he soft
parts are external, attached to the bony framework comprising the
skeleton, the wing bones being directly connected with the central back
bone; so that while these two sorts of animated flying machines are so
different in structure, they yet act in much the same manner when on the
wing. The difference between them is clearly stated by Marey, some of
whose conclusions we now give almost word for word.

The flight of butterflies and moths differs from that of birds in the
almost vertical direction of the stroke of their wings, and in their
faculty of sailing in the air without making any movements; though
sometimes in the course they pursue they seem to resemble birds in their
flight.

The flight of insects and birds moreover differs in the form of the
trajectory in space; in the inclination of the plane in which the wings
beat; in the role of each of the two alternating (and in an inverse
sense) movements that the wings execute; as also in the facility with
which the air is decomposed during these different movements. As the
wings of a fly are adorned with a brilliant array of colors, we can
follow the trajectory or figure that each wing writes in the air. It is
of the form of a figure of eight (Fig. 9), first discovered by Professor
J. Bell Pettigrew of Edinburgh.

[Illustration: 9. Figure cut by an insect's wing.]

[Illustration: 10. Figure cut by a bird's wing.]

By an ingenious machine, specially devised for the purpose, Professor
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 horizontal
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 lowering 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.

[Illustration: 11. Trajectory of an insect's wing.]

[Illustration: 12. Trajectory of a bird's wing.]

In an insect, an energetic movement is equally necessary to strike the
air at bo