r, 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 total 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 dropped the bottom
of the =V= lower down, and as that's the centre of the machine, where all
the Weight is, of course that put the Centre of Gravity in its right
place. But now there is too much Keel Surface above, and the whole
thing's a Bad Compromise, not at all like Our Efficiency."

And Efficiency, blushing very prettily at the compliment, then asked,
"And how does the Centre of Gravity affect matters?"

"That's easy," said Grandfather Gravity. "I'm so heavy that if I am too
low down I act like a pendulum and cause the Aeroplane to roll about
sideways, and if I am too high I'm like a stick balanced on your finger,
and then if I'm disturbed, over I go and the Aeroplane with me; and, in
addition to that, there are the tricks I play with the Aeroplane when
it's banked up,[6] _i.e._, tilted sideways for a turn, and Centrifugal
Force sets me going the way I'm not wanted to go. No; I get on best with
Lateral Stability when my Centre is right on the centre of drift, or, at
any rate, not much below it." And with that he settled back into the
Lecturer's Chair and went sound asleep again, for he was so very, very