the bird. Fig. 12 gives an idea of this disposition 

 of the wing at the active and passive time in a bird. 



The inner half of the wing is the passive part of the organ, 

 while the external half, that which strikes the air, is the active 

 part. A fly's wing makes 330 revolutions in a second, executing 

 consequently 660 simple oscillations ; it ought at each time to 

 impress a lateral deviation of the body of the insect, and destroy 

 the velocity that the preceding oscillation has given it in a con- 

 trary direction. ' So that by this hypothesis the insect in its 

 'flight only utilizes fifty to one hundred parts (or one-half) of 

 the resistance that the air furnishes it. 



In the bird (Fig. 13), at the time of lowering the wings, 

 the oblique plane which strikes the air, -in decomposing the 

 resistance, produces a vertical component which resists the 

 weight of the body, and a horizontal component which imparts 

 swiftness. The horizontal component is not lost, but is utilized 

 during the rise of the wing, as in a paper kite when held in the 



13. A bird on the wing. 



air against the wind. Thus the bird utilizes seventy-five out 

 of one hundred parts of the resistance that the air furnishes. 

 The style of flight of birds is, therefore, theoretically superior 

 to that of insects. As to the division of the muscular force 

 between the resistance of the air and the mass of the body of 

 the bird, we should compare the exertion made in walking on 



