g always an even and
efficient curvature to the steady currents of the air."

High Winds Threaten Stability.

To ensure perfect stability, without control, either human or automatic,
it is asserted that the aeroplane must move faster than the wind is
blowing. So long as the wind is blowing at the rate of 30 miles an hour,
and the machine is traveling 40 or more, there will be little trouble as
regards equilibrium so far as wind disturbance goes, provided the wind
blows evenly and does not come in gusts or eddying currents. But when
conditions are reversed--when the machine travels only 30 miles an hour
and the wind blows at the rate of 50, look out for loss of equilibrium.

One of the main reasons for this is that high winds are rarely steady;
they seldom blow for any length of time at the same speed. They are
usually "gusty," the gusts being a momentary movement at a higher speed.
Tornadic gusts are also formed by the meeting of two opposing currents,
causing a whirling motion, which makes stability uncertain. Besides, it
is not unusual for wind of high speed to suddenly change its direction
without warning.

Trouble With Vertical Columns.

Vertical currents--columns of ascending air--are frequently encountered
in unexpected places and have more or less tendency, according to their
strength, to make it difficult to keep the machine within a reasonable
distance from the ground.

These vertical currents are most generally noticeable in the vicinity
of steep cliffs, or deep ravines. In such instances they are usually of
considerable strength, being caused by the deflection of strong winds
blowing against the face of the cliffs. This deflection exerts a back
pressure which is felt quite a distance away from the point of origin,
so that the vertical current exerts an influence in forcing the machine
upward long before the cliff is reached.

CHAPTER XV. THE ELEMENT OF DANGER.

That there is an element of danger in aviation is undeniable, but it is
nowhere so great as the public imagines. Men are killed and injured in
the operation of flying machines just as they are killed and injured
in the operation of railways. Considering the character of aviation the
percentage of casualties is surprisingly small.

This is because the results following a collapse in the air are very
much different from what might be imagined. Instead of dropping to the
ground like a bullet an aeroplane, under ordinary conditions will, when