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