PROGRESSION IN OR THROUGH THE AIR. 107 



'niarkable property, that it creates the current on which it rises 

 and progresses. It literally flies on a whirlwind of its own 

 forming. 



These remarks apply more especially to the wings of bats 

 and birds, and those insects whose wings are made to vibrate 

 in a more or less vertical direction. The action of the wing 

 is readily imitated, as a reference to fig. 53 will show. 



Fig. 53. 



If, for example, I take a tapering elastic reed, as represented 

 at a b, and supply it with a flexible elastic sail (c d), and a 

 ball-and-socket joint (x), I have only to seize the reed at a 

 and cause it to oscillate upon x to elicit all the wing move- 

 ments. By depressing the root of the reed in the direction 

 n e, the wing flies up as a kite in the direction j f. During 

 the upward movement the wing flies upwards and forwards, 

 and describes a double curve. By elevating the root of the 

 reed in the direction m a, the wing flies down as a kite in 

 the direction i h. During the downward movement the 

 wing flies downwards and forwards, and describes a double 

 curve. These curves, when united, form a waved track, 

 which represents progressive flight. During the rise and fall 

 of the wing a large amount of tractile force is evolved, and 

 if the wrings and the body of the flying creature are inclined 

 slightly upwards, kite-fashion, as they invariably are in ordi- 

 nary flight, the whole mass of necessity moves upwards and 



