THE MACHINERY OF FLIGHT 79 
Aggregate length 
Humerus. of wing-bones. 
Girth of | Actual | Length | Actual | Length 
humerus.| length. | propor- | length. | propor- 
tionate to tionate to 
girth. girth of 
humerus. 
Ins. Ins. Ins. Ins. Ins. 
Skua .. 38 42 4 3 133 133 
Sea Eagle | 1% 63 Tos 20175 224, 
_ of all birds. The huge hollow beak is characteristic 
of the whole skeleton (see fig. 22). 
Stiffness of Wing. Expanse of Bone. 
It is highly important that the bird’s wing when 
extended should be rigid—its bony framework, that 
is. It has elasticity where it is wanted, the elas- 
ticity of the great feathers, which tends, as I have 
shown, to make the bird automatically stable during 
flight. The stiffness of the scaffolding of the bird’s 
extended wing is most remarkable when we compare 
it with the human arm. If we extend one of our 
arms horizontally to its full length with the palm 
of the hand downward, we can, while still keeping 
the upper arm lifted, bend the elbow-joint and 
point the fore-arm vertically upward. As _ the 
bird’s wing descends with great velocity through 
the air, it does not give at the elbow, but remains 
rigid. But the stiffness of the bird’s wrist is still 
more striking; when the wing is extended hardly 
any up-and-down movement at that point can 
