disadvantage of being
invisible. An aviator cannot see the dangerous currents and eddies
into which he may be steering his craft; and so it was not surprising,
in those days when aircraft were frailer than they should have been,
and cross-country flights were first being made, that machines broke
often while in flight and that the airman's enemy, the wind, claimed
many victims.

Wind fluctuations that are dangerous, those which possess for one
reason or another an abnormal strength, are encountered frequently
when a pilot is fairly near the earth; and his peril is all the
greater in consequence. On a windy day, one on which there are heavy
gusts followed by comparative lulls, it is when he is close to the
ground, either in ascending or before alighting, that a pilot has most
to fear. If he is well aloft, with plenty of air space beneath him,
and particularly if he has a machine that is inherently stable, he has
little to fear from the wind; save, perhaps, should his engine fail
him, or should he find--as has been the case in war flying--that the
force of the wind, blowing heavily against him, and reducing the speed
of his machine, has prevented him from regaining his own lines before
his petrol has become exhausted. The modern aeroplane, when its
engine-power is ample, and it is at a suitable altitude, can wage
battle successfully even with a gale. But it must rise from the earth
when it begins a flight, and return to earth again when its journey is
done; and here, in the areas of wind that are disturbed by hills,
woods, and contours of the land, there are often grave dangers. The
wind at these low altitudes blows flukily. Its direction may be
affected, for instance, owing to the influence of a hill or ridge. A
side gust, blowing powerfully and unexpectedly against a machine, just
as it is nearing the ground before alighting, may cause it to tilt to
such an angle that it begins a side-slip. If the craft was
sufficiently high in the air, when this happened, the pilot would be
able, probably, to convert the side-slip into a dive, and the dive
into a renewal of his normal flight. But if such a side-slip begins
near the ground, and there is an insufficient amount of clear space
below the machine, it may strike the ground in its fall, and become a
wreck, before there is time for the pilot, or for the machine itself,
to exercise a righting influence. The fact that a craft may be forced
temporarily from its equilibrium, say