hich is normally my function. Surely our _roles_ have changed one with
the other, and I'm then the Elevator and the Elevator is me!"

[Illustration]

Said Lateral Stability to the Rudder, "That's altogether the wrong way
of looking at it, though I admit"--and this rather sarcastically--"that
the way you put it sounds rather fine when you are talking of your
experiences in the air to those 'interested in aviation' but knowing
little about it; but it won't go down here! You are a Controlling
Surface designed to turn the Aeroplane about a certain axis of the
machine, and the Elevator is a Controlling Surface designed to turn the
Aeroplane about another axis. Those are your respective jobs, and you
can't possibly change them about. Such talk only leads to confusion, and
I hope we shall hear no more of it."

"Thanks," said Efficiency to Lateral Stability. "And now, please, will
you explain your duties?"

"My duty is to keep the Aeroplane horizontal from Wing-tip to Wing-tip.
First of all, I sometimes arrange with the Rigger to _wash-out_, that
is decrease, the Angle of Incidence on one side of the Aeroplane, and
to effect the reverse condition, if it is not too much trouble, on the
other side."

"But," objected Efficiency, "the Lift varies with the Angle of 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:

[Illustration: H.E., Horizontal equivalent.]

"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 large