MOTIVE POWER 



39 



of the near part of the wing will produce a movement 

 of astonishing rapidity towards the further end, near 

 the tips of the great primary- feathers. 



If x (see the figure) moves half an inch, x will 

 move two inches. Preparatory to the down-stroke, 

 the wing is lifted till it points straight upward, its 

 anterior margin being turned in the direction of the 

 bird's flight. During the first part of its descent it 



X 



Diagram to show the velocity with which the extremity 

 of the wing moves. 



cuts edgeways through the air. But soon it turns 

 face downwards and, the air opposing its descent, 

 it finds a fulcrum. But, of course, there is some give. 

 The fulcrum that the air supplies to the wing is, like 

 the fulcrum that the oar finds in water, an imperfect 

 one. When, however, the bird is taking full-length 

 strokes, the wings appear to move with a far longer 

 sweep than is really the case. With each stroke the 



