MECHANISM OP FLIGHT. 369 



wishes to strike the air, it raises the humerus, and with it the 

 wing still folded ; it then unfolds this by extending the fore-arm, 

 as well as the hand, and suddenly depresses it ; the air, resisting 

 this movement, furnishes the Bird with a point of support on 



which it raises itself; it then 

 darts forward like a projectile ; 

 and an impulse being once given 

 to the body, it slants or folds 

 back the wing, to diminish as 

 much as possible the new resist- 

 ance, which the surrounding fluid 



FTG. 182.-WIHO OF FALCON. makeg to ; tg courge> This re- 



a, primaries ; b, secondaries ; c, tertiaries ; sistance, and gravitation which 

 d, spurious quills, arising from the thumb. , . ,. . ... 



tends to make all bodies fall 



towards the centre of the earth, gradually diminish the speed 

 which the Bird has acquired by this stroke upon the air ; and 

 without new movements it will soon fall ; but before the speed 

 acquired by the first stroke of the wing is lost, the Bird gives a 

 second, which adds new speed to that which it already had, and 

 thus proceeds with an accelerated motion. This is, in fact, the 

 mechanism of flight. While the Bird is thus suspended in the 

 air, its wings support all the weight of its body ; and to preserve 

 its equilibrium in this position, its centre of gravity must be 

 placed nearly beneath its shoulders, and as low as possible ; on 

 this account, during flight, it generally projects its head forwards 

 by stretching out its neck ; and its trunk, instead of being 

 elongated like that of the Mammalia, is short and compact. It 

 is evident that the resistance of the air must be greater, in pro- 

 portion to the mass of that fluid struck at once by the wings ; 

 and consequently, that the greater the extent of the wings, other 

 things being equal, the greater will be the 'speed acquired by 

 their down-stroke ; thus Birds with long wings will not only 

 be able to fly quicker than those with short ones, but also they 

 will be able to support themselves longer in the air ; for they 

 will not be obliged to repeat so often the movement of these 

 organs, and consequently they will be less quickly fatigued. In 

 fact, all the Birds remarkable for their rapid and sustained flight 



