opped 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
old, in fact the father of all the Principles.

And the Blackboard had been busy, and now showed them a picture of
the Aeroplane as far as they knew it, and you will see that there is
a slight Dihedral Angle, and also, fixed to the tail, a vertical Keel
Surface or fin, as is very often the case in order to ensure the greater
effect of such surface being behind the vertical turning axis.

But Efficiency, growing rather critical with her newly gained knowledge,
cried out: "But where's the horizontal Tail Surface? It doesn't look
right like that!"

"This is when I have the pleasure of meeting you, my dear," said
Longitudinal Stability. "Here's the Tail Surface," he said, "and in
order to help me it must be set IN EFFECT at a much less Angle of
Incidence than the Main Surface. To explain we must trouble the
Blackboard again," and this was his effort:

"I have tried to make that as clear as possible," he said. "It may
appear a bit complicated at first, but if you will take the trouble to
look at it for a minute you will find it quite simple. A is the normal
and proper direction of motion of the Aeroplane, but, owing to a gust of
air, it takes up the new nose-down position. Owing to Momentum, however,
it does not fly straight along in that direction, but moves more or less
in the direction B, which is the resultant of the