AERONAUTICS. 



249 



down and up strokes. 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 communicated to it at the beginning of 

 the down stroke, the wing darts downwards and forwards in 

 a curve {vide ac, of fig. 81, p. 157), and in doing so it elevates 

 and carries the piston and cyKnder forwards. The force 

 employed in depressing the wing is partly expended in 

 stretching the superior elastic band, the wing being slowed 

 towards the end of the down stroke. The instant the depress- 

 ing force ceases to act, the superior elastic band 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 ce of 

 fig. 81, p. 157. The ascent of the wing stretches the inferior 

 elastic band in the same way tliat the descent of the wing 

 stretched the superior band. The superior and inferior 

 elastic bands antagonize 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-8 curves along its margins (p. 239, fig. 

 122, ab, cd), and throughout its substance similar to what 

 are observed under like circumstances in the natural wing 

 (vide fig. 86, p. 161 ; fig. 103, p. 186). 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. 88, p. 166). As the elastic 

 bands, as has been partly explained, are antagonistic in their 

 action, the wing is constantly oscillating in some directions- 

 there being no dead point either at the end of the down or 

 up strokes. As a consequence, the curves made by the wing 

 during the down and up strokes respectively, run into each 

 other to form a continuous waved track, as represented at fig. 

 12 



