CONTRIVANCE A NECESSITY. 145 



air are combined in this wonderful implement of 

 flight. 



But perhaps the simplest test of the action and 

 reaction of the air and the wing feathers in pro- 

 ducing forward motion is an actual experiment. If 

 we take in the hand the stretched wing of a Heron, 

 which has been dried in that position, and strike it 

 quickly downwards in the air, we shall find that it 

 is very difficult indeed to maintain the perpen- 

 dicular direction of the stroke, requiring, in fact, 

 much force to do so ; and that if we do not apply 

 this force, the hand is carried irresistibly forward, 

 from the impetus in that direction which the air 

 communicates to the wing in its escape backwards 

 from the blow. 



Another test is one of reasoning and observation. 



If the explanation now given be correct, it must 



follow that since no bird can flap its wings in any 



other direction than the vertical — i.e., perpendicular 



to its own axis, (which is ordinarily horizontal,) 



and, as this motion has been shown to produce 



necessarily a forward motion, no bird can ever fly 



backwards. Accordingly no bird ever does so — no 



man ever saw a bird, even for an instant, fly tail 



K 



