CONTRIVANCE A NECESSITY. 143 



length, is a stiff hard edge, wholly unelastic and 

 unyielding to the air. The anterior and posterior 

 webs of each feather are adjusted on the same 

 principle. The consequence of this disposition of 

 the parts as a whole, and of this construction of 

 each of the parts, is, that the air which is struck 

 and compressed in the hollow of the wing, being 

 unable to escape tJirongh the wing, owing to the 

 closing upwards of the feathers against each other, 

 and being also unable to escape forzvards owing to 

 the rigidity of the bones and of the quills in that 

 direction, finds its easiest escape backzvards. In 

 passing backwards it lifts by its force the elastic 

 ends of the feathers ; and thus whilst effecting this 

 escape, in obedience to the law of action and re- 

 action, it communicates, in its passage along the 

 whole line of both wings, a corresponding push 

 forwards to the body of the bird. By this elabo- 

 rate mechanical contrivance the same volume of 

 air is made to perform the double duty of yielding 

 pressure enough to sustain the bird's weight against 

 the Force of Gravity, and also of communicating 

 to it a forward impulse. The bird, therefore, has 

 nothing to do but to repeat with the requisite 



