and consequent density of air they at present experience,
I increase at about the Square of the Speed.

``Oh, I'm a most complex and interesting personality, I
assure you--in fact, a dual personality, a sort of aeronautical
Dr. Jekyll and Mr. Hyde. There's Lift, my vertical part or
COMPONENT, as those who prefer long words would say; he
always acts vertically upwards, and hates Gravity like poison.
He's the useful and admirable part of me. Then there's Drift,
my horizontal component, sometimes, though rather erroneously,
called Head Resistance; he's a villain of the deepest
dye, and must be overcome before flight can be secured.''

``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 of 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