434 DR PETTIGREW ON THE PHYSIOLOGY OF WINGS. 



during its descent and ascent the precise angles made by the wing with the 

 horizon in natural flight. The superior bands are stronger than the inferior 

 ones, and are put upon the stretch during the down stroke. They thus help 

 the wing over the dead point at the end of the down stroke, and assist, in con- 

 junction with the reaction obtained from the air, in elevating it. The jDOsterior 

 bands are stronger than the anterior ones to restrain within certain limits the 

 strong tendency which the wing has to leap forward in curves towards the end 

 of the down and up strokes. The oblique bands, aided by the air, give the 

 necessary degree of rotation to the wing in the direction of its length. This effect 

 can, however, also be produced independently by the four principal bands. From 

 what has been stated it will be evident that the elastic bands exercise a restrain- 

 ing influence, and that they act in unison with the driving power and with the 

 reaction supplied by the air. They powerfully contribute to the continuous vibra- 

 tion of the wing, the vibration being peculiar in this that it varies in rapidity at 

 every successive stage. I derive the motor power, as has been stated, from a 

 direct piston action, the piston being urged either by steam worked expansively 

 or by the hand, if it is merely a question of illustration. In the hand models the 

 " muscular sense " at once informs the operator as to what is being done. Thus 

 if one of the wave wings supplied with a ball and socket joint, and a cross 

 system of elastic bands as explained, has a sudden vertical impulse communi- 

 cated to it at the beginning of the down stroke, the wing darts downivards and 

 forwards in a curve {vide a, c, of fig. 14, page 344), and in doing so it elevates and 

 carries the piston and cylinder forwards. The force employed in depressing the 

 wing is partly expended in stretching the superior elastic band (y of fig. 63, page 

 424), the wing being slowed towards the end of the down stroke. The instant 

 the depressing force ceases to act the superior elastic band (y) contracts, and 

 the air reacts ; the two together, coupled with the tendency which the model has 

 to fall downwards and forwards during the up stroke, elevating the wing. The 

 wing when it ascends describes an upward and forward curve, as shown at e e 

 of fig. 14, page 344. The ascent of the wing stretches the inferior elastic band 

 (z of fig. 63, page 424) in the same way that the descent of the wing stretched 

 the superior band. The superior and inferior elastic bands antagonise each other 

 and reciprocate with vivacity. While those changes are occurring the wing is 

 twisting and untwisting in the direction of its length and developing figure of eight 

 curves along its margins (page 423, fig. 62, a b, c d), and throughout its sub- 

 stance similar to what are observed under like circumstances in the natural wing 

 [vide figs. 39, 40, 41, 42, and 43, page 362). The angles, moreover, made by the 

 under surface of the wing with the horizon during the down and up strokes are 

 continually varying — the wing all the while acting as a kite, which flies steadily 

 upwards and forwards (fig. 15, page 345). As the elastic bands, as has been 

 partly explained, are antagonistic in their action the wing is constantly oscillating 



