lled Head Resistance; he's a villain of the deepest dye,
and must be overcome before flight can be secured."

[Illustration]

"And I," said the Propeller, "I screw through the air and produce the
Thrust. I thrust the Aeroplane through the air and overcome the Drift;
and the Lift increases with the Speed, and when it equals the Gravity
or Weight, then--there you are--Flight! And nothing mysterious about
it at all."

"I hope you'll excuse me interrupting," said a very beautiful young
lady, "my name is Efficiency, and, while, no doubt, all you have said is
quite true, and that, as my young man the Designer says, 'You can make a
tea-tray fly if you slap on Power enough,' I can assure you that I'm not
to be won quite so easily."

"Well," eagerly replied the Lift and the Thrust, "let's be friends. Do
tell us what we can do to help you to overcome Gravity and Drift with
the least possible Power. That obviously seems the game to play, for
more Power means heavier engines, and that in a way plays into the hands
of our enemy, Gravity, besides necessitating a larger Surface or Angle
to lift the Weight, and that increases the Drift."

"Very well," from Efficiency, "I'll do my best, though I'm so shy, and
I've just had such a bad time at the Factory, and I'm terribly afraid
you'll find it awfully dry."

[Illustration]

"Buck up, old dear!" This from several new-comers, who had just
appeared. "We'll help you," and one of them, so lean and long that
he took up the whole height of the lecture room, introduced himself.

"I'm the High Aspect Ratio," he said, "and what we have got to do to
help this young lady is to improve the proportion of Lift to Drift.
The more Lift we can get for a certain area of Surface, the greater
the Weight the latter can carry; and the less the Drift, then the less
Thrust and Power required to overcome it. Now it is a fact that, if the
Surface is shaped to have the greatest possible span, _i.e._, distance
from wing-tip to wing-tip, it then engages more air and produces both
a maximum Reaction and a better proportion of Lift to Drift.

"That being so, we can then well afford to lose a little Reaction by
reducing the Angle of Incidence to a degree giving a still better
proportion of Lift to Drift than would otherwise be the case; for you
must understand that the Lift-Drift Ratio depends very much upon the
size of the Angle of Incidence, which should be as small as possible
within certain limits. So wha