orizontal and thrusting downwards,"
grumbled the Propeller, "and that's not efficient, though I suppose it's
the best we can do until that Inventor fellow finds his Mechanics."

"Thank you so much," said Efficiency. "I think I have now at any rate
an idea of the Elementary Principles of Flight, and I don't know that I
care to delve much deeper, for sums always give me a headache; but isn't
there something about Stability and Control? Don't you think I ought to
have a glimmering of them too?"

"Well, I should smile," said a spruce Spar, who had come all the way
from America. "And that, as the Lecturer says, 'will be the subject of
our next lecture,' so be here again to-morrow, and you will be glad to
hear that it will be distinctly more lively than the subject we have
covered to-day."

[Footnote 1: Propeller Slip: As the propeller screws through the air,
the latter to a certain extent gives back to the thrust of the propeller
blades, just as the shingle on the beach slips back as you ascend it.
Such "give-back" is known as "slip," and anyone behind the propeller
will feel the slip as a strong draught of air.]

[Footnote 2: Helicopter: An air-screw revolving upon a vertical axis.
If driven with sufficient power, it will lift vertically, but, having
regard to the mechanical difficulties of such construction, it is a
most inefficient way of securing lift compared with the arrangement
of an inclined surface driven by a propeller revolving about a
horizontal axis.]

[Footnote 3: Pancakes: Pilot's slang for stalling an aeroplane and
dropping like a pancake.]

PART II

THE PRINCIPLES, HAVING SETTLED THEIR DIFFERENCES, FINISH THE JOB

Another day had passed, and the Flight Folk had again gathered together
and were awaiting the arrival of Efficiency who, as usual, was rather
late in making an appearance.

The crowd was larger than ever, and among the newcomers some of the most
important were the three Stabilities, named Directional, Longitudinal,
and Lateral, with their assistants, the Rudder, Elevator, and Ailerons.
There was Centrifugal Force, too, who would not sit still and created a
most unfavourable impression, and Keel-Surface, the Dihedral Angle, and
several other lesser fry.

"Well," said Centrifugal Force, "I wish this Efficiency I've heard so
much about would get a move on. Sitting still doesn't agree with me at
all. Motion I believe in. There's nothing like motion--the more the
better."

"We are