AIR CURRENTS AND THE FLIGHT OF BIRDS.
In Mr. Darwin’s “Voyage of the Beagle” I find:--
“When the condors are wheeling in a flock round and round any spot their
flight is beautiful. Except when rising from the ground, I do not
remember ever having seen one of these birds flap its wings. Near Lima I
watched several for nearly half an hour, without taking off my eyes;
they moved in large curves, sweeping in circles, descending and
ascending without giving a single flap. As they glided close over my
head I intently watched from an oblique position, the outlines of the
separate and great terminal feathers of each wing, and these separate
feathers, if there had been the least vibratory movement, would have
appeared as if blended together; but they were seen distinct against the
blue sky.”
Man is essentially a land animal, and it is quite possible if Nature had
not placed before him numerous examples of birds and insects that are
able to fly, he would never have thought of attempting it himself. But
birds are very much in evidence, and mankind from the very earliest
times has not only admired the ease and rapidity with which they are
able to move from place to place, but has always aspired to imitate
them. The number of attempts that have been made to solve this problem
has been very great; but it was not until quite recently that science
and mechanics had advanced far enough to put in the hands of
experimenters suitable material to attack the problem. Perhaps nothing
better has ever been written regarding our aspirations to imitate the
flight of birds than what Prof. Langley has said:--
“Nature has made her flying machine in the bird, which is nearly a
thousand times as heavy as the air its bulk displaces, and only those
who have tried to rival it know how inimitable her work is, for ‘the way
of a bird in the air’ remains as wonderful to us as it was to Solomon,
and the sight of the bird has constantly held this wonder before men’s
eyes, and in some men’s minds, and kept the flame of hope from utter
extinction, in spite of long disappointment. I well remember how, as a
child, when lying in a New England pasture, I watched a hawk soaring far
up in the blue, and sailing for a long time without any motion of its
wings, as though it needed no work to sustain it, but was kept up there
by some miracle. But, however sustained, I saw it sweep, in a few
seconds of its leisurely flight, over a distance that to me was
encumbered with every sort of obstacle, which did not exist for it. The
wall over which I had climbed when I left the road, the ravine I had
crossed, the patch of undergrowth through which I had pushed my way--all
these were nothing to the bird--and while the road had only taken me in
one direction, the bird’s level highway led everywhere, and opened the
way into every nook and corner of the landscape. How wonderfully easy,
too, was its flight. There was not a flutter of its pinions as it swept
over the field, in a motion which seemed as effortless as that of its
shadow.”
During the last 50 years a great deal has been said and written in
regard to the flight of birds; no other natural phenomenon has excited
so much interest and been so imperfectly understood. Learned treatises
have been written to prove that a bird is able to develop from ten to
twenty times as much power for its weight as other animals, while other
equally learned works have shown most conclusively that no greater
amount of energy is exerted by a bird in flying than by land animals in
running or jumping.
Prof. Langley, who was certainly a very clever observer and a
mathematician of the first order, in discussing the subject relating to
the power exerted by birds in flight and the old formula relating to the
subject, expresses himself as follows:--
“After many years and in mature life, I was brought to think of these
things again, and to ask myself whether the problem of artificial flight
was as hopeless and as absurd as it was then thought to be. Nature had
solved it, and why not man? Perhaps it was because he had begun at the
wrong end, and attempted to construct machines to fly before knowing the
principles on which flight rested. I turned for these principles to my
books and got no help. Sir Isaac Newton had indicated a rule for finding
the resistance to advance through the air, which seemed, if correct, to
call for enormous mechanical power, and a distinguished French
mathematician had given a formula showing how rapidly the power must
increase with the velocity of flight, and according to which a swallow,
to attain a speed it is known to reach, must be possessed of the
strength of a man.
“Remembering the effortless flight of the soaring bird, it seemed that
the first thing to do was to discard rules which led to such results,
and to commence new experiments, not to build a flying machine at once,
but to find the principles upon which one should be built; to find, for
instance, with certainty by direct trial how much horse-power was needed
to sustain a surface of given weight by means of its motion through the
air.”
There is no question but what a bird has a higher physical development,
as far as the generation of power is concerned, than any other animal we
know of. Nevertheless, I think that everyone who has made a study of the
question will agree that some animals, such as hares and rabbits, exert
quite as much power in running, in proportion to their weight, as a
sea-gull or an eagle does in flying.
The amount of power which a land animal has to exert is always a fixed
and definite quantity. If an animal weighing 100 lbs. has to ascend a
hill 100 feet high, it always means the development of 10,000 foot-lbs.
