28 THE FLIGHT OF BIRDS 
two extremities. APP nnesent the aviator is at a 
great disadvantage as compared with a bird. The 
great expanse of his planes is in itself a danger, 
and his machine has less power of automatic adjust- 
ment. Moreover, the most experienced pilot is 
a mere novice compared with a bird that has flown 
many times every day since he left the nest. Still, 
birds as flyers have already reached their zenith; 
for aviators greater things are still possible. 
Leaving the possibilities or impossibilities of the 
future, I must return to the bird as he is, and call 
attention to another point which promotes stability. 
The extremities of the wings of the best flyers 
have very little downward curve. If they are 
travelling very fast through the air, the primary 
wing-feathers may even bend slightly upwards, 
and aviators have found that the upward incline 
of their ‘‘ planes ’’ reduces the amount of rolling. 
Thus inclined they give less support than if curved. 
downwards at the extremities, but the very fact 
that the air is allowed to escape easily to right and 
left is favourable to equilibrium. If an attempt 
is made by bending the extremities downward to 
check its escape, to coop it up, it may make a 
sudden rush from the right or from the left con- 
cavity and cause risk of a capsize. The wing, 
amply concave as it is till it begins to taper, and 
shallowing as it tapers, keeps in view the questions 
of lift and equilibrium; it holds the air, yet provides 
for its escape. Sometimes when a bird wishes to 
glide downward he will point his wings steeply 
upward—a method often adopted by pigeons— 
much more steeply than the right and left surfaces 
