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. vn). 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 



