remarkable circumstance) they are always adapted to the speed at which the wing is travelling for the time being. The increase and decrease in the angles made by the wing as it hastens to and fro are due partly to the resistance offered by the air, and partly to the mechanism and mode of application of the wing to the air. The wing, during its vibrations, rotates upon two separate centres, the tip rotating round the root of the wing as an axis (short axis of wing), the posterior margin rotating around the anterior margin (long axis of wing). The wing is really eccentric in its nature, a remark which applies also to the rowing feathers of the bird’s wing. The compound rotation goes on throughout the entire down and up strokes, and is intimately associated with the power which the wing enjoys of alternately seizing and evading the air.
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| Fig.20 Fig.21 |
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| Fig.22 Fig.23 |
| Figs. 20, 21, 22 and 23 show the area mapped out by the left wing of the Wasp when the insect is fixed and the wing made to vibrate. These figures illustrate the various angles made by the wing with the horizon as it hastens to and fro, and show how the wing reverses and reciprocates, and how it twists upon itself in opposite directions, and describes a figure-of-8 track in space. Figs. 20 and 22 represent the forward or down stroke (a b c d e f g), figs. 21 and 23 the backward or up stroke (g h i j k l a). The terms forward and back strokes are here employed with reference to the head of the insect, x, x′, line to represent the horizon. If fig. 22, representing the down or forward stroke, be placed upon fig. 23, representing the up or backward stroke, it will be seen that the wing crosses its own track more or less completely at every stage of the down and up strokes. |
The compound rotation of the wing is greatly facilitated by the wing being elastic and flexible. It is this which causes the wing to twist and untwist diagonally on its long axis when it is made to vibrate. The twisting referred to is partly a vital and partly a mechanical act;—that is, it is occasioned in part by the action of the muscles and in part by the greater resistance experienced from the air by the tip and posterior margin of the wing as compared with the root and anterior margin,—the resistance experienced by the tip and posterior margin causing them to reverse always subsequently to the root and anterior margin, which has the effect of throwing the anterior and posterior margins of the wing into figure-of-8 curves, as shown at figs. 9, 11, 12, 16, 18, 20, 21, 22 and 23.
The compound rotation of the wing, as seen in the bird, is represented in fig. 24.
Not the least curious feature of the wing movements is the remarkable power which the wing possesses of making and utilizing its own currents. Thus, when the wing descends it draws after it a strong current, which, being met by the wing during its ascent, greatly increases the efficacy of the up stroke. Similarly and conversely, when the wing ascends, it creates an upward current, which, being met by the wing when it descends, powerfully contributes to the efficiency of the down stroke. This statement can be readily verified by experiment both with natural and artificial wings. Neither the up nor the down strokes are complete in themselves.
The wing to act efficiently must be driven at a certain speed, and in such a manner that the down and up strokes shall glide into each other. It is only in this way that the air can be made to pulsate, and that the rhythm of the wing and the air waves can be made to correspond. The air must be seized and let go in a certain order and at a certain speed to extract a maximum recoil. The rapidity of the wing movements is regulated by the size of the wing, small wings being driven at a very much higher speed than larger ones. The different parts of the wing, moreover, travel at different degrees of velocity—the tip and posterior margin of the wing always rushing through a much greater space, in a given time, than the root and anterior margin.
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| Fig. 24.—Wing of the Bird with its root (a, b) cranked forwards. | |
| a, b, Short axis of the wing (axis for tip of wing, h). c, d, Long axis (axis for posterior margin of wing, h, i, j, k, l). m, n, Short axis of rowing feathers of wing. |
r, s, Long axis of rowing feathers of wing. The rotation of the rowing feathers on their long axis (they are eccentrics) enables them to open or separate during the up, and close or come together during the down strokes. e f, g p, concave shape presented by the under surface of the wing. |
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| Fig. 25 shows how different portions of the wing travel at different degrees of speed. In this figure the rod a, b, hinged at x, represents the wing. When the wing is made to vibrate, its several portions travel through the spaces d b f, j k l, g h i, and e a c in exactly the same interval of time. The part of the wing marked b, which corresponds with the tip, consequently travels very much more rapidly than the part marked a, which corresponds with the root. m n, o p, curves made by the wing at the end of the up and down strokes; r, position of the wing at the middle of the stroke. |
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| Fig. 26.—In this figure f, f ′ represent the movable fulcra furnished by the air, p p′ the power residing in the wing, and b the body to be moved. In order to make the problem of flight more intelligible, the lever formed by the wing is prolonged beyond the body (b), and to the root of the wing so extended the weight (w, w′) is attached; x represents the universal joint by which the wing is attached to the body. When the wing ascends as shown at p, the air (fulcrum f ) resists its upward passage, and forces the body (b) or its representative (w) slightly downwards. When the wing descends as shown at p′, the air (fulcrum f ′) resists its downward passage, and forces the body (b) or its representative (w′) slightly upwards. From this it follows that when the wing rises the body falls, and vice versa—the wing describing the arc of a large circle (f f ′), the body (b), or the weights (w, w′) representing it, describing the arc of a small circle. |
The rapidity of travel of the insect wing is in some cases enormous. The wasp, for instance, is said to ply its wings at the rate of 110, and the common house-fly at the rate of 330 beats per second. Quick as are the vibrations of natural wings, the speed of certain parts of the wing is amazingly increased. Wings as a rule are long and narrow. As a consequence, a comparatively slow and very limited movement at the root confers great range and immense speed at the tip, the speed of each portion of the wing increasing as the root of the wing is receded from. This is explained on a principle well understood in mechanics, viz. that when a wing or rod hinged at one end is made to move in a circle, the tip or free end of the wing or rod describes a much wider circle in a given time than a portion of the wing or rod nearer the hinge (fig. 25).
One naturally inquires why the high speed of wings, and why the progressive increase of speed at their tips and posterior margins? The answer is not far to seek. If the wings were not driven at a high speed, and if they were not eccentrics made to revolve upon two separate axes, they would of necessity be large cumbrous structures; but large heavy wings would be difficult to work, and what is worse, they would (if too large), instead of controlling the air, be controlled by it, and so cease to be flying organs.
There is, however, another reason why wings should be made to vibrate at high speeds. The air, as explained, is a very light, thin, elastic medium, which yields on the slightest pressure, and unless the wings attacked it with great violence the necessary recoil or resistance could not be obtained. The atmosphere, because of its great tenuity, mobility and comparative imponderability, presents little resistance to bodies passing through it at low velocities. If, however, the speed be greatly accelerated,
