From the day of Solomon onward the way of a bird in the air has been
a subject of general interest, and the attention given to the problem
of aerial navigation of late years has caused the flight of birds to
be carefully studied in the hope that it might throw some light on the
subject. There have been many conceptions, not to say misconceptions,
regarding the flight of birds; it has been assumed that their muscles
exerted a power quite beyond that of other animals, that the air
sacs of some birds and the hollow bones of others gave them a degree
of lightness quite unattainable by the use of ordinary materials,
while some have even gone so far as to suggest the presence of some
mysterious power, something like Stockton’s negative gravity, whereby
birds could set at naught the law of gravitation and rise at will like
a balloon. The strength of a bird’s muscles, of some birds’ muscles at
least, is not to be underrated; a hawk will plant its talons in a bird
of nearly its own size and weight and bear the victim bodily away, and
an osprey will carry a fish for a long distance. But a tiger has been
known to fell a bullock with a single blow of the paw, to carry a man
as a cat would carry a rat and to drag an Indian buffalo heavier than
himself. On the other hand, some of the petrels, birds which can pass
a day or so on the wing with ease, cannot rise from the water after a
hearty meal, and the humming-bird, unsurpassed in aerial evolutions,
may be trapped in a spider’s web. This shows no great power, and
long ago Marey found that the pulling force of a hawk’s great breast
muscle, applied through the humerus, amounted to 1,298 grams per square
centimeter, something like seven pounds to the square inch; not a very
heavy pull. So it seems fair to assume that while the power exerted
by a bird is great, it is very far from marvelous, probably far less
in proportion to size than the engine of Maxim’s great aeroplane, or
the naphtha motor of Professor Langley, which weighs less than ten
pounds per horse power. We may get a fair idea of what this means by
remembering that a bald eagle weighs from nine to fifteen pounds and
that he exerts but a small fraction of a horse power.
Turning to the question of the part played by the air sacs it may
be said that their value is not proved; some of the fastest birds
get along without them, while birds of the most labored flight are
sometimes well provided. In birds like the gannet and brown pelican
the air sacs and cellular tissue about the body undoubtedly serve as
buffers to break the shock of a headlong plunge into the water from
a height of a hundred to a hundred and fifty feet. Or, again, they
equalize the internal and external pressure when a soaring bird drops
suddenly from a great height, or still more often aid in oxygenating
the blood.
The hollow bones of birds are frequently cited as beautiful instances
of providential mechanics in building the strongest and largest
possible limb with the least expenditure of material, and this is
largely true. And yet birds like ducks, which cleave the air with the
speed of an express train, have the long bones filled with marrow or
saturated with fat, while the lumbering hornbill that fairly hurtles
over the tree tops has one of the most completely pneumatic skeletons
imaginable, permeated with air to the very toe tips; and the ungainly
pelican is nearly as well off. Still it is but fair to say that the
frigate bird and turkey buzzards, creatures which are most at ease when
on the wing, have extremely light and hollow bones, but comparing one
bird with another the paramount importance of a pneumatic skeleton to a
bird is not as evident as that of a pneumatic tire to a bicycle.
While it may not be easy to disprove Herr Gätke’s assertion that birds
sustain themselves in the air by the exercise of some power beyond our
own, it is pretty safe to assume that they do not, and it would seem
that the burden of proof should lie with those who take the affirmative
side of the question.
If we have nothing to learn from birds in the way of building an engine
that shall exert great power for its size and weight we may still have
something to gain in the matter of speed, although here the popular
idea is apt to be exaggerated. We often read that ducks fly at the rate
of a mile a minute, or that the swallow has a speed of two hundred
miles an hour, but it is very difficult to lay hands upon any facts
that will sustain these assertions.
So, too, homing pigeons are frequently stated to have travelled for
long distances at the rate of sixty miles an hour, but some of the
published records show that one hundred and twenty miles in two hours
and a quarter is unusually fast traveling, and this is at the rate of
only nine-tenths of a mile per minute, a speed not unusual for express
trains. However, it may be said that actual observations show that
ducks do travel from forty to fifty miles an hour, and any sportsman
will readily believe that under some conditions they attain a velocity
of a mile and a quarter a minute, although a confession of faith is not
a demonstration of an assured fact.
So far the lesson taught by the bird is that a machine of low power may
attain a very considerable speed and it remains to be seen if there is
anything to be learned concerning methods of flight. Broadly speaking,
there are two, possibly three, distinct modes of flight, by repeated
strokes of the wings and by soaring or sailing, although we find every
intermediate stage between the two, or combinations of flapping and
sailing, and as a matter of fact no bird can entirely dispense with
strokes of the wing.
