STABILITY. 25 
body rises and falls, its centre of gravity travels 
onward almost in a straight line.* 
Points that make for Automatic Stability. 
Though the position of the centre of gravity is 
of no avail, the general build of the bird and the 
elasticity of the feathers make for automatic stabil- 
ity. To begin with, thereis the question of curves. I 
have already pointed out that a curved surface gives 
more lift and less drift thana flatone. But anyone 
who experiments with gliders soon finds out that 
an excessive curve is fatal to stability. In the 
preface to the Aero Manual (1910) it is stated that 
the curve (the depth of the concavity) should not 
be more than one-twelfth of the breadth. Imagine 
a surface with a more considerable curve. The wind 
Ww—_— > eave 
Fra. 11. Rs 
Diagram to show the disadvantage of an excessive curve. 
will impinge upon its upper side and the glider will 
duck and descend rapidly to earth. Butif the curve 
is only slight, though the tendency to duck and dive 
may, no doubt, arise, yet it tends to correct itself. 
Imagine the glider launched on its way and develop- 
ing a tendency to lower its head and raise its tail 
unduly. In proportion as the tendency develops, in 
* There is no need to discuss the question of pendulum stability. 
It is possible to suspend a heavy weight from an aeroplane, and 
there are theorists who hold that automatic stability may be thus 
attained. It is obvious that in the case of a bird nothing of the 
nature of a pendulum is possible. See Flight, Feb. 25, March 18, 
and March 25, 1911. 
