Incidence, and surely such a condition will result in one side of the
Aeroplane lifting more than the other side?'

"That's all right," said the Propeller, "it's meant to off-set the
tendency of the Aeroplane to turn over sideways in the opposite
direction to which I revolve."

"That's quite clear, though rather unexpected; but how do you counteract
the effect of the gusts when they try to overturn the Aeroplane
sideways?" said she, turning to Lateral Stability again.

"Well," he replied, rather miserably, "I'm not nearly so perfect as the
Longitudinal and Directional Stabilities. The Dihedral Angle--that is,
the upward inclination of the Surfaces towards their wing-tips--does
what it can for me, but, in my opinion, it's a more or less futile
effort. The Blackboard will show you the argument." And he at once
showed them two Surfaces, each set at a Dihedral Angle like this:

"Please imagine," said the Blackboard, "that the top V is the front
view of a Surface flying towards you. Now if a gust blows it into the
position of the lower V you see that the horizontal equivalent of the
Surface on one side becomes larger, and on the other side it becomes
smaller. That results in more Lift on the lower side and less on the
higher side, and if the V is large enough it should produce such a
difference in the Lift of one side to the other as to quickly turn the
Aeroplane back to its former and normal position."

"Yes," said the Dihedral Angle, "that's what would happen if they would
only make me large enough; but they won't do it because it would too
greatly decrease the horizontal equivalent, and therefore the Lift, and
incidentally it would, as Aeroplanes are built to-day, produce an excess
of Keel Surface above the turning axis, and that in itself would spoil
the Lateral Stability. The Keel Surface should be equally divided above
and below the longitudinal turning axis (upon which the Aeroplane rolls
sideways), or the side upon which there is an excess will get blown over
by the gusts. It strikes me that my future isn't very promising, and
about my only chance is when the Junior Draughtsman makes a mistake, as
he did the other day. And just think of it, they call him a Designer now
that he's got a job at the Factory! What did he do? Why, he calculated
the weights wrong and got the Centre of Gravity too high, and they
didn't discover it until the machine was built. Then all they could do
was to give me a larger Angle. That dr