The humming-bird represents the perfection of one method, the frigate
bird of the other, and in his own line each is unrivaled. These two
modes of flight are associated with equally distinct modifications
of structure, and just as we have every intermediate state of flight
between flapping and soaring so the two structural extremes are merged
into one another. The humming-bird flies as the Irishman played the
fiddle, by main strength, the frigate bird relies on his skill in
taking advantage of every varying current of air, and the skeleton of
the one indicates great muscular power while that of the other shows
its absence. No other bird has such proportionately great muscles as
the humming-bird, the keel of the sternum or breast bone from which
these muscles arise runs from one end of the body to the other while
at the same time it projects downward like the keel of a modern racing
yacht. These muscles drive at the rate of several hundred strokes a
minute a pair of small, rigid wings, the outermost bones of which are
very long while the innermost are very short, a feature calculated to
give the greatest amount of motion at the tip of the wing with the
least movement of the bones of the upper arm, to which the driving
muscles are attached. Another peculiar feature is that the outermost
feathers, the flight feathers or primaries, are long and strong, while
the innermost, those attached to the forearm, are few and weak; so far
as flight is concerned the bird could dispense with these secondaries
and not feel their loss. Finally the heart, which we may look upon
as the boiler that supplies steam for this machinery, is large and
powerful, as is necessary for such a high-pressure engine as the
little humming-bird. It is hardly to him that we would look for aid in
constructing a flying machine, the expenditure of force is too great
for the results attained, the space required for boiler and engine
leaves no room for carrying freight.
As just intimated the frigate bird is exactly the reverse of his tiny
relative; the body is a mere appendage to a pair of wings, while the
breast muscles are so small as to show at a glance that of all flying
creatures the frigate bird is the one which has most successfully
solved the problem of the conservation of energy and can obtain the
greatest amount of power with the least expenditure of muscle.
There is also a great difference between the hummer and the frigate
bird, or between flapping and sailing birds generally, in the
complexity of what may be termed the muscles of adjustment, the little
muscles that run from the shoulder to the elbow and forearm and, among
other duties, are concerned in keeping free from wrinkles that portion
of the wing which lies between the shoulder and the wrist, forming a
triangular flap with the base forming the front edge of the wing and
the apex lying in the elbow joint.
The wing of the frigate bird, too, is quite the opposite of that of
the hummer, for it is the inner portion of the wing, the upper arm and
forearm, which is elongated, and instead of the six feeble secondaries
of the humming-bird there are no less than twenty-four; instead of
a short, stiff, rounded wing we have one that is long, flexible and
pointed. Instead of a wing driven at the rate of several hundred
strokes a minute there is a wing that may be held outstretched and
apparently motionless for minutes at a time, the muscles of the frigate
bird being almost as constantly in repose as those of the other are
perpetually in motion.
If the frigate bird represents the highest type of soaring flight
two more familiar birds, the turkey buzzard and albatross, are not
far behind, and these represent two methods of sailing flight and
two distinct modifications in the type of wings. The albatross is
continually on the move, ever quartering the water as a well-trained
setter does the ground, and yet with all this movement rarely mounting
higher than fifty feet above the water and never wheeling in great
circles in mid-air. This bird has that type of wing which best fulfills
the conditions necessary for an aeroplane, being long and narrow,
so that while a fully grown albatross may spread from ten to twelve
feet from tip to tip, this wing is not more than nine inches wide.
This spread of wing, like that of the frigate bird, is gained by the
elongation of the inner bones of the wing and by increasing the number
of secondaries, there being about forty of these feathers in the wing
of the albatross.
The turkey buzzard is emphatically a high flyer, wheeling slowly about,
half a mile or a mile above the earth, while his cousin, the condor, so
Humboldt tells us, has been seen above the summit of Chimborazo. If any
bird knows how to utilize every breath of wind to the utmost that bird
is the albatross, and it is equally a delight and a marvel to see this
bird apparently setting at naught all natural laws as he sails with
outstretched pinions almost into the eye of the wind or hangs just off
the lee quarter of a ship reeling off ten or twelve knots an hour. In
this last trick, however, the gull is almost equally expert, evidently
making use of the draft from the sails as well as of the eddies caused
by the passage of the vessel.
It has long been evident that if man is to navigate the air it must
be done after the method of the albatross rather than that of the
humming-bird, by the aeroplane and not by any device to imitate the
strokes of a bird’s wings, for not only do the largest birds and
those of the longest flight for the most part sail or soar, but it
is apparent that the limit of size in a vibrating wing must soon be
reached, since in a strong wind with its varying eddies it would be
quite out of the question to manipulate such a piece of mechanism.