With a bird, however, there is no such thing as a fixed quantity. If a
bird weighing 100 lbs. should raise itself into the air 100 feet during
a perfect calm, the amount of energy developed would be 10,000 foot lbs.
plus the slip of the wings. But, as a matter of fact, the air in which a
bird flies is never stationary, as I propose to show; it is always
moving either up or down, and soaring birds, by a very delicate sense of
feeling, always take advantage of a rising column. If a bird finds
itself in a column of air which is descending, it is necessary for it to
work its wings very rapidly in order to prevent a descent to the earth.
I have often observed the flight of hawks and eagles. They seem to glide
through the air with hardly any movement of their wings. Sometimes,
however, they stop and hold themselves in a stationary position directly
over a certain spot, carefully watching something on the earth
immediately below. In such cases they often work their wings with great
rapidity, evidently expending an enormous amount of energy. When,
however, they cease to hover and commence to move again through the air,
they appear to keep themselves at the same height with an almost
imperceptible expenditure of power.
[Illustration: Fig. 3.--While in the Pyrenees I often observed eagles
balancing themselves on an ascending current of air produced by the wind
blowing over large masses of rock.]
Many unscientific observers of the flight of birds have imagined that a
wind or a _horizontal_ movement of the air is all that is necessary to
sustain the weight of a bird in the air after the manner of a kite. If,
however, the wind, which is only air in motion, should be blowing
everywhere at exactly the same velocity, and in the same
direction--horizontally--it would offer no more sustaining power to a
bird than a dead calm, because there is nothing to prevent the body of
the bird from being blown along with the air, and whenever it attained
the same velocity as the air, no possible arrangement of the wings could
prevent it from falling to the earth.
It is well known that only a short distance above the earth’s surface,
say 30 or 40 miles, we find an extremely low temperature sometimes
referred to as interstellar temperature or absolute zero. In order to
illustrate the extremely low temperature of space, I would cite the
following instance:--
One evening, in the State of Ohio, a farmer saw a very brilliant meteor;
it struck in one of his fields not more than 100 feet from his house. He
at once rushed to the spot, and, pushing his arm down the hole,
succeeded in touching it; but he very quickly withdrew his hand, as he
found it extremely hot. Some of the neighbours rushed to the spot, and
he told them what had occurred, whereupon one of them put his hand in
the hole, expecting to be burnt, but, much to his surprise, the tips of
his wet fingers were instantly frozen to the meteor. The meteor had been
travelling at such an exceedingly high velocity that the resistance of
the intensely cold and highly attenuated outer atmosphere was sufficient
to bring its temperature up to the melting point of iron; but the heat
did not have time to pass into the interior, it only extended inwards
perhaps 1/8 inch, so that when the meteor came to a state of rest, the
heat of the exterior was soon absorbed by the intensely cold interior,
thus reducing the surface to a temperature much below any natural
temperature that we find at the surface of the earth.
Nothing can be more certain than that the temperature is extremely low a
slight distance above the earth’s surface. As the air near the earth
never falls in temperature to anything like the absolute zero, it
follows that there is a constant change going on, the relatively warm
air near the surface of the earth always ascending, and, in some cases,
doing sufficient work in expanding to render a portion of the water it
contains visible, forming clouds, rain, or snow, while the very cold air
is constantly descending to take the place of the rising column of warm
air. I have noticed a considerable degree of regularity in the movement
of the air, especially at a long distance from land, where the
regularity of the up and down currents is, at times, very marked.
On one occasion while crossing the Atlantic in fine weather I noticed,
some miles directly ahead of the ship, a long line of glassy water.
Small waves indicated that the wind was blowing in the exact direction
in which the ship was moving, and as we approached the glassy line, the
waves became smaller and smaller until they completely disappeared in a
mirror-like surface, which was about 300 or 400 feet wide, and extended
both to the port and starboard in approximately a straight line as far
as the eye could reach. After passing the centre of this zone, I noticed
that small waves began to show themselves, but in the exact opposite
direction to those through which we had already passed, and these waves
became larger and larger for nearly half an hour. Then they began to get
gradually smaller, when I observed another glassy line directly ahead of
the ship. As we approached it, the waves again completely disappeared,
but after passing through it, the wind was blowing in the opposite
direction, and the waves increased in size exactly in the same manner
that they had diminished on the opposite side of the glassy streak (Fig.
4).
[Illustration: Fig. 4.--Air currents observed in mid Atlantic, warm air
ascending at _a_, _a_, _a_, and cold air descending at _b_, _b_, _b_.
