to indicate the place on the plane where the air
acts with the greatest force. It has reference to
a point between the front and rear margins only
of the plane.

_Fig. 24. Air lines below a concaved Plane._

UTILIZING THE RAREFIED AREA.--This structure,
however, has another important advantage, as it
utilizes the rarefied area which is produced, and
which may be understood by reference to Fig. 25.

The plane B, with its upward curve, and at the
same angle as the straight plane, has its lower
end so curved, with relation to the forward movement,
that the air, in rushing past the upper end,
cannot follow the curve rapidly enough to maintain
the same density along C, hence this exerts

an upward pull, due to the rarefied area, which
serves as a lifting force, as well as the compressed
mass beneath the plane.

CHANGING CENTER OF PRESSURE.--The center of
pressure is not constant. It changes with the
angle of the plane, but the range is considerably
less on a concave surface than on a flat plane.

_Fig. 25. Air lines above a convex Plane._