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 Main Surface, although set at the same angle on
the machine. It may then be said to have IN EFFECT a less Angle of
Incidence. I'll just show you on the Blackboard."

"And now," said Efficiency, "I have only to meet the Ailerons and the
Rudder, haven't I?"

"Here we are," replied the Ailerons, or little wings. "Please hinge us
on to the back of the Main Surfaces, one of us at each Wing-tip, and
join us up to the Pilot's joystick by means of the control cables. When
the Pilot wishes to tilt the Aeroplane sideways, he will move the stick
and depress us upon one side, thus giving us a larger Angle of Incidence
and so creating more Lift on that side of the Aeroplane; and, by means
of a cable connecting us with the Ailerons on the other side of the
Aeroplane, we shall, as we are depressed, pull them up and give them a
reverse or negative Angle of Incidence, and that side will then get
a reverse Lift or downward thrust, and so we are able to tilt the
Aeroplane sideways.

"And we work best when the Angle of Incidence of the Surface in front
of us is very small, for which reason it is sometimes decreased or
washed-out towards the Wing-tips. The reason of that is that by the
time the air reaches us it has been deflected downwards--the greater the
Angle of Incidence the more it is driven downwards--and in order for
us to secure a Reaction from it, we have to take such a l