FLIGHT OF BIRDS AND INSECTS. 31 
Marey found that a bird’s wing moves in an ellipse, with a pointed 
summit (Fig. 10). The insect beats the air in a distinctly horizon- 
tal plane, but the bird in a vertical plane. The wing of an insect 
is impervious to the air; while the bird’s wing 
resists the air only on its under side. Hence, 
there are two sorts of effects ; in the insect, the up 
and down strokes are active; in the bird, the low- 
ering of the wing is the only active period, though 
the return stroke seems to sustain the bird, the air 
acting on the wing. ‘The bird’s body is horizontal 
when the wing gives a downward stroke; but 
when the beat is upward, the bird is placed in an inclined plane 
like a winged projectile, and mounts up on the air by means of the 
inclined surfaces that it passively offers to the resistance of this 
fluid. 
In an insect, an energetic movement is equally necessary to 
strike the air at both beats up and down. In the bird, on the con- 
trary, one active beat, only, is necessary, the down beat. It cre- 
ates 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 destined, then, 
simply to strike the air. But in the bird the wing plays also 
a passive role, i. e., 
it receives the pres- 
sure of the air on its 
under side, when the 
bird is projected rap- 
idly onward by its ac- 
quired swiftness. In . 
these conditions the Trajectory of an insect’s wing. 
Fig. 11. 
whole animal is carried onward in space; all the points of its 
wing have the same velocity (vitesse). The neighboring regions 
of the body are useful to press upon the air which acts as on a 
paper kite (cerf-volant). The base of the wing also in the bird, is 
broad and provided with feathers, which form a broad surface on 
