Deserted, did I say? But not so. The lecture that day had been upon
the Elementary Principles of Flight, and they lingered yet. Upon the
Blackboard was an illustration thus:

[Illustration]

"I am the side view of a Surface," it said, mimicking the tones of the
lecturer. "Flight is secured by driving me through the air at an angle
inclined to the direction of motion."

"Quite right," said the Angle. "That's me, and I'm the famous Angle
of Incidence."

"And," continued the Surface, "my action is to deflect the air
downwards, and also, by fleeing from the air behind, to create a
semi-vacuum or rarefied area over most of the top of my surface."

"This is where I come in," a thick, gruff voice was heard, and went
on: "I'm the Reaction. You can't have action without me. I'm a very
considerable force, and my direction is at right-angles to you," and
he looked heavily at the Surface. "Like this," said he, picking up
the chalk with his Lift, and drifting to the Blackboard.

[Illustration: The action of the surface upon the air.]

"I act in the direction of the arrow R, that is, more or less, for the
direction varies somewhat with the Angle of Incidence and the curvature
of the Surface; and, strange but true, I'm stronger on the top of the
Surface than at the bottom of it. The Wind Tunnel has proved that by
exhaustive research--and don't forget how quickly I can grow! As the
speed through the air increases my strength increases more rapidly than
you might think--approximately, as the Square of the Speed; so you see
that if the Speed of the Surface through the air is, for instance,
doubled, then I am a good deal more than doubled. That's because I am
the result of not only the mass of air displaced, but also the result of
the Speed and consequent Force with which the Surface engages the Air. I
am a product of those two factors, and at the speeds at which Aeroplanes
fly to-day, and at the altitudes 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, ca