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.

[Illustration]

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:

[Illustration]

"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 two forces, Momentum
and Thrust. And so you will note that the Angle of Incidence, which
is the inclination of the Surfaces to the Direction of Motion, has
decreased, and of course the Lift decreases with it. You will also
see, and this is the point, that the Tail Surface has lost a higher
proportion of its Angle, and consequently its Lift, than has the Main
Surface. Then, such being the case, the Tail must fall and the Aeroplane
assume its normal position again, though probably at a slightly lower
altitude."

"I'm afraid I'm very stupid," said Efficiency, "but please tell me why
you lay stress upon the words '_in effect_.'"

"Ah! I was wondering if you would spot that," he replied. "And there is
a very good reason for it. You see, in some Aeroplanes the Tail Surface
may be actually set at the same Angle on the machine as the Main
Surface, but owing to the air being deflected downwards by the front
Main Surface it meets the Tail Surface at a lesser angle, and indeed in
some cases at no angle at all. The Tail is then for its surface getting
less Lift than the Mai