1^ 



3o6 



ANIMAL MOVEMENT 



such circumstances the upward pressure of the air is greatest on 

 the front of the wings, tending to tilt up the fore part of the 

 bird, and thus giving an upward bias. Unless this is corrected 

 by some alteration of position an upward glide will result. A 

 comparison may be made with a kite which a boy is sending 

 up by the familiar process of running rapidly in front of it, a 

 friend having previously "let go" at the psychological moment. 

 At first the kite is dragged through the air at a slight inclination 



Fig. 837. — A Falcon Swooping 



with the ground, but as the pull of the string gradually gives it 

 horizontal velocity an upward bias is developed, the string is 

 paid out, and it glides up obliquely into the air. Birds which 

 take wing from the ground, or from the water, always face the 

 wind, and the assistance which this gives them is partly to be 

 explained on the kite principle, the horizontal velocity required 

 for a start being given by a preliminary leap aided by vigorous 

 wing-flapping. The analogy is only an imperfect one, but the 

 details are too complicated to give here. 



It remains to be noticed that a bird which ceases active flight 

 and commences gliding can easily adjust itself so as to correct 

 the upward bias described above. The commonest methods em- 

 ployed appear to be slight folding of the wings and spreading 



