88 STRUCTURE AXD COMPARISON. 



does he get his start? How does he guide his course? How- 

 does he stop ? 



Watch different birds taking flight. The okl crow on the 

 fence-rail, if there is no breeze, throws himself forward and 

 drops a little, which gives him his first wing stroke with 

 all the momentum of his falling body. When there is a 

 wind he turns to face it, even if he intends to fly in the other 

 direction, stretches up on his legs to his full height, and lifts 

 his wings. The wind fills them. He leans down upon it, and 

 his first stroke gives him headway and bears him up. 



Many birds give a little leap in air as a help in rising from 

 the ground. From a tree it is easy for any bird to get upon 

 the wing, but starting from a level surface the difficulties are 

 greater and they increase with the weight of the bird, whether 

 he be a good flyer or not. The turkey-buzzard, a majestic 

 bird on the wdng, makes a slow, ungraceful start. The eagle, 

 even when in danger of his life, has been reported to stop 

 to run in awkward leaps several rods because he could not at 

 once gain momentum enough for his wings to get their 

 leverage. The loon is habitually in a worse plight, for he 

 can get no chance to spring from the water, and must get his 

 momentum by running along the surface, flapping his wings. 

 Even then his wings are too small to lift his heavy body unless 

 there is a breeze blowing. 



What is momentum ? — an impulse to go ahead. A body 

 at rest has only a tendency to stay still, its inertia, until some- 

 thing sets it moving. The bird starting to fly must overcome 

 its inertia. If it can once get the going-ahead motion, all it 

 needs to do is to hold its body in the right position and lift 

 itself with its wings. 



Holding the body in this or that position alters the direction 

 of the bird's flight. If he wishes to rise he throws the body 



