1872.] Natural and Artificial Flight. 193 
understood, completely occupy the space contained within 
the rim or circumference of the wheel. . . . From these 
remarks it will appear that not only the margins, but also 
the direction of the planes of the wing, are more or less: 
completely reversed at each complete flexion and extension ; 
and it is this reversing, or screwing and unscrewing, which 
enables the wing to lay hold of the air with such avidity 
during extension, and to disentangle itself with such facility 
during flexion,—to present, in fact, a more or less concave, 
oblique, and strongly resisting surface the one instant, and 
a comparatively narrow, non-resisting cutting edge the next. 
The figure-of-8 action of the wing explains how an insect 
or bird may fix itself in the air, the backward and forward 
reciprocating action of the pinion being so regulated as to 
afford support, but no propulsion.” 
“<The Wing, when Advancing with the Body, describes a Waved 
Track.—Although the figure of 8 represents with con- 
siderable fidelity the twisting of the wing upon its axis 
during extension and flexion, when the insect is playing its 
wings before an object, or still better, when it is artificially 
Fic. 6. 
fixed, it is otherwise when the insect is fairly on the wing, 
and progressing rapidly. In this case the wing, in virtue of 
its being carried forwards by the body in motion, describes 
an undulating course.” 
VOL. Ii. (N.S.) 2c 
