STABILITY 29 
of a monoplane. This attitude must tend to 
ensure a comfortable equilibrium (PI. 11, B). 
The question of the escape of air from beneath 
the curved surfaces seems to be of the utmost 
importance. Mr. Pilcher’s celebrated glider was 
punctured with many small holes which were 
intended to encourage a steady escape instead of a 
sudden upsetting rush on one side. I cannot judge 
whether this was a good plan for dealing with the 
difficulty. At any rate, the wing of evena large bird 
presents too small a surface to be treated in this 
way. But it is probable that the notches between 
the feathers at the wing’s posterior margin tend to 
prevent irregular escapes of air from below. The 
great soaring birds, whose steadiness as they circle 
with motionless wings is so marvellous, have the 
great flight-feathers parted like outspread fingers. 
Moreover, the force of the wind sometimes bends 
them conspicuously upward, thus giving them an 
incline that is recognized as favourable to equilibrium. 
Voluntary Adjustments. 
It cannot be gainsaid that there is much in the 
bird’s build that makes for automatic stability. 
Nevertheless, since the wind, at low levels at any 
rate, is a chartered libertine, full of capricious un- 
expected eddies, such automatic adjustments are 
altogether inadequate. The bird must be ready at 
a& moment’s notice to give his mind to the question 
of balance, and make conscious voluntary adjust- 
ments. But even we ourselves find that movements 
we are perpetually making tend, through force of 
habit, to become automatic, and that in some cases 
