are most effective as elevators and propellers, from the fact
(pointed out by Mr. Wenham) that at high speeds, with very oblique
incidences, the supporting effect becomes transferred to the _front
edge_ of the pinion. It is in this way “that the effective propelling
area of the two-bladed screw is tantamount to its entire circle
of revolution.” A similar principle was announced by Sir George
Cayley upwards of fifty years ago. “_In very acute angles with the
current_, it appears that the centre of resistance in the sail does
not coincide with the centre of its surface, _but is considerably
in front of it_. As the obliquity of the current decreases, these
centres approach, and coincide when the current becomes perpendicular
to the plane; hence any heel of the machine backwards or forwards
removes the centre of support behind or before the point of
suspension.”--Nicholson’s Journal, vol. xxv. p. 83. When the speed
attained by the bird is _greatly accelerated_, and _the stratum of
air passed over in any given time enormously increased_, the support
afforded by the air to the inclined planes formed by the wings _is
likewise augmented_. This is proved by the rapid flight of skimming
or sailing birds when the wings are moved at long intervals and
very leisurely. The same principle supports the skater as he rushes
impetuously over insecure ice, and the thin flat stone projected
along the surface of still water. The velocity of the movement in
either case prevents sinking by not giving the supporting particles
time to separate.
_The Flight of the Albatross compared to the Movements of a Compass
set upon Gimbals._--A careful examination of the movements in
skimming birds has led me to conclude that by a judicious twisting or
screw-like action of the wings at the shoulder, in which the pinions
are alternately advanced towards and withdrawn from the head in a
manner analogous to what occurs at the loins in skating without lifting
the feet, birds of this order can not only maintain the motion which
they secure by a few energetic flappings, but, if necessary, actually
increase it, and that without either bending the wing or beating the
air.
The forward and backward screwing action of the pinion referred to, in
no way interferes, I may remark, with the rotation of the wing on its
long axis, the pinion being advanced and screwed down upon the wind,
and retracted and unscrewed alternately. As the movements described
enable the sailing bird to tilt its body from before backwards, or the
converse, and from side to side or laterally, it may be represented
as oscillating on one of two centres, as shown at fig. 105; the one
corresponding with the long axis of the body (fig. 105, _a b_), the
other with the long axis of the wings (_c d_). Between these two
extremes every variety of sailing and gliding motion which is possible
in the mariner’s compass when set upon gimbals may be performed; so
that a skimming or sailing bird may be said to possess perfect command
over itself and over the element in which it moves.
[Illustration: FIG. 105.]
Captain Hutton makes the following remarkable statement regarding the
albatross:--“I have sometimes watched narrowly one of these birds
sailing and wheeling about in all directions for more than an hour,
without seeing the slightest movement of the wings, and have never
witnessed anything to equal the ease and grace of this bird as he
sweeps past, often within a few yards, every part of his body perfectly
motionless except the head and eye, which turn slowly and seem to take
notice of everything.”[98]
[98] “On some of the Birds inhabiting the Southern Ocean.”--Ibis, 2d
series, vol. i. 1865.
“Tranquil its spirit seem’d and floated slow;
Even in its very motion there was rest.”[99]
[99] Professor Wilson’s Sonnet, “A Cloud,” etc.
As an antithesis to the apparently lifeless wings of the albatross,
the ceaseless activity of those of the humming-bird may be adduced. In
those delicate and exquisitely beautiful birds, the wings, according to
Mr. Gould, move so rapidly when the bird is poised before an object,
that it is impossible for the eye to follow each stroke, and a hazy
circle of indistinctness on each side of the bird is all that is
perceptible. When the humming-bird flies in a horizontal direction,
it occasionally proceeds with such velocity as altogether to elude
observation.
_The regular and irregular in Flight._--The coot, diver, duck, and
goose fly with great regularity in nearly a straight line, and with
immense speed; they rarely if ever skim or glide, their wings being too
small for this purpose. The woodpecker, magpie, fieldfare and sparrow,
supply examples of what may be termed the “irregular” in flight. These,
as is well known, fly in curves of greater or less magnitude, by giving
a few vigorous strokes and then desisting, the effect of which is to
project them along a series of parabolic curves. The snipe and woodcock
are irregular in another respect, their flight being sudden, jerky, and
from side to side.
