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. VII, VIII, and Xt). 
