MECHANICAL PARADOXES. 



the obliquity the greater the forward speed 

 in comparison with that of the wind in that 

 case, of the wings in this, so long as the resist- 

 ance is not too great. Consequently it is often 

 impossible, without the closest observation, to 

 see that obliquity in the wings of birds which 

 produces the forward movement. 



In the return or up beat, shown at B and 

 D, the wing surface is purely passive, being 

 merely carried forwards and upwards by the 

 front edge, which cleaves a way for it through 

 the air till it gets into position to act in the 

 next down beat. 



