FIG. 121.--Gives the appearance presented by the artificial wing (fig. 120) when made to vibrate by the hand. It is thrown into longitudinal and transverse waves. The longitudinal waves are represented by the arrows _c d e_, and the transverse waves by the arrows _f g h_. A wing constructed on this principle gives a continuous elevating and propelling power. It develops figure-of-8 curves during its action in longitudinal, transverse, and oblique directions. It literally floats upon the air. It has no dead points--is vibrated with amazingly little power, and has apparently no slip. It can fly in an upward, downward, or horizontal direction by merely altering its angle of inclination to the horizon. It is applied to the air by an irregular motion--the movement being most sudden and vigorous always at the beginning of the down stroke.--_Original._] The wave wing is endowed with the very remarkable property that it will fly in any direction, demonstrating more or less clearly that flight is essentially a progressive movement, _i.e._ a horizontal rather than a vertical movement. Thus, if the anterior or thick margin of the wing be directed upwards, so that the under surface of the wing makes a _forward_ angle with the horizon of 45°, the wing will, when made to vibrate by the hand, fly with an undulating motion _in an upward direction_, like a pigeon to its dovecot. If the under surface of the wing makes no angle, or a very small _forward_ angle, with the horizon, it will dart forward in a series of curves in a _horizontal direction_, like a crow in rapid horizontal flight. If the anterior or thick margin of the wing be directed downwards, so that the under surface of the wing makes a _backward_ angle of 45° with the horizon, the wing will describe a waved track, and _fly downwards_, as a sparrow from a house-top or from a tree (p. 230). In all those movements progression is a necessity. The movements 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 realization 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, are largest when the vibration is slow, and least when it is quick. In like manner, the air is thrown into large waves by the slow