144 THE REIGN OF LAW. 



velocity and strength its perpendicular blows upon 

 the air, and by virtue of the structure of its wings 

 the same blow both sustains and propels it* 



The truth of this explanation of the mechanical 

 theory of flight may be tested in various ways. 

 In the first place it is quite visible to the eye. In 

 many birds flying straight to us, or straight from 

 us, the effect of aerial resistance in bending up- 

 wards the ends of the quill feathers is very con- 

 spicuous. The flight of the common Rook affords 

 an excellent example — where the bird is seen fore- 

 shortened. In Eagles the same effect is very 

 marked — the wing tips forming a sharp upward 

 curve. I have seen it equally obvious in that 

 splendid bird the Gannet, or Solan Goose ; and 

 when we recollect the great weight which those 

 few quill feathers are thus seen sustaining, we 

 begin to appreciate the degree in which lightness, 

 strength, and imperviousness to the passage of 



* The upward stroke has no sustaining power, but has consider- 

 able propelling power ; because some air, failing to escape be- 

 tween the feathers, must always pass along the convex surface of 

 the wing, and escaping backwards, must exert upon the ends of the 

 quills a similar reactive force to that which is exerted in the down- 

 ward stroke. 



