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 
Xi 
Fie. 15. 
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 afulcrum. 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 
