sive positions of the wing in tlie down-stroke and re- 

 covery. 



Fig. 5. 



71. I say the down-stroke is as effective as the bird 

 chooses ; that is to say, it can be given with exactly 

 the quantity of imjDulse, and exactly the quantity of 

 supporting power, required at the moment. Thus, when 

 the bird wants to ily slowly, the wings are fluttered fast, 

 giving vertical blows; if it wants to pause absolutely 

 in still air, (this large birds cannot do, not being able to 

 move their wings fast enough,) the velocity becomes 

 vibration, as in the humming-bird : bnt if there is wind, 

 any of the larger birds can lay themselves on it like a 

 kite, their own weight answering the purpose of the 

 string, while they keep the wings and tail in an inclined 

 plane, giving them as much gliding ascent as counteracts 

 the fall. They nearly all, however, nse some slightly 

 fi^lidino^ force at the same time ; a sinHe stroke of the 

 wing, with forward intent, seeming enough to enable 

 them to glide on for half a minute or more without 

 stirring a plume. A circling eagle floats an inconceiva- 

 ble time without visible stroke : (fancy the pretty action 

 of the inner wing, hacking air instead of water, which 



