_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. 80, p. 149; fig. 83, p. 158, and fig. 122, p. 239), and owes much of its elevating and propelling, seizing, and disentangling power to its different portions travelling at different rates of speed (see fig. 56, p. 120), 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 versâ_; 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_, _g_, _i_, of fig. 81, p. 157. This result is favoured if the operator runs forward so as not to impede or limit the action of the wing. [Illustration: FIG. 129.] _How the Wave Wing creates currents, and rises upon them, and how the Air assists in elevating the Wing._--In order to ascertain in what way the air contributes to the elevation of the wing, I made a series of experiments with natural and artificial wings. These experiments led me to conclude that when the wing descends, as in the bat and bird, it compresses and pushes before it, in a downward and forward direction, a column of air represented by _a_, _b_, _c_ of fig. 129, p. 253.[119] 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_, _w_. 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 difficult 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 _l_, _m_, _o_, _p_, _q_, _r_, but seeing the wing is always advancing, this need not be taken into account.