there is in front of it."

Efficiency looking a little puzzled, he added: "Just like a weathercock,
and by Keel-Surface I mean everything you can see when you view the
Aeroplane from the side of it--the sides of the body, struts, wires,
etc."

"Oh, now I begin to see light," said she: "but just exactly how does it
work?"

"I'll answer that," said Momentum. "When perhaps by a gust of air the
Aeroplane is blown out of its course and points in another direction, it
doesn't immediately fly off on that new course. I'm so strong I pull it
off the new course to a certain extent, and towards the direction of the
old course. And so it travels, as long as my strength lasts, in a more
or less sideways position."

"Then," said the Keel-Surface, "I get a pressure of air all on one side,
and as there is, in effect, most of me towards the tail, the latter
gets pressed sideways, and the Aeroplane thus tends to assume its first
position and course."

"I see," said Efficiency, and, daintily holding the Chalk, she
approached the Blackboard. "Is this what you mean?"

"Yes, that's right enough," said the Keel-Surface, "and you might
remember, too, that I always make the Aeroplane nose into the gusts
rather than away from them."

"If that was not the case," broke in Lateral Stability, and affecting
the fashionable Flying Corps stammer, "it would be a h-h-h-o-r-rible
affair! If there were too much Keel-Surface in front, then that gust
would blow the Aeroplane round the other way a very considerable
distance. And the right-hand Surface being on the outside of the turn
would have more speed, and consequently more Lift, than the Surface
on the other side. That means a greater proportion of the Lift on that
side, and before you could say Warp to the Ailerons over the Aeroplane
would go--probable result a bad side-slip"

"And what can the Pilot do to save such a situation as that?" said
Efficiency.

"Well," replied Lateral Stability, "he will try to turn the Aeroplane
sideways and back to an even keel by means of warping the Ailerons or
little wings which are hinged on to the Wing-tips, and about which you
will hear more later on; but if the side-slip is very bad he may not be
able to right the Aeroplane by means of the Ailerons, and then the only
thing for him to do is to use the Rudder and to turn the nose of the
Aeroplane down and head-on to the direction of motion. The Aeroplane
will then be meeting the air in the direction it is d