cable. Flat lifting surfaces are, however, never used.

The surface acts upon the air in the following manner:

[Illustration]

As the bottom of the surface meets the air, it compresses it and
accelerates it _downwards_. As a result of this definite action there
is, of course, an equal and opposite reaction _upwards_.

The top surface, in moving forward, tends to leave the air behind
it, thus creating a semi-vacuum or rarefied area over the top of the
surface. Consequently the pressure of air on the top of the surface
is decreased, thus assisting the reaction below to lift the surface
_upwards_.

The reaction increases approximately as the square of the velocity. It
is the result of (1) the mass of air engaged, and (2) the velocity and
consequent force with which the surface engages the air. If the reaction
was produced by only one of those factors it would increase in direct
proportion to the velocity, but, since it is the product of both
factors, it increases as V^2.

Approximately three-fifths of the reaction is due to the decrease of
density (and consequent decrease of downward pressure) on the top of the
surface; and only some two-fifths is due to the upward reaction secured
by the action of the bottom surface upon the air. A practical point in
respect of this is that, in the event of the fabric covering the surface
getting into bad condition, it is more likely to strip off the top than
off the bottom.

[Illustration]

The direction of the reaction is, at efficient angles of incidence,
approximately at right-angles to the neutral lift line of the surface,
as illustrated above; and it is, in considering flight, convenient to
divide it into two component parts or values, thus:

1. The vertical component of the reaction, _i.e._, Lift, which is
opposed to Gravity, _i.e._, the weight of the aeroplane.

2. The horizontal component, _i.e._, Drift (sometimes called
Resistance), to which is opposed the thrust of the propeller.

The direction of the reaction is, of course, the resultant of the forces
Lift and Drift. The Lift is the useful part of the reaction, for it
lifts the weight of the aeroplane.

The Drift is the villain of the piece, and must be overcome by the
Thrust in order to secure the necessary velocity to produce the
requisite lift for flight.

DRIFT.--The drift of the whole aeroplane (we have considered only the
lifting surface heretofore) may be conveniently divided into three
parts, as follo