curved one, means that it must be acted upon by
some external force.

We have explained that power is something
which is inherent in the thing itself. Now, in order
that there may be a change imparted to a
moving mass, advantage must be taken of the medium
through which it moves,--the atmosphere.

VERTICAL CONTROL PLANES.--If vertically-arranged
planes are provided, either fore or aft of
the machine, or at both ends, the angles of incidence
may be such as to cause the machine to
turn from its straight course.

In practice, therefore, since it is difficult to supply
sufficient power to a machine to keep it in motion
horizontally, at all times, aeroplanes are provided
with supporting surfaces, and this aid in
holding it up grows less and less as its speed increases.

But, however strong the power, or great the
speed, its control from side to side is not dependent
on the power of the engine, or the speed
at which it travels through the air.

Here the size of the vertical planes, and their
angles, are the only factors to be considered, and
these questions will be considered in their proper
places.