*Reprinted by permission from "The Evening Post," New York.
The question why man cannot fly may be answered in a very simple and
yet satisfactory manner: He has not been organically constructed for
that purpose. That may seem like cutting the Gordian knot, but, after
all, it is the only explanation that can be given. You might as well
ask why man cannot clutch a perch with his foot after the manner of a
bird or a monkey, for the response would be the same--his foot was made
for walking, and not for prehensile purposes. On the other hand, the
bird cannot grasp an object with its wings, while a man's hand is well
adapted for the performance of such a function. Nature's motto in her
whole realm seems to be: "Every creature after its kind."
When we look at the structure of the flying birds, we see at once that
they were formed for swift locomotion through the air, just as plainly
as the lithe skiff was made to glide over the water or the carriage to
spin over the land. In the first place, the body of the bird is
comparatively light--that is, in proportion to the width, strength, and
extent of its wings. By its thick, light, airy covering of feathers
its body is made still more buoyant, besides presenting a larger
surface to the supporting air with very little additional weight. The
tail, too, with its long, closely woven quills spread out like a fan,
not only serves the purpose of a rudder for guiding the aërial craft,
but is still more useful in helping to sustain the bird's weight in the
up-buoying element.
It is interesting to note that the feathers on the bodies of the flying
birds are arranged in tracts, with intervals here and there of quite,
or almost, bare skin, called "apteria." Now, when a bird is carefully
skinned, it will be seen that the feathered spaces have their own
special slips of muscles inserted into the roots of the feathers, and
when these muscles are contracted, they serve to raise the feathers,
and must, therefore, be of some subsidiary value in flying, by making
the bird's body more buoyant. Suggestive, indeed, is the fact that the
plumes of the non-flyers are not arranged in tracts, but are evenly
distributed over the body.
Nor is that all that Nature has done to carry out her evident purpose
of making the bird a natural "flying machine." The body of the bird
contains numerous air sacs, all connected with the lungs, and these,
when inflated, are a great help in flying by making the bird light.
More than that, many of the bones, though strong, have thin walls and
are hollow, the cavities being connected with the lungs and air sacs,
from which they are also filled with air, contributing another element
of lightness to the aërial navigator. That the bird's bones are
capable of being permeated with air can be demonstrated by actual
experiment, and is, therefore, a scientifically established fact. It
is easy enough to prove it in this way: Take a dead bird that has been
beheaded, pass a syringe into its windpipe, tie it carefully so that
the air cannot escape at the sides, then blow the air down through the
tube, and you will be able to follow the passage of the air into the
skin and other parts of the body. Now, if you will cut off one of the
bones, you can detect the air passing from the cut surface; and, more
than that, as a scientific English writer says, "if the experiment be
made by using colored fluid instead of air--which is pumped in by a
syringe--the fluid can be seen to ooze from the ends of any bone or
muscle that has been cut across." Thus it is seen that the whole body
of the fowl is so constructed that it can be pervaded with air.
However, while all parts of the bird's organism combine to produce the
end in view, the special instruments of flight are the wings. They are
really the fore limbs of the fowl, but differ in many respects from the
fore limbs of the mammals. They are under the control of muscles of
great comparative strength, as every one knows who has ever been beaten
by the wings of even an ordinary barnyard fowl, which has meagre powers
of flight. What a powerful stroke a large hawk or an eagle must be
able to deliver! If man's arm muscles were as strong in proportion, he
might have some hope of one day navigating the air on artificial wings,
but it is due principally to this muscular weakness that Darius Green
has never been able to make a success of his flying machine, and
perhaps never will. He would not have the strength to wield wings
large enough to sustain so much avoirdupois on the yielding air.
The wings are highly specialized members of the avicular organism, and
hence differ in many important respects from the fore or pectoral limbs
of the mammals. Beginning at the point nearest the body, let us
examine one of these wonderful instruments. The wing proper begins at
the shoulder joint, which hinges freely upon the shoulder in a shallow
socket, into which the globular head of the first bone fits closely,
and in which it is firmly held by the powerful muscles that control the
organs of flight. The first bone is called the humerus, and is the
largest and strongest bone of the wing, extending from the shoulder to
the elbow. At the elbow, which is the first angle of the wing,
reaching backward when the wing is folded, the humerus articulates in a
wisely designed way with two other bones, called the ulna and radius,
which together constitute the forearm and extend to the wrist joint.
