VII 



FLIGHT 



177 



apply it to a bird's wing we understate the rate of 

 increase of the air's resistance. 



It will now be well to take a particular instance. 

 Let W A and W B represent the same wing in 

 different postures, a and b the same point in it. Let 

 a b be one inch in length, and A B three times as long. 

 When the wing descends, a passes through one inch 

 of air, A through three inches. But the resistance of 

 the air will be, at the lowest estimate, as the squares of 

 ! anc | 3_that is, at A it will be nine times as great as 

 it is at a. It is by rapid movement of its wings, 



Fig. 46. 



then, that the bird obtains a fulcrum on which they 

 can work as levers. However large an expanse a 

 wing might offer it would be useless, unless it were 

 driven through the air at a great speed ; it could not 

 possibly obtain the comparatively fixed point that 

 every lever must have. Though there is still much 

 to be said about the shape of the wing and the way 

 in which the air acts upon it, we have advanced far 

 enough to understand the system of leverage. The 

 weight to be raised is the body of the bird, the 

 power lies in the breast muscles and is applied not far 



N 



