surface. It is derived from the Greek _planos_, wandering, therefore the
entire word signifies an air wanderer.

The surfaces are really aero curves arched in the rear of the front
edge, thus allowing the supporting surface of the aeroplane in passing
forward with its backward side set at an angle to the direction of its
motion, to act upon the air in such a way as to tend to compress it
on the under side.

After the surfaces come the rudders in importance. It is of vital
consequence that the machine be balanced by the operator. In the present
method of balancing an aeroplane the idea in mind is to raise the lower
side of the machine and make the higher side lower in order that it
can be quickly righted when it tips to one side from a gust of wind,
or when making angle at a sudden turn. To accomplish this, two methods
can be employed. 1. Changing the form of the wing. 2. Using separate
surfaces. One side can be made to lift more than the other by giving
it a greater curve or extending the extremity.

In balancing by means of separate surfaces, which can be turned up or
down on each side of the machine, the horizontal balancing rudders are
so connected that they will work in an opposite direction--while one
is turned to lift one side, the other will act to lower the other side
so as to strike an even balance.

The motors and propellers next claim attention. It is the motor that
makes aviation possible. It was owing in a very large measure to the
introduction of the petrol motor that progress became rapid. Hitherto
many had laid the blame of everything on the motor. They had
said,--"give us a light and powerful engine and we will show you how