stable currents will be found from 50 to 100 feet from the
earth, provided the wind is not diverted by such objects as trees,
rocks, etc. That there are equally stable currents higher up is true,
but they are generally to be found at excessive altitudes.

How a Bird Meets Currents.

Observe a bird in action on a windy day and you will find it continually
changing the position of its wings. This is done to meet the varying
gusts and eddies of the air so that sustentation may be maintained and
headway made. One second the bird is bending its wings, altering the
angle of incidence; the next it is lifting or depressing one wing at a
time. Still again it will extend one wing tip in advance of the other,
or be spreading or folding, lowering or raising its tail.

All these motions have a meaning, a purpose. They assist the bird in
preserving its equilibrium. Without them the bird would be just as
helpless in the air as a human being and could not remain afloat.

When the wind is still, or comparatively so, a bird, having secured the
desired altitude by flight at an angle, may sail or soar with no wing
action beyond an occasional stroke when it desires to advance. But, in a
gusty, uncertain wind it must use its wings or alight somewhere.

Trying to Imitate the Bird.

Writing in _Fly_, Mr. William E. White says:

"The bird's flight suggests a number of ways in which the equilibrium
of a mechanical bird may be controlled. Each of these methods of control
may be effected by several different forms of mechanism.

"Placing the two wings of an aeroplane at an angle of three to five
degrees to each other is perhaps the oldest way of securing lateral
balance. This way readily occurs to anyone who watches a sea gull
soaring. The theory of the dihedral angle is that when one wing is
lifted by a gust of wind, the air is spilled from under it; while
the other wing, being correspondingly depressed, presents a greater
resistance to the gust and is lifted restoring the balance. A fixed
angle of three to five degrees, however, will only be sufficient for
very light puffs of wind and to mount the wings so that the whole
wing may be moved to change the dihedral angle presents mechanical
difficulties which would be better avoided.

"The objection of mechanical impracticability applies to any plan
to preserve the balance by shifting weight or ballast. The center of
gravity should be lower than the center of the supporting surfaces,
but