th beats up and down. In the bird, on the contrary, one active
beat only is necessary, the down beat. It creates at that time all the
motive force that will be dispensed during the entire revolution of the
wing. This difference is due to the difference in form of the wing. The
difference between the two forms of flight is shown by an inspection of
the two accompanying figures (11, 12). An insect's wing is small at the
base and broad at the end. This breadth would be useless near the body,
because at this point the wing does not move swiftly enough to strike
the air effectively. The type of the insectean wing is designed, then,
simply to strike the air. But in the bird the wing plays also a passive
role, _i. e._, it receives the pressure of the air on its under side
when the bird is projected rapidly onward by its acquired swiftness. In
these conditions the whole animal is carried onward in space; all the
points of its wing have the same velocity. The neighboring regions 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 contrary 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.

[Illustration: 13. A bird on the wing.]

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 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 f