78 love's meinie. 



arrow, W is the locus of the leading feather of 

 its wing, and a and b^ roughly, the successive 

 positions of the wing in the down-stroke and 

 recovery. 



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 impulse, and 

 exactly the quantity of supporting power, re- 

 quired at the moment. Thus, when the bird 

 wants to fly 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 : but 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, use some 

 slightly gliding force at the same time ; a single 

 stroke of the wing, with forward intent, seeming 

 enough to enable them to ghde on for half a 

 minute or more without stirring a plume. A 



* See App. p. 197, § 145- 



