CHAPTER II. 
STABILITY. 
CENTRE OF GRAVITY—POINTS THAT MAKE FOR AUTOMATIC 
STABILITY—VOLUNTARY ADJUSTMENTS. 
There are two things that we must carefully 
distinguish—stability and the maintenance of equi- 
librium. An aeroplane, when it has once completely 
lost its balance, cannot recover it, though some 
swaying or pitching can, no doubt, be corrected. The 
stability of a bird is a very different thing from 
the aviator’s careful maintenance of equilibrium. <A 
very strong and sudden gust may throw the bird on 
his side, or even on his back, and yet he will very 
quickly right himself. 
Centre of Gravity. 
But we want to know whether a bird in ordinary 
flight, when there are no very sudden gusts, is auto- 
matically stable, or whether he has to be perpetually 
making small adjustments. It is sometimes main- 
tained that a bird need take no trouble about the 
question of balance, since his centre of gravity is 
low down. The great flight muscles, which are 
massed upon the breast, form a great part of his 
weight. I have weighed the three pairs of breast 
muscles of two Wood-Pigeons and those of two 