It must be remembered that, when the wing is closed, the forearm is the
segment that reaches obliquely forward. The wrist joint is the second
angle of the wing. In the wrist there are two small bones (the radiale
and ulnare) which serve an important purpose in joining the forearm
with what is known as the hand, and make possible the specialized
movement of the two parts upon each other. The hand is the terminal
segment of the wing, composed of the metacarpal bones and the digits or
fingers. Of the last-named organs there are ordinarily three, forming
a graceful tapering point to the wing, and giving to it the symmetry
and proportion that are required for effective use. When the wing is
folded, the hand extends obliquely downward and backward.
Now, these bones and their attendant ligatures are wonderfully and
wisely contrived. The humerus moves freely in its socket in the
shoulder, so that it can be swung in every required direction, and yet,
as should be the case, its principal movement is up and down in a
vertical line--the precise movement required for the effective
wingstrokes in flight. But note further. The elbow joint, unlike that
of the shoulder, is a rigid hinge, permitting motion in only one plane,
that of the wing itself, or nearly so. The same is true of the wrist
joint, which holds the hand firmly, allowing no motion save that which
opens and closes the wing. The wisdom of this arrangement will be seen
at a glance.
In the human arm the hand can be moved in every direction with the
greatest freedom, and, moreover, the wrist may be turned and the hand
laid on its back, its palm, its edge, or at almost any conceivable
angle. This is a very convenient contrivance for man, but it would be
a great misfortune for our avian friends if their wings would rotate so
readily; for in that case they would not have sufficient rigidity to
answer the purposes of flight, but would be twisted into every position
by the assaults of the air currents. Besides, even in ordinary flight
it would require a constant muscular effort to keep the wings in the
proper position. How wisely Nature has devised the bird's flying
apparatus! When outstretched, it is held firmly by the power of its
own mechanism, with its broad under surface lying horizontally, and no
breezy current can bend or twist it from its normal position.
The set of muscles that open the wing are called the extensors, and
those that close it, the flexors. The former lie upon the back of the
upper arm and the front of the forearm and the hand, their tendons
passing over the convexities of the elbow and wrist, while the flexors
occupy the opposite sides, and their tendons run up into the
concavities of the joints. There are several powerful pectoral muscles
which run out from the shoulder and breast, and operate upon the upper
end of the humerus, and with these the wing is lifted and the strokes
are made during flight.
Another mechanical contrivance deserves attention. An extremely
elastic cord reaches over from the shoulder to the wrist joint,
supporting a fold of skin that occupies the deep angle of the elbow,
and that is covered with short, fluffy feathers. When the bird is
flying, this cord is stretched and forms the front edge of that section
of the wing. But, now, suppose the wing is closed, will not this cord
make a cumbersome fold, flapping loosely in the angle of the elbow?
Such would, indeed, be the case, did not its extreme elasticity enable
it to contract to the proper length, so as to keep the wing's border
straight and smooth.
Without the feathers the wing would be useless as an instrument of
flight. The shorter plumes that shield the bases of the long quill
feathers are called the coverts, which are found on both the upper and
under surfaces of the wing. They are divided into several sets,
according to the position they occupy, and are called the "primary
coverts" (because they overlie the bases of the primaries), the
"greater coverts," the "middle coverts," and the "lesser coverts."
Forming a vast expansion of the bony and fleshy framework are the
quills, or flight-feathers, called collectively the "remiges." These
plumes mainly determine the contour of the wing, and constitute a thin,
elastic surface for striking the air--one that is sufficiently
resilient to give the proper rebound and yet firm enough to support the
bird's weight. The longest quills are those that grow on the hand or
outer extremity of the wing and are known as the primaries. What are
called the secondaries are attached to the ulna of the forearm, while
the tertiaries occupy the humerus and are next to the body. All these
feathers are so placed relatively that the stiff outer vane of each
quill overlaps the more flexible inner vane of its successor, like the
leaves of certain kinds of fans, thus presenting an unbroken surface to
the air. As to the structure of these plumes, they combine firmness,
lightness, and mobility, the barbs and barbules knitting the more
flexible parts together, so that they do not separate, but only expand,
when the wing is unfolded.