_c_, _c_, _c_ represent the lines where the waves were the largest].
This, of course, shows that directly over the centre of the first
glassy streak, the air was meeting from both sides and ascending in
practically a straight line from the surface of the water, and then
spreading out high above the sea, setting up a light wind in both
directions.
I spent the winter of 1890-91 on the Riviera, between Hyères les
Palmiers and Monte Carlo. The weather for the most part was very fine,
and I often had the opportunity of observing the peculiar phenomena
which I had already noticed in the Atlantic, only on a much smaller
scale. Whereas, in the Atlantic, the glassy zones were from 8 to 15
miles apart, I often found them not more than 500 feet apart in the bays
of the Mediterranean. This was most noticeable at Antibes (Fig. 5), very
good photographs of which I obtained. It will be observed that the whole
surface of the water is streaked like a block of marble.
[Illustration: Fig. 5.--Glassy streaks showing the centres of ascending
and descending columns of air in the Bay of Antibes, Alpes Maritimes.]
At Nice and Monte Carlo this phenomena was also very marked. On one
occasion, while making observations from the highest part of the
promontory of Monaco on a perfectly calm day, I noticed that the whole
of the sea presented this peculiar effect as far as the eye could
reach, and that the lines which marked the descending air were never
more than 1,000 feet from those which marked the centre of the ascending
column. At about three o’clock one afternoon, a large black steamer
passed along the coast in a perfectly straight line, and its wake was at
once marked by a glassy line, which indicated the centre of an ascending
column. This line remained almost straight for two hours, when finally
it became crooked and broken. The heat of the steamer had been
sufficient to determine this upward current of air.
[Illustration: Fig. 6.--Air currents observed in the Mediterranean,
ascending currents at _a_, _a_, _a_, and descending currents at _b_,
_b_, _b_.]
In 1893 I spent two weeks in the Mediterranean, going and returning by a
slow steamer from Marseilles to Constantinople, and I had many
opportunities of observing the peculiar phenomena to which I have
referred. The steamer passed over thousands of square miles of calm sea,
the surface being only disturbed by large patches of small ripples (Fig.
6), separated from each other by glassy streaks, which, however, were
not straight as on the Atlantic, and I found that in no case was the
wind blowing in the same direction on both sides of these streaks, every
one of which indicated the centre of an ascending or a descending column
of air. If we should investigate these phenomena in what might be called
a dead calm, we should find that the air was rising very nearly
straight up over the centres of some of these streaks and descending in
a vertical line over the centres of others. But, as a matter of fact,
there is no such thing as a _dead calm_. The movement of the air is the
resultant of more than one force. The air is not only rising in some
places and descending in others, but at the same time, the whole mass is
moving forward with more or less rapidity from one part of the earth to
another, so we must consider that, instead of the air ascending directly
from the relatively hot surface of the earth and descending vertically
in other places, in reality the whole mass of rotating air is moving
horizontally at the same time.
Suppose that the local influence which causes the up and down motion of
the air should be sufficiently great to cause the air to rise at the
rate of 2 miles an hour, and that the wind at the same time should be
blowing at the rate of 10 miles an hour, the motion of the air would
then be the resultant of these two velocities. In other words, it would
be blowing up an incline of 1 in 5. Suppose, now, that a bird should be
able to so adjust its wings that it advanced 5 miles in falling 1 mile
through a perfectly calm atmosphere, it would then be able to sustain
itself in an inclined wind, such as I have described, without any
movement at all of its wings. If it were possible to adjust its wings in
such a manner that it could advance 6 miles by falling through 1 mile of
air, it would then be able to rise as relates to the earth while in
reality falling as relates to the surrounding air.
In conducting a series of experiments with artillery and small guns on a
large and level plain just out of Madrid, I often observed the same
phenomena, as relates to the wind, that I have already spoken of as
having observed at sea, except that the lines marking the centre of an
ascending or a descending column of air were not so stationary as they
were over the water. It was not an uncommon thing, when adjusting the
sights of a gun to fire at a target at a very long range, making due
allowances for the wind, to have the wind change and blow in the
opposite direction before the word of command was given to fire. While
conducting these experiments, I often noticed the flight of eagles. On
one occasion a pair of eagles came into sight on one side of the plain,
passed directly over our heads, and disappeared on the opposite side.
They were apparently always at the same height from the earth, and in
soaring completely across the plain they never once moved their wings.
