230 



by a continuation of equal vibrations, keep at 

 the same height. A bird sometimes ceases al- 

 together to move his wings, closes them against 

 his sides, and falls with a precipitate motion, 

 like any other weighty body. Observe a hawk 

 drop suddenly on a poultry yard ; if, on the point 

 of reaching the ground, he perceive danger, he 

 immediately spreads his wings, and thus saves 

 himself from falling ; he then rises anew, and 

 takes to flight. 



The oblique motion differs from the vertical 

 motion which has just been described, in this, 

 that the bird rises by a series of curves which are 

 more or less extended, as the motion is more 

 horizontal or vertical. In consequence of the 

 peculiar strength of their wings, birds of prey 

 have a very powerful horizontal motion, so that 

 in soaring, the curves which they describe are so 

 slight, that the motion seems quite horizontal. 



Those visionaries who have conceived it pos- 

 sible for man to support himself in the air, by 

 rendering his body specifically lighter, have not 

 considered, that it is impossible to give to the 

 muscles which move the arms a sufficient degree 

 of strength to enable them to move the machines 

 which are adapted to them ; and hence, all who 

 have thus Icarus-like, attempted to fly, have suf- 

 fered for their rashness.* 



The throat-bone of a bird, which furnishes an 

 attachment for the principal muscles moving the 

 wings, presents an area much greater than that 



* Richerand's Physiology. 



