STRUCTURE OF BIRDS, 43 



direction, or by being stretched out to break its descent. In 

 proportion to their bulk birds are also much lighter than any 

 vertebrate animals, and their lightness is produced by the intro- 

 duction of air into their tissue, and even into the bones, as well 

 as by the great bulk of the feathers, which in those having a 

 very buoyant flight, as Owls and Gulls, is much greater than 

 that of the body. 



When a bird intends to fly, it loosens its wings from their 

 ordinary position, throws its body forward, and gives it a sudden 

 impulse by means of the legs, which would merely produce a 

 leap, but the w^ngs being in the meantime spread out and ele- 

 vated, they are again brought down with force, so that their 

 points generally strike against the ground. Whether or not, 

 they act as levers, and by repeated strokes, carry the bird up- 

 wards. AVere its ascent vertical, the rapid action of the wings 

 in the same plane would suflice to raise it, provided the down- 

 ward stroke w^ere much more powerful than the upward, the 

 wing, moreover, being drawn in during the latter, and stretched 

 out during the former. But, for progression in a horizontal direc- 

 tion, it is necessary that the downward stroke should be modi- 

 fied by the elevation in a certain degree of the free extremities 

 of the quills, and that the pinion should be pulled backwards. 

 The best subject in which to study the motions of the wrings 

 during flight, is one of the larger gulls, in which the wings 

 being very long and the flight remarkably buoyant, and per- 

 formed by slow beats, one may trace their alternations with ease, 

 provided he be near enough. In these birds, the wings are 

 never extended to their full length, the elbow-joint being always 

 more or less bent, and the hand or pinion always inclined back- 

 w^ards. During the elevation of the wing, it is drawn in a little, 

 the humerus is directly raised, but the cubitus inclines down- 

 wards to some extent, and the fore-edge of the hand is depressed, 

 so that the primary quills are elevated, by which arrangement 

 little resistance is offered to the air. Then the wing is stretched 

 out, brought strongly down, first with its whole concave surface 

 direct, but presently with the quills raised a little behind, so as 

 to procure an oblique stroke on the air. The Gannet, when 

 viewed at hand, shews these alternate motions still more decided- 



