``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.

``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 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