e is no one
between us?"

And Efficiency, smiling and blushing, but practical as ever, says:

"And you will never throw those Compromises in my face?"

"My dear, I love you for them! Haven't they been my life ever since I
began striving for you ten long years ago?"

And so they walked off very happily, arm-in-arm together; and if this
hasn't bored you and you'd like some more of the same sort of thing, I'd
just love to tell you some day of the wonderful things they accomplish
together, and of what they dream the future holds in store.

And that's the end of the Prologue.

CHAPTER I. FLIGHT

Air has weight (about 13 cubic feet = 1 lb.), inertia, and momentum.
It therefore obeys Newton's laws[14] and resists movement. It is that
resistance or reaction which makes flight possible.

Flight is secured by driving through the air a surface[15] inclined
upwards and towards the direction of motion.

S = Side view of surface.

M = Direction of motion.

CHORD.--The Chord is, for practical purposes, taken to be a straight
line from the leading edge of the surface to its trailing edge.

N = A line through the surface starting from its trailing edge. The
position of this line, which I call the Neutral Lift Line, is found by
means of wind-tunnel research, and it varies with differences in
the camber (curvature) of surfaces. In order to secure flight, the
inclination of the surface must be such that the neutral lift line makes
an angle with and ABOVE the line of motion. If it is coincident with M,
there is no lift. If it makes an angle with M and BELOW it, then there
is a pressure tending to force the surface down.

I = Angle of Incidence. This angle is generally defined as the angle the
chord makes with the direction of motion, but that is a bad definition,
as it leads to misconception. The angle of incidence is best described
as the angle the neutral lift line makes with the direction of motion
relative to the air. You will, however, find that in nearly all rigging
specifications the angle of incidence is taken to mean the angle the
chord makes with a line parallel to the propeller thrust. This is
necessary from the point of view of the practical mechanic who has to
rig the aeroplane, for he could not find the neutral lift line, whereas
he can easily find the chord. Again, he would certainly be in doubt as
to "the direction of motion relative to the air," whereas he can easily
find a line parallel to the propell