MOTIVE POWER 47 



towards the extremity of the wing, and this gives 

 propulsion enough. This short stroke is the one 

 adopted by gulls in their ordinary leisurely flight. 

 The wings give the air a sharp slap, and this with a 

 bird so well built for flight and so skilled is very 

 effective. It is to be noticed that, when gulls are 

 taking short, leisurely strokes, the wings during the 

 down- strokes are very distinctly curved from their 

 base to their extremity. This curve prevents a too 

 easy slipping away of the air and so increases the 

 wing's lifting power ; but when a stronger stroke 

 is taken the primary- feathers bend upward. 



The question how a bird lifts and propels himself 

 I have now briefly answered. In other chapters I 

 shall try to describe the build of this living flying- 

 machine, to my thinking the noblest of all craft that 

 sail the air (see Chaps, vn, vm, and xi). 



