STABILITY 29 



of a monoplane. This attitude must tend to 

 ensure a comfortable equilibrium (PI. in, 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 even a 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 



