from above; an effect that is secured by the
downward slope of the plane towards its rear edge. The air, sweeping
above the raised front section of the plane, is deflected upward, and
with such force that it cannot descend again immediately and follow
the downward curve of the surface. So, between this swiftly-moving air
stream, and the slope to the rear of the plane, a partial vacuum is
formed, and this sucks powerfully upward. With a single wing,
therefore, it is possible to gain a double lifting influence--one
above and one below.

The building of aeroplanes, once their wing lift is known, becomes a
matter of precision. According to the speed at which they fly, and the
size and curve of their planes, machines will sustain varying loads.
In some machines, as a general illustration--craft which fly fast--the
planes may bear a load equal to 10 lbs. per square foot. In others the
loading may be less than 3 lbs. per square foot.

Apart from raising a craft into the air, by the lifting power of its
wings, there is the problem of controlling it when in flight. The air
is treacherous, quickly moving. Gusts of abnormal strength, sweeping
up as they do invisibly, may threaten to overturn a machine and dash
it to earth. Eddies are formed between layers of warm and cold air.
There are, as a craft flies, constant increases or lessenings of
pressure in the air-stream that is sweeping under and over its wings;
and all these fluctuations influence its equilibrium. Unless,
therefore, a machine is automatically stable--and with craft of this
type we shall deal later--the pilot must be ready, by a movement of
the surfaces which govern the flight of the machine, to counteract
quickly, with a suitable action of his levers, the overturning
influence that may be exercised by a gust of wind. Here lies the art
of flying. A man is given a machine which, by the action of its motor
and propeller, will raise itself into the air; and it is his task,
when the craft is once aloft, to manipulate it accurately and without
accident, and to bring it to earth safely after he has made a flight.

In the description of controlling movements which follows we shall,
for the sake of convenience, and for the sake also of brevity, deal
only with the type of "pusher" biplane to which reference has been
made already, and on which large numbers of pupils have been, and are
being, trained to fly. This casts no aspersion whatever on tractor
machines or on monopla