l68 THE REIGN OF LAW. 



partially folded inwards ; and this contraction of 

 the area is constantly resorted to. But a bird 

 which has wings so small and scanty as to compel 

 it to strike them always at full stretch, and with 

 great velocity in order to fly at all, is incapable of 

 standing still in the air. No man ever saw a Diver 

 or a Duck performing the evolution which the 

 Kestrel may be seen performing every hour over 

 so many English fields. The cause of this is ob- 

 vious if we refer to the principles which have 

 already been explained. We have seen that the 

 perpendicular stroke of a bird's wing has the 

 double effect of both propelling and sustaining. 

 The reaction from such a stroke brings two dif- 

 ferent forces to bear upon the bird — one whose 

 direction is upwards, and another whose direction 

 is forwards. How can these two effects be sepa- 

 rated from each other ? How can the wing be so 

 moved as to keep up just enough of the sustaining 

 force without allowing the propelling force to come 

 into play ? The answer to this, although it in- 

 volves some very complicated laws connected with 

 what mechanicians call the " parallelogram of 

 forces," is practically a simple one. It can only 



