438 



DR PETTIGEEW ON THE PHYSIOLOGY OF WINGS. 



If fig. 72 be made to assume a horizontal position, instead of the oblique 

 position which it at present occupies, the manner in which an artificial current 

 is produced by one sweep of the wing from right to left, and utilised by it in a 

 subsequent sweep from left to right, will be readily understood. The artificial 

 wave wing makes a horizontal sweep from right to left, i.e., it passes from the 

 point a to the point c of fig. 72. During its passage it has displaced a column 

 of air. To fill the void so created, the air rushes in from all sides, viz., from 

 d, <?> f> 9, K i; I, m, o, p, q, r. The currents marked g, h, i; p, q, r, repre- 

 sent the reflex or artificial currents. These are the currents which, after a 

 brief interval, force the flame of the candle from right to left. It is those 

 same currents which encounter the wing, and contribute so powerfully to its 



, 1/ 



Fig. 72. 



elevation, when it sweeps from left to right. The wing, when it rushes from 

 left to right, produces a new series of artificial currents, which are equally 

 powerful in elevating the wing when it passes a second time from right to 

 left, and thus the process of making and utilising currents goes on so long 

 as the wing is made to oscillate. In waving the artificial wing to and fro, 

 I found the best results were obtained when the range of the wing and the 

 speed with which it was urged were so regulated as to produce a perfect 

 reciprocation. Thus, if the range of the wing be great, the speed should also 

 be high, otherwise the air set in motion by the right stroke will not be utilised 

 by the left stroke, and vice versa. If, on the other hand, the range of the 

 wing be small, the speed should also be low, as the short stroke will enable 

 the wing to reciprocate as perfectly as when the stroke is longer and the ■ 



