DP PETTIGREW ON THE PHYSIOLOGY OF WINGS. 427 



During the clown stroke of the piston the wing is flattened out in every direction, 

 and its extremities twisted in such a manner as to form two screws, as shown 

 at a' b' c f d' ; e'f g' h' ; B, B' of figure. The active area of the wing is by this 

 means augmented, so that it seizes the air with great avidity during the down 

 stroke. The area of the wing may be still further increased and diminished 

 during the down and up strokes by adding joints to the body of the wing on the 

 principle recommended at pages 428, 429, 430, and 431, figs. 67, 68, and 69. 

 The degree of convexity given to the upper surface of the wing can be increased 

 or diminished at pleasure by causing a cord (ij; 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 con- 

 tinued play. The compound wave wing produces the currents on which it 

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

 rent which forms an eddy, and on this eddy the wing rises, as explained at page 

 438, fig. 72. The ascent of the wing is favoured by the superimposed air play- 

 ing 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 surface 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. 

 14, page 344. The compound wave wing possesses most of the peculiarities of 

 single wings when made to vibrate simultaneously. It forms a most admirable 

 elevator and propeller, and has this advantage 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 superimposed, 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 gearing apparatus is avoided, and 

 the number of joints and working points reduced — a matter of no small import- 

 ance when it is desirable to conserve the motor power and keep clown the 

 weight. 



How to Construct a Wave Wing on the Bat and Bird type. — In order to 



VOL. XXVI. PART II. 5 S 



