referred.

Then heat will expand air, making it less dense,
or lighter, so that it will move upwardly, to be
replaced by a colder body of air. In aeronautics
neither of these conditions is of any importance
in considering the lifting power of aeroplane surfaces.

RAREFACTION PRODUCED BY MOTION.--The third
rarefied condition is produced by motion, and generally
the area is very limited when brought about
by this means. If, for instance, a plane is held
horizontally and allowed to fall toward the earth,
it will be retarded by two forces, namely, compression
and rarefaction, the former acting on the
under side of the plane, and the latter on the upper
side.

Of the two rarefaction is the most effectual,
and produces a greater effect than compression.
This may be proven by compressing air in a long
pipe, and noting the difference in gauge pressure
between the ends, and then using a suction pump
on the same pipe.

When a plane is forced through the air at any
angle, a rarefied area is formed on the side which
is opposite the one having the positive angle of
incidence.