The morning news. [volume] (Savannah, Ga.) 1887-1900, 1898-03-03
HOW WELL KNOWN BIRDS FLY.
Their Use of the Force of the Wind
in Flight.
F. W. Headley in knowledge.


res, and they turn them ali to account.
Many problems, which men Of science are
compelled to class among those which they
hope to solve some day or, perhaps, de-
spair of solving, a young bird begins to
work out directly he is fiedged. l will take
some typical case in which d bird derives
help from the wind 1t will be found that
the wind If it is to lift a bird supple
menting the work of his wings, or even
doing all the lifting, must either move in
an upward direction, or if it does not de-
viate from the horizontal. it must be of
unequal velocity. A wind thai is unequal,
or not uniform, may either be made up of
currents moving at unequal rates, or It
may blow now gently. now strongly. A
uniform horizontal breeze cannot lift or
maintain a heavy body in the air unless
that body has momentum of its own. It
is no better than still air, since the bird
1--WE must try to put ourselves in his po-
sitlon) becomes part of the moving cur
rent The momentum required he derives
either from his own wing beats or from
the inequalities of the wind When a bird
rise with wings outstretched and motion
less there must be inequality. supposlng
that the wind is horizontal.
A lark when he is flying upward in
variably faces the wind. When he turns
away from it he ceases to rise and gem
again to rise he turns his head toward
the wind. To take another instance: A
cormorant stated at last. wishes to join
his comrades, who are drying their wings
on a rock not far off. The wind is blow
ing toward the rock in question; accord
ingly ne begins by flying in the opposite
direction, so that the wind may assist
him as he struggles to rise from the wa
ter This feat accomplished. he turns
!, t. Il Y=Y1s
ton The lark and the cormorant are
quite right in thinking that the wind viii
help to lift them, but it does so only be
cause 1t increases in velocity as they rise.
Near the surface of the land or the sea
il is retarded by friction. An thermometer
reveals a very marked. increase in velocity
a It successive elevations not far from the
ground.
BY means of kites experiments have
been made up to a hight of a thousand
feet. and even there the increase conttn-
ues though it is much more gradual.
Were II not for this increase of velocity
with altitude the lark, after his first jump
from the ground, would, but for his own
efforts, move backward with the wind. II
would not lift him any more than it would
a balloon from which the gas has escaped.
But perpetually passing from a slower to
a more rapid current Qn theory it is con-
STRATA, he has always an inertia which
enables the wind to lift him. He is still,
so to speak, part of the slowly moving
stratum below which he energies into the
the rapidly moving stratum above. In
fact, he illustrates the principle of the
kite inertia, which is equivalent to mo-
mentum, taking the place of the string.
AS long as the kite is held by the string
il will rise; cut the string and the wind
Wi a IN fha safs Of
gravitation.
We see, then that if the lark wishes the
wind TO help him to rise, he must face it
incline the body forward as a kite is in
clined. and further. that the inequality of
the wind must give him inertia, or mo-
mentum, or else his wings will have to
raise him without assistance. When Sulls
advance to right angles with the wind
without any beating of the wings, the
principle applied is the same. From some
slight elevation, they glide downward till
they are near the ground or the surface
of the sea, then possibly by a twist at the
WaisD they turn suddenly and face the
wind, which lifts them to their former ele-
walton; after which the process begins
again, It may be that when birds soar,
i. e., rise in spirals without moving their
wings, they are availing themselves at
each upward step of the increasing veloc-
ity of the wind. But they sometimes st
tain an elevation of mile or more. and
there the increase must be neglible guam
tity, if it has not at length reached the
vanishing point; and, therefore. we have
to look elsewhere for an explanation of
soaring. .t
This brings us to the subject of up-cur-
rents, obviously much to be desired if
wing labor is to be saved, and birds when
ever such assistance is TO be had, are al
ways ready to profit by il. When the wind
blows against a cliff, it is. of course, de-
Elected upward, and gulls may sometime
be seen enjoying the fun of being lifted
like foam bubble of the up-current, then
descending TO be lifted again. Oftener
they may be seen soaring over cliffs or
hills near the shore ascending in noble spi-
rails without a motion of their wings. The
secret lies in the upward slant of the wind
But soaring may be seen under quite
different circumstances. The adjutant
bird, one of the noblest of soarers. rises
till he looks mere speck or vanshes all
together. over the plain of Burmah. Can
there be up-currents there? 1t is possible
that, owing TO the ground being unequally
heated by the sun, there may be such cur
rents here and there. But in Egypt I have
seen kites soaring over fields where the
young corn was growing thick and green.
and where, consequently. the earth's sur
face was not much heated. Probably the
adjutant depends on che irregularity of the
wind
Even what we call steady wind, we