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of support with the minimum of slip, and the minimum of force. It supplies a degree of buoying and propelling power which is truly remarkable. Its buoying area is nearly equal to half a circle. It can act upon still air, and it can create and utilize its own currents. I proved this in the following manner. I caused the wing to make a horizontal sweep from right to left over a candle; the wing rose steadily as a kite would, and after a brief interval, the flame of the candle was persistently blown from right to left. I then waited until the flame of the candle assumed its normal perpendicular position, after which I caused the wing to make another and opposite sweep from left to right. The wing again rose kite fashion, and the flame was a second time affected, being blown in this case from left to right. I now caused the wing to vibrate steadily and rapidly above the candle, with this curious result, that the flame did not incline alternately from right to left and from left to right. On the contrary, it was blown steadily away from me, i.e. in the direction of the tip of the wing, thus showing that the artificial currents made by the wing, met and neutralized each other always at mid stroke. I also found that under these circumstances the buoying power of the wing was remarkably increased.
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 (ab, cd 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 versd; 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.
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. 253.1 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.
If fig. 129 be made to assume a horizontal position, instead of the oblique position which it at present occupies^ the manner in which an artificial current is produced by one sweep of the wing from right to left, and utilized by it in a subsequent sweep from left to right, will be readily understood. The artificial wave wing makes a horizontal sweep from right to left, i.e. it passes from the point a to the point c of fig. 129. During its passage it has displaced a column of air. To fill the void so created, the air rushes in from all sides, viz. from d,e,f,g,h,i; l, m, o, p, q, r. The currents marked g, h, i; p, q, r, represent the reflex or artificial currents. These are the currents which, after a brief interval, force the flame of the candle from right to left. It is those same currents which the wing encounters, and which contribute so powerfully to its elevation, when it sweeps from left to right. The wing, when it rushes from left to right, produces a new series of artificial currents, which are equally powerful in elevating the wing when it passes a second time from right to left, and thus the process of making and utilizing currents goes on so long as the wing is made to oscillate. In waving the artificial wing to and fro, I found the best results were obtained when the range of the wing and the speed with which it was •urged were so regulated as to produce a perfect reciprocation. Thus, if the range of the wing be great, the speed should also be high, otherwise the air set in motion by the right stroke will not be utilized by the left stroke, and vice versd. If, on the other hand, the range of the wing be small, the speed should also be low, as the short stroke will enable the wing to reciprocate as perfectly as when the stroke is longer and the speed quicker. When the speed attained is high, the angles made by the under surface of the wing with the horizon are diminished; when it is low, the angles are increased. From these remarks it will be evident that the artificial wave wing reciprocates in the same way that the natural wing reciprocates; the reciprocation being most perfect when the wing is vibrating in a given spot, and least perfect when it is travelling at a high horizontal speed.
1 The artificial currents produced by the wing during its descent may he readily seen hy 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, g, h, i, I, in, o,p, q, r of fig. 129, p. 253.
The Artificial Wing propelled at various degrees of speed during the Down and Up Strokes.—The tendency which the artificial wave wing has to rise again when suddenly and vigorously depressed, explains why the elevator muscles of the wing should be so small when compared with the depressor muscles—the latter being something like seven times larger than the former. That the contraction of the elevator muscles is necessary to the elevation of the wing, is abundantly proved by their presence, and that there should be so great a difference between the volume of the elevator and depressor muscles is not to be wondered at, when we remember that the whole weight of the body is to be elevated by the rapid descent of the wings—the descent of the wing being entirely due to the vigorous contraction of the powerful pectoral muscles. If, however, the wing was elevated with as great a force as it was depressed, no advantage would be gained, as the wing, during its ascent (it acts against gravity) would experience a much greater resistance from the air than it did during its descent. The wing is consequently elevated more slowly than it is depressed; the elevator muscles exercising a controlling and restraining influence. By slowing the wing during the up stroke, the air has an opportunity of reacting on its under surface.
The Artificial Wave Wing as a Propeller.—The wave wing makes an admirable propeller if its tip be directed vertically downwards, and the wing lashed from side to side with a sculling figure-of-8 motion, similar to that executed by the tail of the fish. Three wave wings may be made to act in concert, and with a very good result; two of them being made to vibrate figure-of-8 fashion in a more or less horizontal direction with a view to elevating; the third being turned in a downward direction, and made to act vertically for the purpose of propelling.
Fig. 130.— Aerial wave screw, whose blades are slightly twisted (ab,cd; ef, (fh), so that those portions nearest the root (d ft) make a greater angle with the horizon than those parts nearer the tip (bf). The angle is thus adjusted to the speed attained by the different portions of the screw. The angle admits of further adjustment by means of the steel springs z, s, these exercising a restraining, and to a certain extent a regulating, influence which effectually prevents shock.
It will be at once perceived from this figure that the portions of the screw marked m and n travel at a much lower speed than those portions marked o and p, and these again more slowly than those marked q and r (compare with fig. 56, p. 120). As, however. the angle which a wing or a portion of a wing, as 1 have pointed out, varies to accommodate itself to the speed attained by the wing, or a portion thereof, it follows, that to make the wave screw mechanically perfect, the angles made by its several portions must be accurately adapted to the travel of its several parts as indicated above.
x, Vertical tube for receiving driving shaft, v, w. Sockets in which the roots of the blades of the screw rotate, the degree of rotation being limited by the steel springs z, s. ab, ef, Tapering elastic reeds forming anterior or thick margins of blades of screw, d c, hg, Posterior or thin elastic margins of blades of screw, m n,op,q r, Radii formed by the different portions of the blades of the screw when in operation. The arrows indicate the direction of travel.—Original.
A New Form of Aerial Screw.—If two of the wave wings represented at fig. 122, p. 239, be placed end to end, and united to a vertical portion of tube to form a two-bladed screw, similar to that employed in navigation, a most powerful elastic aerial screw is at once produced, as seen at fig. 130.