DR PETTIGREW ON THE PHYSIOLOGY OF WINGS. 437 



Compound rotation of the Artificial Wave Wing: the different parts of the 

 Wing travel at different speeds. — The artificial wave wing, like the natural wing, 

 revolves upon two centres {a b, c d of fig. 45, page 376, and x, I of fig. 63, page 

 424), and owes much of its elevating and propelling, seizing and disentangling 

 power to its different portions travelling at different rates of speed (see fig. 51, 

 page 399), and to its storing up and giving off energy as it hastens to and fro. 

 Thus the tip of the wing moves through a very much greater space in a given 

 time than the root, and so also of the posterior margin as compared with the 

 anterior. This is readily understood, by bearing in mind that the root of the 

 wing forms the centre or axis of rotation for the tip ; while the anterior margin 

 is the centre or axis of rotation for the posterior margin. The momentum, 

 moreover, acquired by the wing during the stroke from right to left is expended 

 in reversing the wing, and in preparing it for the stroke from left to right, and 

 vice versa ; a continuous to and fro movement devoid of dead points being thus 

 established. If the artificial wave wing be taken in the hand and suddenly 

 depressed in a more or less vertical direction, it immediately springs up again, 

 and carries the hand with it. It, in fact, describes a curve whose convexity is 

 directed downwards, and in doing so, carries the hand upwards and forwards. 

 If a second down stroke be added, a second curve is formed ; the curves 

 running into each other, and producing a progressive waved track similar to 

 what is represented at a, c, e, j, i of fig. 14, page 344. This result is favoured if 

 the operator runs forward so as not to impede or limit the action of the wing. 



How the Wave Wing creates currents, and rises upon them, and hoiv the air 

 assists in elevating the Wing. — In order to ascertain in what way the air contri- 

 butes to the elevation of the wing, I made a series of experiments with natural 

 and artificial wings. On concluding these experiments, I felt convinced that 

 when the wing descends it compresses and pushes before it, in a downward and 

 forward direction, a column of air represented by a, b, c of fig. 72, p. 438.* The 

 air rushes in from all sides to replace the displaced air, as shown at d, e,f g, h, i, 

 and so produces a circle of motion indicated by the dotted line s, t, v, w. The 

 wing rises upon the outside of the circle referred to, as more particularly seen 

 at d, e, v, iv. The arrows, it will be observed, are all pointing upwards, and as 

 these arrows indicate the direction of the reflex or back current, it is not diffi- 

 cult to comprehend how the air comes indirectly to assist in elevating the wing. 

 A similar current is produced to the right of the figure, as indicated by /, m, 

 °> P> <I> r > but seeing the wing is always advancing, this need not be taken into 

 account. 



* The artificial currents produced by the wing during its descent may be readily seen by partially 

 filling a chamber with steam, smoke, or some impalpable white powder, and causing the wing to 

 descend in its midst. By a little practice, the eye will not fail to detect the currents represented at 

 d > e >f, ff, h, i, I, m, n, o, p, q, r of fig. 72, p. 438. 



