l which left room for the play of the screw. In this ugly
arrangement the loss of efficiency is easy to see. The screw works in a
disturbed medium, and the complicated metal-work presents a large
resistance to the passage of the machine through the air. The monoplane,
from the first, was a 'tractor' machine; its airscrew was in front of
the planes, and its body, or fuselage, was covered in and streamlined,
so as to offer the least possible resistance to the air. A later
difficulty caused by the forward position of the airscrew had nothing to
do with flying. When the war came, and machine-guns were mounted on
aeroplanes, a clear field was needed for forward firing. This difficulty
was ultimately met by the invention of a synchronizing gear, which timed
the bullets between the strokes of the airscrew-blades. In all but a few
types of machine the airscrew is now retained in the forward position.
The debate between monoplane and biplane is not yet concluded; the
biplane holds its own because with the same area of supporting surface
it is much stronger and more compact than the monoplane.

Instead of wing-warping, which puts a strain on the supporting surfaces
and is liable to distort them, the French (to whom Bleriot is believed
to have shown the way) introduced ailerons, that is, small subsidiary
hinged planes attached to the extremities of the wings. By controlling
these, one up and the other down, in conjunction with the rudder, the
pilot can preserve his lateral balance, and turn the machine to right or
left. Later on, these ailerons, when they were borrowed by the Voisin
and Farman biplanes, were not fitted to the extremities of the planes,
but became hinged flaps forming the extreme section of the trailing
edge; and this position they have kept in all modern aeroplanes. An even
greater advance was made by the French school in its device for the
control of the machine. The machine which Wilbur Wright flew in France
was controlled by two upright levers, grasped by the pilot, one in
either hand. The left-hand lever moved only backwards and forwards; it
controlled the elevator and directed the machine upwards or downwards.
The right-hand lever controlled the rudder and the warping of the wings.
By moving it backwards or forwards the pilot turned the machine to right
or left; by moving it sideways he warped the wings. There is nothing
instinctive or natural in these correspondences; the backward and
forward movement which i