SAFETY DEVICES IN BIRDS WINGS — GRAHAM 



277 



the mercy of the rush of air which they feel, the air stream. Hav- 

 ing wider webs behind than in front of their shafts, the feather 

 blades can not help twisting somewhat into line with the upward- 

 slanting stream, because it has more effect on the broad than on the 

 narrow webs. Thus the angle at which the separated blades of 

 the feathers lie to the line of the air stream becomes less than that 

 at which the main wing lies. This is a matter of decided advantage 

 to a bird, because it means that he can afford to put his wings at 

 such a large angle of incidence that, though they may stall and be- 

 come comparatively ineffective, he will yet be safe, because their 



Figure 6. — A marsh-harrier descending. (From a photograph.) 



Figure 7.- — The right wing of a crane seen from 

 below. (From a photograph lent by Col. R. 

 Meinertzhagen.) 



very important outer j)arts (important because they are most favor- 

 ably situated for controlling) will automatically remain effective 

 and in an unstalled condition. Further, they will remain so even if 

 he increase the incidence of the main wing to several degrees beyond 

 the stalling angle. 



That the separated feathers should bend upward is only natural 

 since the airstream is striking them at an angle from below, but that 

 they should also bend forward seems a trifle odd. The explanation 

 is that they are yielding to the reaction of the displaced air, which 

 acts, according to the accepted theory of flight, in a direction apjDrox- 

 imately at right angles to the surface of their blades; and after they 

 have been twisted, that direction, as can be seen in Figure 8, must 

 be upward and forward relative to the parent wing. While the slots 



