FLYING ON A LEVEL PLANE 8i 

 the wing displaces the air in the same way as if 

 it were a fan — a. fan, however, so constructed 

 as to work with a single or one-sided action. 

 When the wing or pair of wings are thus set 

 in motion at high velocity, it is easy to under- 

 stand what the result must be when these 

 wings are attached to a. free body, hght and so 

 shaped as to present the least possible resist- 

 ance to the air. 



Flying on a level plane, unaided by the 

 wind, a duck travels at the rate of, say, 50 

 miles per hour ; this is the limit of the bird's 

 flying powers which can only be accelerated 

 by the assistance of a favourable wind. Con- 

 versely, the flight is retarded by an adverse 

 wind. Speed is again accelerated when the 

 bird, still using its wings, leaves the mean 

 level and flies earthward ; in this event 

 acceleration is due to gravitation. But the 

 test of the wing-driving power is what a bird 

 can do on a level plane without assistance from 

 the wind. Flying at an angle above the mean 

 level reduces speed inversely as the angle of 

 inclination becomes steeper. It must, how- 

 ever, be understood, that though the wings are 

 now lifting as well as propelling the bird, their 

 position and scope of action in relation to the 

 body is still, as always, the same. Like the 

 paddle-wheels of a steamer which act in concert 



