LOCOMOTION. 17 



[ Every time a bird wishes to strike the air, he first raises the 

 humerus with the wing still folded; next he expands the wing 

 extending the fore-arm and hand, and then suddenly depresses it ; 

 the air which resists this movement now affords him a point of 

 support, upon which he rises: he launches himself forward like 

 a projectile, and the moment an impulse is given to his body, he 

 folds the wing to diminish as much as possible the new resistance 

 which the ambient air opposes to his course. This resistance, 

 and the attraction of gravitation, (which tends to cause all bodies 

 to fall towards the centre of the earth,) gradually diminish the 

 swiftness the bird has acquired by this blow or stroke upon the 

 air, and if he made no new movement, he must soon descend, 

 but if, before losing the swiftness acquired by the first blow of 

 the wing, he gives a second, he will add a new impulse to that 

 which he had already, and gain an accelerated movement, 

 Such is, in fact, the mechanism of flight. 



While the bird is thus suspended in the air, the whole weight 

 of his body is supported by his wings, and to enable him to 

 preserve his equilibrium in this position, the centre of gravity 

 must be placed very nearly beneath the shoulders and as low as 

 possible ; it is for this reason that, while flying, he generally 

 carries his head in advance by stretching out the neck, and that the 

 body, instead of being elongated, like that of mammals, is always 

 gathered up and oval. In this necessity for lowering, as much as 

 possible, this centre of gravity, we also find the reason for a 

 peculiarity of structure, which at first sight appears singular; 

 the principal elevating muscles of the wings, instead of being 

 placed upon the back, as is ordinarily the case in other animals, 

 are found upon the chest with the depressors, and they produce 

 an effect opposite to the latter, because their tendons pass over 

 a sort of pulley before reaching the humerus ; this arrangement 

 is injurious to their action, but it has the advantage of accumu- 

 lating, at the most depending part of the thorax, all the most 

 weighty organs of the body, and, consequently of lowering thus 

 far the centre of gravity. (See, First Book of Natural History, 

 Page 92.) 



It is evident that the resistance of the air is in proportion to 

 the mass of this fluid, struck at one time by the wings, and con- 

 sequently, that the greater the surface of the wings, alJ things be- 

 ing equal, the greater will be the swiftness acquired by depressing 

 these oars ; hence it follows, that birds with long wings are not 

 only able to fly with greater rapidity, than birds with short wings, 

 but they are also able to support themselves for a longer time in 

 the air, because they are not obliged to repeat the movements of 



