32 FLIGHT OF BIRDS AND INSECTS. 
which the air presses with a force and method very effi cacious in 
supporting the bird. Fig. 13 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 fys wing 
99 
makes 330 revolutions 
Fig. 12. 
in a second, executing 
consequently 660 sim- 
ple oscillations ; it 
ought at each time to 
impress a lateral devi- 
Trajectory of a bird’s wing. ation 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 resist- 
ance that the air furnishes it. 
In the bird, at the moment 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 bird's body, 
and a horizontal component which imparts swiftness. The hori- 
zontal component is not lost, but is utilized during the rise of the 
wing, as in a paper kite when held in the 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 
Fig. 13. 
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 
sre made in walking on sand, for example, as compared with 
walking on marble. This is easy to measure. When a fish strikes 
