180 THE STRUCTURE AND LIFE OF BIRDS chap. 



perhaps, would be the only advantage. But it is quite 

 possible to propel a boat in this way, and such a 

 system of rowing would illustrate what takes place in 

 flight. True, the blades of a bird's "oars" face 

 differently, so that, while they propel him, they at 

 the same time raise or maintain him in the air. But 

 the system of leverage is the same. This diagram is 

 a further illustration. 



Fig. 47. 



A* and Z are rigid rods representing the bird's wings hinged at I 'to V V the bird's 

 body ; a b and c b are the muscles which lower the wings. The shortening of a b 

 and c b will cause Y 3" to rise, since the air resists the descent of X and Z. After 

 Alix, Appareil Locoiiioteur des Oiscaicx. 



Horizontal Flight. 



Why does a bird advance horizontally when it 

 works its wings up and down ? The common metaphor 

 which makes them oars is picturesque, but may be, as 

 I have shown, misleading. Vergil, who, in describing 

 Daedalus' wings, uses the expression " Remigium 

 alarum," doubtless never intended to commit himself 

 to any theory of flight. If in a scientific work, wings 



