PEOGKESSION IN OK THROUGH THP: AIK. 



143 



during its vibrations from above, it will be found that the 

 blur or impression produced on the eye by its action is more 

 or less concave (fig, 66, p. 139). This is due to the fact 

 that the wing is spiral in its nature, and because during its 

 action it twists upon itself in such a manner as to describe a 

 double curve, — the one curve being directed upwards, the 

 other downwards. The double curve referred to is particularly 

 evident in the flight of birds from the greater size of their 

 wings. The wing, both when at rest and in motion, may not 

 inaptly be compared to the blade of an ordinary screw pro- 

 peller as employed in navigation. Thus the general outline 

 of the wing corresponds closely with the outline of the blade 

 of the propeller, and the track described by the wing in 

 space is twisted upon itself propeller fashion. The great 

 velocity with which the wing is driven converts the impres- 

 sion or blur into what is equivalent to a solid for the time 

 being, in the same way that the spokes of a wheel in violent 

 motion, as is well understood, completely occupy the space 

 contained within the rim or circumference of the wheel (figs. 

 64, 65, and 66, p. 139). 



The figure-of-8 action of the wing explains how an insect, 

 bat, or bird, may fix itself in the air, the backward and for- 

 ward reciprocating action of the pinion affording support, but 

 no propulsion. In these instances, the backward and forward 

 strokes are made to counterbalance each other. 



The JVing, when advancing with the Body, describes a Looped 

 and JVaved Tra.ck. — Although the figure-of-8 represents with 

 considerable fidelity the twisting of the wing upon its long axis 

 during extension and flexion, and during the down and up 

 strokes when the volant animal is playing its wings before an 

 object, or still better, when it is artificially fixed, it is other- 

 wise when it is free and progressing rapidly. In this case the 

 wing, in virtue of its being carried forward by the body in 

 motion, describes first a looped and then a waved track. This 

 looped and waved track made by the wing of the insect is re- 

 presented at figs. 71 and 72, and that made by the wing of 

 the bat and bird at fig. 73, p. 144. 



The loops made by the wing of the insect, owing to the 

 more oblique stroke, are more horizontal than those made by 