These phenomena, I think, can only be accounted for on the hypothesis
that these birds were able to feel out with their wings an ascending
column of air, that the centre of this column of air was approximately a
straight line running completely across the plain, that they found
upward movement more than sufficient to sustain their weight in the air,
and that whereas, as relates to the earth, they were not falling at all,
they were in reality falling some 4 or 5 miles an hour in the air which
supported them.
Again, at Cadiz in Spain, when the wind was blowing in strongly from the
sea, I observed that the sea-gulls always took advantage of an ascending
column of air. As the wind rose to pass over the fortifications, the
gulls selected a place where they would glide on the ascending current
of air, keeping themselves always approximately in the same place
without any apparent exertion. When, however, they left this ascending
column, it was necessary for them to work their wings with great vigour
until they again found the proper place to encounter a favourable
current.
I have often noticed that gulls are able to follow a ship without any
apparent exertion; they simply balance themselves on an ascending column
of air, where they seem to be quite as much at ease as they would have
been roosting on a solid support. If, however, they are driven out of
this position, they generally commence at once to work their passage. If
anything is thrown overboard which is too heavy for them to lift, the
ship soon leaves them behind, and in order to catch up with it again
they move their wings very much as other birds do; but when once
established in the ascending column of air, they manage to keep up with
the ship by doing little or no work. In a calm or head wind we find them
directly aft of the ship; if the wind is from the port side they may
always be found on the starboard quarter, and _vice versâ_.
One Sunday morning, while living at Kensington, I noticed some very
curious atmospheric effects. The weather had been intensely cold for
about a week, when suddenly the atmosphere became warm and very humid.
The earth being much colder than the atmosphere, water was condensing on
everything that it touched. I went to the bridge over the Serpentine in
Hyde Park, and was not disappointed in finding a large number of
sea-gulls waiting about the bridge to be fed. On all ordinary occasions
these birds manage to move about with the expenditure of very little
energy, but on this occasion every one of them, without a single
exception, no matter in what position he might be, was working his
passage like any other bird, just as I had expected. It is only on very
rare occasions that the surface of the earth is sufficiently cold as
relates to the atmosphere to prevent all upward currents of air.
Everyone who has passed a winter on the northern shores of the
Mediterranean must have observed the cold wind which is generally called
the mistral. One may be out driving, the sun may be shining brightly,
and the air warm and balmy, when suddenly, without any apparent cause,
one finds himself in a cold descending wind. This is the much-dreaded
_mistral_, and if at sea it would be marked by a glassy line on the
surface of the water. On land, however, there is nothing to render its
presence visible. The ascending column of air is, of course, always very
much warmer than the descending column, and this is taking place in a
greater or lesser degree everywhere and at all times. A decided upward
trend of air is often encountered by those who are experimenting with
kites, the kite often mounting higher than can be accounted for on the
hypothesis that the wind is moving in a horizontal direction. I have
heard this discussed at considerable length. When a kite is flown in an
upward current, it behaves in many respects like a soaring bird.
From the foregoing, I think, we may safely draw the following
conclusions:--
First, that there is a constant interchange of air taking place, the
cold air descending, spreading itself out over the surface of the earth,
becoming warm, and ascending in other places.
Second, that the centres of the two columns are generally separated from
each other by a distance which may be from 500 feet to 20 miles.
Third, that the centres of greatest action are not in spots, but in
lines which may be approximately straight, but sometimes abound in many
sinuosities.
Fourth, that this action is constantly taking place over both the sea
and the land; that the soaring of birds, the phenomenon which has
heretofore been so little understood, may be accounted for on the
hypothesis that the bird seeks out an ascending column of air, and while
sustaining itself at the same height in the air, without any muscular
exertion, it is in reality falling at a considerable velocity through
the air that surrounds it.
It has been supposed by some scientists that birds may take advantage of
some vibratory or rolling action of the air. I find, however, from
careful observation and experiment, that the motion of the wind is
comparatively steady, and that the short vibratory or rolling action is
always very near to the earth and is produced by the air flowing over
hills, high buildings, trees, etc.
Tools and instruments used by mechanicians are very often made of the
material most used in their profession; for instance, a blacksmith’s
tools are generally of iron, a carpenter’s tools largely of wood, and a
glass-blower uses many things made of glass, and so on. Mathematicians
are no exception to this general rule, and seem to imagine that
everything can be accomplished by pure mathematical formulæ.
