150 TEXT-BOOK OF ZOOLOGY 



The structure and position of the flight-feathers play an important 

 part in this process, since the two parts of the vane of a flight-feather 

 are of unequal width, the anterior vane being much narrower than the 

 posterior vane. Moreover, the flight-feathers do not lie close by the side 

 of each other, but overlap each other in such a manner that all the 

 anterior vanes come to lie on the upper surface, and all the posterior 

 vanes on the under surface of the wing (something after the manner of 

 the bars of a Venetian blind). 



In the downward stroke of the wing the posterior vanes, in conse- 

 quence of the pressure of the air thus produced, are applied so closely to 

 the anterior vanes, which in part cover them, that the flight-feathers 

 come to form an almost air-tight surface, ivhich prevents the passage of air. 



This is an absolute necessity for the downward stroke of the wing, 

 but would have a damaging effect in the case of the upward stroke, in 

 which the wings must encounter the least possible resistance of air, as 

 otherwise each upward stroke would cause the bird to sink. To reduce 

 this resistance to a minimum, the bird, during the upward stroke, bends 

 the wing in the wrist- joint so that the hand (see Section 3), with its 

 attached flight-feathers, is drawn almost vertically upwards. But even 

 this movement does not sufficiently diminish the resistance of the air, 

 and an additional one is called into play. In the upward stroke of the 

 wing the air pressing on its upper surface strikes the narrow anterior 

 vanes and the free-lying portions of the broad posterior vanes. These 

 latter, being long and very flexible and unsupported below, and the 

 whole feather being capable of a certain amount of rotation on its longi- 

 tudinal axis, are accordingly forced downwards, so that the air is enabled 

 to effect a passage between the feathers. 



(c) How does a bird progress in the air ? In the downward stroke of. 

 the wings the elastic feathers are, by the pressure of the air, bent 

 upwards to some extent, so that the wings form a surface inclined 

 obliquely forwards and downwards (A B). Now, the stroke has the 

 same effect as a current of air (c) striking against the surface of the 

 wing from below. We are, in fact, dealing with a force impinging upon 

 the lower surface of the wing at an oblique angle, and resolvable (accord- 

 ing to the law of the resolution of forces) into two components, of which 

 one (d) passes obliquely upwards towards the edge of the wing, and is 

 without effect, whilst the second (e} strikes the surface of the wing at 

 right angles to the first component (d). The component (d) alone is 

 effectual in the forward movement of the bird ; being itself resolvable 

 into two components (drawn for the sake of clearness above the wing) 

 (g) and (/), of which one (g) acts in a forward, the other (/) in an up- 

 ward direction. By the first of these forces (g), accordingly, the bird is 



