entirely opposed to that," objected the three Stabilities, all
in a breath. "Unless it's in a perfectly straight line or a perfect
circle. Nothing but perfectly straight lines or, upon occasion, perfect
circles satisfy us, and we are strongly suspicious of your tendencies."

"Well, we shall see what we shall see," said the Force darkly. "But who
in the name of blue sky is this?"

And in tripped Efficiency, in a beautifully "doped" dress of the latest
fashionable shade of khaki-coloured fabric, a perfectly stream-lined
bonnet, and a bewitching little Morane parasol,[4] smiling as usual, and
airily exclaiming, "I'm so sorry I'm late, but you see the Designer's
such a funny man. He objects to skin friction,[5] and insisted upon me
changing my fabric for one of a smoother surface, and that delayed me.
Dear me, there are a lot more of us to-day, aren't there? I think I
had better meet one at a time." And turning to Directional Stability,
she politely asked him what he preferred to do.

"My purpose in life, miss," said he, "is to keep the Aeroplane on its
course, and to achieve that there must be, in effect, more Keel-Surface
behind the Vertical Turning Axis than there is in front of it."

[Illustration]

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,"