vil 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 4 the same point in it. Let 
a 6 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 
1 and 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, 
A 
Fic. 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 