[Illustration: Barn Swallow]
While the primary purpose of wings is flight, there is quite a number
of notable exceptions. A concrete example is the ostrich, whose wings
are too feeble to lift it from the ground, but evidently aid the great
fowl in running, as it holds them outspread while it skims over the
plain, perhaps using them mainly as outriggers or balancing poles in
its swift passage on its stilt-like legs. The penguins convert their
wings into fins while swimming through the water, the feathers closely
resembling scales.
There are birds of many kinds, and therefore a great variety of wings
and modes of flight. Birds with short, broad, rounded wings, with the
under surface slightly concave and the upper surface correspondingly
convex, usually have comparatively heavy bodies, and race through the
air with rapid wing-beats and rather labored flight, and compass only
short distances. Among the birds of this kind of aërial movement may
be mentioned the American meadowlark, the bob-white, and the pheasant.
Other species propel themselves in rapid, gliding, and continued flight
by means of long, narrow, and pointed wings, like the swifts, swallows,
and goatsuckers, while many others, notably herons, hawks, vultures,
and eagles, are distinguished by a vast alar expansion in proportion to
their weight, and hence are able to sustain themselves in the air by
sailing, with only a slight stroke at rare intervals. Such birds as
the stormy petrel and the frigate-bird have wings that are broad,
convex, and of great length in contrast with the lightness and small
bulk of their bodies, for which reason they are able to sustain
themselves in the air for days without rest. It is even thought that
some of these wonderful birds of the limitless ocean sleep on the wing,
though how such an hypothesis could be proved it would be difficult to
say.
Even in this day of scientific research and astuteness, it must not be
supposed that everything about the mechanics of avicular flight is
understood. We may readily comprehend how a bird, without fluttering
its wings, can poise in the air; but how can it move forward or in a
circle, and even mount upward, without a visible movement of a pinion?
And this some birds are able to do without reference to the direction
of the ethereal currents. That, I venture to say, is still a mystery.
It almost seems as if some of the masters of aërial navigation in the
bird world were gifted with the ability to propel themselves forward by
a mere act of volition.
An interesting article on the subject of bird flight appeared not long
ago in one of the foremost periodicals of the country, a part of which
is here quoted to show what a puzzling problem we have before us:
Recent developments in aërial navigation have renewed interest in the
comparative study of the mechanical principles involved in the flying
of birds. There is one exceedingly puzzling law in regard to birds and
all flying creatures, the solution of which may work far-reaching
influences in the construction of flying craft.
"This law, which has thus far perplexed scientists, is that the heavier
and bigger the bird or insect, the less relative wing area is required
for its support. Thus the area of wing surface of a gnat is forty-nine
units of area to every one of weight. In graphic contrast to that, a
condor (_Sarcorhamphus gryphus_) which weighed 16.52 pounds had a wing
surface of 9.80 square feet. In other words, though the gnat needs
wing surface in a ratio of forty-nine square feet per pound of weight,
a great condor manages to sail along majestically with .59 of a square
foot to at least a pound of weight. The unexplained phenomenon
persists consistently throughout the whole domain of entomology and
ornithology. Going up the scale from the gnat, it is found that with
the dragon fly this ratio is 30 to 1, with the _tipula_, or
daddy-longlegs, 14.5 to 1, the cockchafer only 5.15 to 1, the
rhinoceros beetle 3.14 to 1.
"Among birds the paradoxical law that the smaller the creature the
bigger the relative supporting wings holds good. A screech owl (_Scops
zorca_) weighing one-third of a pound had 2.35 square feet of wing
surface per pound of weight. A fish hawk (_Pandion haliaetus_)
weighing nearly three pounds had a wing area of 1.08 square feet to
each pound. A turkey buzzard weighing 5.6 pounds had a little less
than one square foot of wing surface to each pound. A griffon vulture
(_Gyps fulvus_) weighing 16.52 pounds had a wing surface of only .68
square feet to the pound.
"Students of aërial navigation who are devoting much attention to
observations of birds say that if the peculiar law governing extant
flying creatures could be fathomed the problem of human flight might be
solved."