But in spite of the fact that sailing flight calls for the exercise
of comparatively little muscular power, the structure of the
skeleton suggests that the wing of a soaring or sailing bird needs a
particularly strong point of support, for birds which sail or soar have
the bones which sustain the direct pull of the wing strengthened or
braced as other birds do not. The shoulder joint of a bird is formed
by the shoulder blade and coracoid, this last being the bone which is
attached to the breast bone and on which comes the direct pull of the
wing, and in front of the coracoids, running downwards towards the
sternum, is the wishbone or furcula, corresponding to our collar bones
or clavicles. It is evident that the greater the length of the coracoid
the less able would it be to resist the strain brought upon it, and it
is also evident that the simultaneous downward stroke of the wings must
have a tendency to force the coracoids inwards, or towards one another.
Obviously the greater the strain the greater the need of strengthening
or bracing the coracoid to resist it, and there are in the shoulder
girdle of a bird various devices looking towards this end. In some
birds, the albatross, for example, the coracoid is short and stout,
while in others extra bracing is obtained from the wishbone.
In the humming-bird the wishbone is light and weak and so short that it
does not come near the sternum; the pigeon, a bird of powerful flight,
is little better off, for the wishbone is so long and slender that it
does little or nothing towards strengthening the shoulder joint, and
in both these birds which fly by rapid wing strokes the entire pull
of the wing is taken by the coracoid. In the frigate bird, on the
contrary, the wishbone is not only strong, but it rests upon and is
firmly soldered to the breastbone, while at its upper end it fuses with
the coracoid, thus making the firmest possible support to the wing. The
cranes, which soar well, also have the wishbone united with sternum,
and in the albatrosses and petrels the wishbone touches the breastbone
and is so curved forward as to gain strength in this way while, as
previously noted, the strength of the coracoid is increased by its
shortness. The turkey buzzard and birds of prey, some of which both
soar and flap, have the wishbone strengthened by having more material
added to make the furcula thick and strong while at the same time it is
shaped like a wide U instead of a V.
Either there is more force exerted in sailing than is at first sight
apparent or else extra strength is called for in making sudden turns,
or when it becomes necessary, as it does more or less frequently, to
take a sudden wing stroke. As wings are levers of the third order the
longer the wing the more force is required to move it and more strength
is needed at the fulcrum or shoulder joint, and since sailing birds
have long wings the need of strength is evident.
Neither birds nor any creatures that live or have lived afford us any
criterion as to the limit of size that must be placed on an aeroplane.
The largest of whales is weak and insignificant beside an ocean liner,
and the condor and albatross, with their spread of ten or twelve feet
and weight of ten to twenty pounds, tell us nothing of what may be the
possibilities of size and weight.
Among the various problems confronting the would-be navigator of the
air is that of at times making headway against a medium moving at the
rate of ten, twenty, or thirty miles an hour, sometimes even more, a
difficulty that neither locomotive nor steamer is called upon to meet.
True, an aeroplane would, to use a technical term, probably lie within
two and one-half points of the wind and could thus advantageously beat
to windward, but any deviation from a straight course means loss of
time, and nowadays time is everything.
The mode of propulsion may be, undoubtedly will be, as entirely
different from a wing as the propeller is unlike the tail of a fish,
and as the study of fish has thrown little or no light on the problems
of the proper form or best motor for a ship, it is doubtful if the
study of birds will do more for the aerodrome. Nor does it seem likely
that a study of the bird will suggest any new devices in the way of
joints, braces, or rudders, for what must be discouraging to those
engaged in solving the problems of flight is the utter inadequacy of
the bird’s wing, from a mechanical standpoint, for the work it is
called upon to do, for in all its articulations there is a freedom
of movement, an amount of play that would be inadmissible in any
machine. The shoulder, elbow and wrist joints are but loose affairs,
depending for their efficiency on the pull of the muscles; subtract
the element of life from the wing of a bird and it becomes at once
limp and useless. And herein is the key to the bird’s success as a
flying machine; it has _life_, and while the wing may reveal certain
principles of balancing, it cannot teach us all the art, for it is
done instinctively. The bird has back of it untold ages of experience
and its actions during flight demand no thought; the muscles respond
instinctively to each change in the pressure and direction of the wind,
and the bird need take no thought as to how it shall fly.
Mr. Chanute has taken the greatest step yet made towards overcoming
the difficulty of responding to changes in the velocity of the fickle
air, but whether or not it will be possible to construct apparatus that
will not only adjust itself to changes in the force of the wind, but
to eddies and changes in direction as well, remains to be seen, the
more that it must act not on planes six feet in length, but on surfaces
infinitely larger. The proper method of constructing the wings of an
aeroplane so as to insure stability and utilize the power of the wind
to the best advantage, and some hints as to balancing and steering are
the main assistance that we seem likely to gain from a study of the
structure and flight of birds.