CHAPTER. III. 
MOTIVE POWER. 
LEVERAGE—PROPULSION—PHASES OF THE WING-STROKE. 
In describing gliding we have taken the motive 
power for granted, assuming that the bird already 
has momentum. We must now investigate his 
method of lifting and propelling himself. The lifting 
is the hard work, for the bird is so shaped that when 
he sets himself at a suitable angle the air offers but 
little resistance to his movement onward in a hori- 
zontal direction. To lift himself he must put great 
force into the downward beat of his wings, making 
their extremities move with such velocity that the 
air, the resistance of which increases as the square 
of the velocity, will very soon offer effective support. 
They will, in fact, become levers, each having its 
fulcrum mainly near the extremity, the weight to 
be lifted being, of course, the bird’s body, or, more 
correctly, the whole bird. The power is applied 
quite close to the body (see fig. 21, Chap. v11)}’ The 
depressor muscle, the great muscle that springs from 
the breastbone and covers it with its great expanse, 
attaches to the humerus (or upper-arm bone) close 
to the nearer end. With a lever like this there is 
no economy of power—very far from it. What is 
gained is rapidity of movement. A quick movement 
