8o FORWARD PROPULSION 



birds are then flying on a level plane (parallel 

 to the earth), the action of the wings is con- 

 tinuous ; should, however, one or more of the 

 birds show an inclination to " coast " it is a 

 sure sign that the " flight " is about to descend, 

 otherwise the wings never cease beating the 

 air. Therefore it is obvious that forward 

 motion results directly from the wing action 

 independently of " gliding " or " undulating " ; 

 in which case, the wings, for the time being, 

 are out of action and serve merely as supports. 



Now we have to consider how this beating 

 of the wings engenders propulsion ? On again 

 examining the wing oi a bird, we observe that 

 the " plumage " gradually tapers away from 

 the " arm " to a thin marginal edge defined 

 by the extremities, in juxtaposition, of single 

 feathers, spring-like and resihent. We have 

 already seen that when the wing is lifted 

 (up-stroke) it meets with practically no resist- 

 ance, therefore it is the down-stroke which 

 counts. 



Now, by way of experiment, we shaU 

 imagine this wing actuated from the shoulder 

 in the natural way, straight up-and-down, 

 but from a stationary body. The effect upon 

 the air, which can be tested, is that a draught 

 is set up which flows away from behind the 

 wing at each successive down-beat ; in short. 



