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on these points； all varying to such a remarkable
degree； we must look elsewhere to find the secret。
Only one other direction offers any opportunity；
and that is in the individual wing movement。
NATURE'S TYPE NOT UNIFORM。When this is
examined； the same obscurity surrounds the issue。
Even the speeds vary to such an extent that when
it is tried to differentiate them； in comparison
with form； shape； and construction； the experimenter
finds himself wrapt in doubt and perplexity。
But birds do fly； notwithstanding this wonderful
array of contradictory exhibitions。 Observation
has not enabled us to learn why these things
are so。 High authorities； and men who are expert
aviators； tell us that the bird flies because
it is able to pick out ascending air currents。
THEORIES ABOUT FLIGHT OF BIRDS。Then we
are offered the theory that the bird has an instinct
which tells it just how to balance in the
air when its wings are once set in motion。
Frequently； what is taken for instinct； is something
entirely different。
It has been assumed； for instance； that a cyclist
making a turn at a rapid speed； and a bird flying
around a circle will throw the upper part of the
body inwardly to counteract the centrifugal force
which tends to throw it outwardly。
Experiments with the monorail car； which is
equipped with a gyroscope to hold it in a vertical
position； show that when the car approaches a
curve the car will lean inwardly； exactly the same
as a bird； or a cyclist； and when a straight stretch
is reached； it will again straighten up。
INSTINCT。Now； either the car； so equipped
possesses instinct； or there must be a principle
in the laws of nature which produces the similarity
of action。
In like manner there must be some principle
that is entirely independent of the form of matter；
or its arrangement； which enables the bird
to perform its evolutions。 We are led to believe
from all the foregoing considerations that it is
the manner or the form of the motion。
MODE OF MOTION。In this respect it seems to
be comparable in every respect to the great and
universal law of the motions in the universe。
Thus； light； heat and electricity are the same； the
manifestations being unlike only because they
have different modes of motion。
Everything in nature manifests itself by motion。
It is the only way in which nature acts。
Every transformation from one thing to another；
is by way of a movement which is characteristic
in itself。
Why； then； should this great mystery of nature；
act unlike the other portions of which it is
a part？
THE WING STRUCTURE。The wing structure of
every flying creature that man has examined； has
one universal point of similarity； and that is the
manner of its connection with the body。 It is a
sort of universal joint； which permits the wing
to swing up and down； perform a gyratory movement
while doing so； and folds to the rear when
at rest。
Some have these movements in a greater or
less degree； or capable of a greater range； but
the joint is the same； with scarcely an exception。
When the stroke of the wing is downwardly the
rear margin is higher than the front edge； so
that the downward beat not only raises the body
upwardly； but also propels it forwardly。
THE WING MOVEMENT。The moment the wing
starts to swing upwardly the rear end is
depressed； and now； as the bird is moving forwardly；
the wing surface has a positive angle of
incidence； and as the wing rises while the forward
motion is taking place； there is no resistance
which is effective enough to counteract the
momentum which has been set up。
The great problem is to put this motion into a
mechanical form。 The trouble is not ascribable
to the inability of the mechanic to describe this
movement。 It is an exceedingly simple one。
The first difficulty is in the material that must
be used。 Lightness and strength for the wing
itself are the first requirements。 Then rigidity
in the joint and in the main rib of the wing； are
the next considerations。
In these respects the ability of man is limited。
The wing ligatures of flying creatures is exceedingly
strong； and flexible； the hollow bone formation
and the feathers are extremely light； compared
with their sustaining powers。
THE HELICOPTER MOTION。The helicopter； or
helix…wing； is a form of flying machine which depends
on revolving screws to maintain it in the
air。 Many propellers are now made； six feet in
length； which have a pull of from 400 to 500
pounds。 If these are placed on vertically…disposed
shafts they would exert a like power to
raise a machine from the earth。
Obviously； it is difficult to equip such a machine
with planes for sustaining it in flight； after it is
once in the air； and unless such means are provided
the propellers themselves must be the
mechanism to propel it horizontally。
This means a change of direction of the shafts
which support the propellers； and the construction
is necessarily more complicated than if they
were held within non…changeable bearings。
This principle； however； affords a safer means
of navigating than the orthopter type； because
the blades of such an instrument can be forced
through the air with infinitely greater speed than
beating wings； and it devolves on the inventor to
devise some form of apparatus which will permit
the change of pull from a vertical to a horizontal
direction while in flight。
CHAPTER VI
THE LIFTING SURFACES OF AEROPLANES
THIS subject includes the form； shape and angle
of planes； used in flight。 It is the direction in
which most of the energy has been expended in
developing machines； and the true form is still
involved in doubt and uncertainty。
RELATIVE SPEED AND ANGLE。The relative
speed and angle； and the camber； or the curved
formation of the plane； have been considered in
all their aspects； so that the art in this respect has
advanced with rapid strides。
NARROW PLATES MOST EFFECTIVE。It was
learned； in the early stages of the development
by practical experiments； that a narrow plane；
fore and aft； produces a greater lift than a wide
one； so that； assuming the plane has 100 square
feet of sustaining surface； it is far better to make
the shape five feet by twenty than ten by ten。
However； it must be observed； that to use the
narrow blade effectively； it must be projected
through the air with the long margin forwardly。
Its sustaining power per square foot of surface
is much less if forced through the air lengthwise。
Experiments have shown why a narrow blade
has proportionally a greater lift； and this may
be more clearly understood by examining the
illustrations which show the movement of planes
through the air at appropriate angles。
_Fig。 22。 Stream lines along a plane。_
STREAM LINES ALONG A PLANE。In Fig。 22； A
is a flat plane； which we