204 Natural and Artificial Flight. (April, 
a progressive movement, 7.¢., a horizontal rather than a 
vertical movement. ‘Thus, if the anterior or thick margin 
of the wing be directed upwards, and the angle which the 
under surface of the wing makes with the horizon be some- 
thing like 45°, the wing will, when made to vibrate by the 
hand, fly with an undulating motion im an upward direction, 
like a pigeon to its dovecot. If the under surface of the 
wing makes no angle, or a very small angle, with the horizon, 
it will dart forward in a series of curves, in a horizontal 
direction, like a crow in rapid horizontal flight. If the angle 
made by the under surface of the wing be reversed, so that 
the thick margin of the wing be dire¢ted downwards, the 
wing will describe a waved track, and fly downwards, as a 
sparrow from a house-top or from a tree. In all those 
movements progression is a necessity. ‘The movements are 
Fic. 15. 
Fig. 15. Elastic spiral wing, which twists and untwists figure-of-8 fashion 
during its action to form a mobile helix or screw. This wing is vibrated 
by a direét piston aétion, and by a slight adjustment may be made to act 
vertically or horizontally, or at any degree of obliquity. 
a, b, Anterior margin of wing, to which the neure or ribs are affixed. c,d, 
Posterior margin of wing crossing anterior one. 4%, Ball-and-socket joint 
at root of wing, the wing being attached to the side of the cylinder by the 
socket. ?t, Cylinder. 7, Piston, with cross heads (w w) and piston head 
(s). 0, 0, Stuffing boxes. e, f, Driving chains. m, Superior elastic band, 
which assists in elevating the wing. m, Inferior elastic band, which 
antagonises m. The superior and inferior elastic bands assist in securing 
continuity of vibration, by removing the dead points at the end of the 
down and up strokes. To the superior and inferior elastic bands an 
anterior and posterior horizontal band is added. These run at right 
angles to the former and limit the action of the wing in an anterior and 
posterior direction. 
continuous gliding forward movements. There is no halt or 
pause between the strokes, and if the angle which the 
under surface of the wing makes with the horizon be properly 
regulated, the amount of steady tractile and buoying power 
developed is trulyastonishing. This form of wing, which may 
be regarded as the realisation of the figure-of-8 or wave theory 
of flight, elevates and propels both during the down and up 
strokes, and its working is accompanied with almost no slip. 
