FEATHERS 



397 



Fig. 312. — A diagram of a developing feather, highly magnified. — From Shipley 



and MacBride. 



der.. Dermis; epid., epidermis; fol., follicle; fth., feather; Mp., Malpighian layer of epidermis; 

 pap., papilla by the growth of whose epidermis the feather is formed. 



FLIGHT 



Bird flight is complex, and no complete dynamic account of 

 it has ever been given. It may be assumed on the principles of 

 Newtonian mechanics that in flapping flight the wing must exert 

 a force on the air which has a downward component to resist 

 the bird's weight, and a backward component to overcome air 

 resistance ; both must be large enough at times to provide 

 acceleration. Photographs of birds in flight leave no doubt that 

 wings go into most peculiar positions, and it may be assumed 

 that they do so in order to produce the necessary force. A recent 

 cinematograph film of the pigeon in slow flight (as at take-off) 

 (Fig. 313) suggests that the downstroke produces nearly all lift 

 (i.e. downward force) and the upstroke mainly thrust (i.e. back- 

 ward force) with some lift in its earlier phases ; in normal flight 

 both thrust and lift come almost entirely from the downstroke 

 and the wing moves in a much simpler up-and-down fashion. It is 

 obvious that the relative proportions of lift and thrust can be 

 altered by alterations in the attitudes of the wings. In gliding, 



