422 DR PETTIGKEW ON THE PHYSIOLOGY OF WINGS. 



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 directed, 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 move- 

 ments are 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 truly astonishing. This form of wing, which may 

 be regarded as the realisation of the figure of 8 theory of flight, elevates and 

 propels both during the down and up strokes, and its working is accompanied 

 with almost no slip. It seems literally to float upon the air. No wing that is 

 rigid in the anterior margin can twist and untwist during its action, and produce 

 the figure of 8 curves generated by the living wing. To produce the curves in 

 question, the wing must be flexible, elastic, and capable of change of form in 

 all its parts. The curves made by the artificial wing, as has been stated (p. 420), 

 are largest when the vibration is slow, and least when it is quick. In hke 

 manner, the air is thrown into large waves by the slow movement of a large 

 wing, and into small waves by the rapid movement of a smaller wing. The size 

 of the wing curves and air waves bear a fixed relation to each other, and both 

 are dependent on the rapidity with which the wing is made to vibrate. This is 

 proved by the fact that insects, in order to fly, require, as a rule, to drive 

 their small wings with immense velocity. It is further proved by the 

 fact that the small humming bird, in order to keep itself stationary before 

 a flower, requires to oscillate its tiny wings with great rapidity, whereas the 

 large humming bird (Patagona gigas), as was pointed out by Darwin, can 

 attain the same object by flapping its large wings with a very slow and powerful 

 movement. In the larger birds the movements are slowed in proportion to the 

 size, and more especially in proportion to the length of the wing, the cranes and 

 vultures moving the wings very leisurely, and the large oceanic birds dispensing 

 in a great measure with the flapping of the wings, and trusting for progression 

 and support to the wings in the expanded position. 



This leads me to conclude that very large wings may be driven with a com- 

 paratively slow motion, a matter of some inportance in artificial flight secured 

 by the flapping of wings. 



How to Construct an Artificial Wave Wing on the Insect Type. — The follow- 

 ing appear to me to be essential features in the construction of an artificial 

 wing : — 



The wing should be of a generally triangular shape. 



It should taper from the root towards the tip, and from the anterior margin 

 in the direction of the posterior margin. 



