244 



AERONAUTICS. 



The degree of convexity given to the upper surface of the 

 wing can be increased or diminished at pleasure by causing a 

 cord (iy; A, A') and elastic band (k) to extend between two 

 points, which may vary according to circumstances. The 

 wing is supplied with vertical springs, which assist in slowing 

 and reversing it towards the end of the down and up strokes, 

 and these, in conjunction with the elastic properties of the 

 wing itself, contribute powerfully to its continued play. The 

 compound wave wing produces the currents on which it 

 rises. Thus during the up stroke it draws after it a current, 

 nvhich being met by the wing during its descent, confers 

 additional elevating and propelling power. During the down 

 stroke the wing in like manner draws after it a current which 

 forms an eddy, and on this eddy the wing rises, as explained 

 at p. 253, fig. 129. The ascent of the wing is favoured by 

 the superimposed air playing on the upper surface of the 

 posterior margin of the organ, in such a manner as to cause 

 the wing to assume a more and more oblique position with 

 reference to the horizon. This change in the plane of the 

 wing enables its upper surface to avoid the superincumbent 

 air during the up stroke, while it confers upon its under sur- 

 face a combined kite and parachute action. The compound 

 wave wing leaps forward in a curve both during the down 

 and up strokes, so that the wing during its vibration describes 

 a waved track, as shown at a, c, e,g, i of fig. 81, p. 157. The 

 compound wave wing possesses most of the peculiarities of 

 single wings when made to vibrate separately. It forms a 

 most admirable elevator and propeller, and has this advan- 

 tage over ordinary wings, that it can be worked without 

 injury to itself, when the machine which it is intended to 

 elevate is resting on the ground. Two or more compound 

 wave wings may be arranged on the same plane, or super- 

 imposed, and made to act in concert. They may also by a 

 slight modification be made to act horizontally instead of 

 vertically. The length of the stroke of the compound wave 

 wing is determined in part, though not entirely by the stroke 

 of the piston — the extremities of the wing, because of their 

 elasticity, moving through a greater space than the centre of 

 the wing. By fixing the wing to the head of the piston all 