It appears that Prof. Langley was at times considerably puzzled by the
extraordinary behaviour of birds, and was led to believe that they took
advantage of some vibratory or oscillating movement of the air; he
called it “the internal work of the air.” I have been very much amused
in a recent mathematical work that I have read, in which the writer
seeks to solve all questions by pure mathematics. In this case,
notwithstanding that all of the factors are unknown and unknowable,
still, with the use of about two pages of closely written algebraic
formulæ, he appears to have solved the whole question. Just how he
arrived at it, however, is more than I am able to understand.
If a kite is flown only a few feet above the ground, it will be found
that the current of air is very unsteady. If it is allowed to mount to
500 feet the unsteadiness nearly all disappears, while if it is allowed
to mount further to a height of 1,500 or 2,000 feet, the pull on the
cord is almost constant, and, if the kite is well made, it remains
practically stationary in the air.
I have often noticed in high winds that light and fleecy clouds come
into view, say, about 2,000 feet above the surface of the earth, and
pass rapidly and steadily by preserving their shape completely. This
would certainly indicate that there is no rapid local disturbance in
the air in their immediate vicinity, but that the whole mass of air in
which these clouds are formed is practically travelling in the same
direction and at the same velocity. Numerous aeronauts have also
testified that, no matter how hard the wind may be blowing, the balloon
is always practically in a dead calm, and if a piece of gold-leaf is
thrown overboard, even in a gale, the gold-leaf and the balloon never
part company in a horizontal direction, though they may in a vertical
direction.
Birds may be divided into two classes. First, the soaring birds, which
practically live upon the wing, and, by some very delicate sense of
touch, are able to feel the exact condition of the air. Many fish which
live near the top of the water are greatly distressed by sinking too
deeply, while others which live at great depths are almost instantly
killed by being raised to the surface. The swim-bladder of a fish is in
reality a delicate barometer provided with sensitive nerves which enable
the fish to feel whether it is sinking or rising in the water. With the
surface fish, if the pressure becomes too great, it involuntarily exerts
itself to rise nearer the surface and so diminish the pressure, and I
have no doubt that the air cells, which are known to be very numerous
and to abound throughout the bodies of birds, are so sensitive as to
enable soaring birds to know at once whether they are in an ascending or
a descending column of air.
The other class of birds consists of those which only employ their wings
occasionally for the purpose of taking them rapidly from one place to
another. Such birds do not expend their power so economically as the
soaring birds. They do not pass much of their time in the air, but what
time they are on the wing they put forth an immense amount of power and
fly very rapidly, generally in a straight line, taking no advantage of
air currents. Partridges, pheasants, wild ducks, geese, and some birds
of passage may be taken as types of this kind. This class of birds has
relatively small wings, and carries about two and a half times as much
weight per square foot of surface as soaring birds do.
We shall never be able to imitate the flight of the soaring birds. We
cannot hope to make a sensitive apparatus that will work quick enough to
take advantage of the rising currents of air, and he who seeks to fly
has this problem to deal with. A successful flying machine, moving at a
high velocity, is likely at any time to encounter downward currents of
air, which will greatly interfere with its action. Therefore flying
machines must, in the very nature of things, be provided with sufficient
power to propel them through various currents of air, after the manner
of ducks, partridges, pheasants, etc.
+------------------+-------+--------+-----------------+
| | | | Corresponding |
|Common Name. |Sq. Ft.|Lbs. per|Speed for a Plane|
| |per Lb.|Sq. Ft. | at 3° in Miles |
| | | | per Hour. |
+------------------+-------+--------+-----------------+
|Bat, | 7·64 | 0·131 | 15·9 |
|Swallow, | 3·62 | 0·276 | 23·1 |
|Lark, | 3·06 | 0·327 | 25·1 |
|Sparrow hawk, | 3·00 | 0·333 | 25·3 |
|Sparrow, | 2·42 | 0·414 | 28·2 |
|Gull, | 2·35 | 0·426 | 28·6 |
|Owl, | 2·26 | 0·443 | 29·2 |
|Crane, | 2·02 | 0·495 | 30·9 |
|Rook, | 1·74 | 0·575 | 33·3 |
|Plover, | 1·38 | 0·725 | 37·4 |
|Balbuzzard, | 1·26 | 0·795 | 39·2 |
|Egyptian vulture, | 1·18 | 0·848 | 40·4 |
|Duck, | 0·864 | 1·158 | 44·2 |
|Grey pelican, | 0·732 | 1·365 | 51·3 |
|Wild goose, | 0·586 | 1·708 | 57·4 |
|Turkey, | 0·523 | 1·910 | 60·6 |
|Duck (female), | 0·498 | 2·008 | 62·2 |
| „ (male), | 0·439 | 2·280 | 66·2 |
+------------------+-------+--------+-----------------+