CONTRIVANCE A NECESSITY. 169 



be done by shortening the stroke, and altering the 

 perpendicularity of its direction. Of course, if a 

 bird, by altering the axis of its own body, can 

 direct its wing-stroke in some degree forwards, it 

 will have the effect of stopping instead of pro- 

 moting progression. But in order to do this, it 

 must have a superabundance of sustaining force, 

 because some of this force is sacrificed when the 

 stroke is off the perpendicular. Hence it follows 

 that birds so heavy as to require the whole action 

 of their wings to sustain them at all, can never 

 afford this sacrifice of the sustaining force, and 

 except for the purpose of arresting their flight, can 

 never strike except directly downwards, that is, 

 directly against the opposing force of gravity. But 

 birds with superabundant sustaining power, and 

 long sharp wings, have nothing to do but to 

 diminish the length of stroke, and direct it off the 

 perpendicular at such an angle as will bring all 

 the forces bearing upon their body to an exact 

 balance, and they will then remain stationary at 

 a fixed point in the air.* 



* Mr Wolfs illustration of a Kestrel hovering shows accurately 

 the position of the Bird when the action is performed in still air. 