_Mode of ascending, descending, turning, etc._--All birds which do
not, like the swallow and humming-birds, drop from a height, raise
themselves at first by a vigorous leap, in which they incline their
bodies in an upward direction, the height thus attained enabling them
to extend and depress their wings without injury to the feathers. By
a few sweeping strokes delivered downwards and forwards, in which the
wings are made nearly to meet above and below the body, they lever
themselves upwards and forwards, and in a surprisingly short time
acquire that degree of momentum which greatly assists them in their
future career. In rising from the ground, as may readily be seen in
the crow, pigeon, and kingfisher (fig. 102, p. 183), the tail is
expanded and the neck stretched out, so that the body is converted
into an inclined plane, and acts mechanically as a kite. The centre of
gravity and the position of the body are changed at the will of the
bird by movements in the neck, feet, and tail, and by increasing or
decreasing the angles which the under surface of the wings makes with
the horizon. When a bird wishes to fly in a horizontal direction, it
causes the under surface of its wings to make a slight _forward_ angle
with the horizon. When it wishes to ascend, the angle is increased.
When it wishes to descend, it causes the under surface of the wings
to make a slight _backward_ angle with the horizon. When a bird flies
up, its wings strike downwards and _forwards_. When it flies down, its
wings strike downwards and _backwards_. When a sufficient altitude
has been attained, the length of the downward stroke is generally
curtailed, the mere extension and flexion of the wing, assisted by the
weight of the body, in such instances sufficing. This is especially
the case if the bird is advancing against a slight breeze, the effort
required under such circumstances being nominal in amount. That little
power is expended is proved by the endless gyrations of rooks and other
birds; these being continued for hours together. In birds which glide
or skim, it has appeared to me that the wing is recovered much more
quickly, and the down stroke delivered more slowly, than in ordinary
flight--in fact, that the rapidity with which the wing acts in an
upward and downward direction is, in some instances, reversed; and this
is what we should naturally expect when we recollect that in gliding,
the wings require to be, for the most part, in the expanded condition.
If this observation be correct, it follows that birds have the power of
modifying the duration of the up and down strokes at pleasure. Although
the wing of the bird usually strikes the air at an angle which varies
from 15° to 30°, the angle may be increased to such an extent as to
subvert the position of the bird. The tumbler pigeon, _e.g._ can, by
slewing its wings forwards and suddenly throwing back its head, turn
a somersault. When birds are fairly on the wing they have the air,
unless when that is greatly agitated by a storm, completely under
control. This arises from their greater specific gravity, and because
they are possessed of independent motion. If they want to turn, they
have simply to tilt their bodies laterally, as a railway carriage would
be tilted in taking a curve,[100] or to increase the number of beats
given by the one wing as compared with the other; or to keep the one
wing extended while the other is partially flexed. The neck, feet, and
tail may or may not contribute to this result. If the bird wishes to
rise, it tilts its entire body (the neck and tail participating) in
an upward direction (fig. 59, p. 126; fig. 102, p. 183); or it rises
principally by the action of the wings and by muscular efforts, as
happens in the lark. The bird can in this manner likewise retain its
position in the air, as may be observed in the hawk when hovering
above its prey. If the bird desires to descend, it may reverse the
direction of the inclined plane formed by the body and wings, and
plunge head foremost with extended pinions (fig. 106); or it may flex
the wings, and so accelerate its pace; or it may raise its wings and
drop parachute-fashion (fig. 55, p. 112; _g_, _g_ of fig. 82, p. 158);
or it may even fly in a downward direction--a few sudden strokes, a
more or less abrupt curve, and a certain degree of horizontal movement
being in either case necessary to break the fall previous to alighting
(fig. 107, below). Birds which fish on the wing, as the osprey and
gannet, precipitate themselves from incredible heights, and drop into
the water with the velocity of a meteorite--the momentum which they
acquire during their descent materially aiding them in their subaqueous
flight. They emerge from the water and are again upon the wing before
the eddies occasioned by their precipitous descent have well subsided,
in some cases rising apparently without effort, and in others running
along and beating the surface of the water for a brief period with
their pinions and feet.
[100] “If the albatross desires to turn to the right he bends
his head and tail slightly upwards, at the same time raising his
left side and wing, and lowering the right in proportion to the
sharpness of the curve he wishes to make, the wings being kept quite
rigid the whole time. To such an extent does he do this, that in
sweeping round, his wings are often pointed in a direction nearly
perpendicular to the sea; and this position of the wings, more or
less inclined to the horizon, is seen always and only when the bird
is turning.”--“On some of the Birds inhabiting the Southern Ocean.”
Ibis, 2d series, vol. i. 1865, p. 227.
[Illustration:
FIG. 106.--The Pigeon (_Treron bicincta_, Jerdon), flying downwards
and turning prior to alighting. The pigeon expands its tail both in
ascending and descending.--_Original._]
[Illustration:
FIG. 107.--The Red-headed Pochard (_Fuligula ferina_, Linn.)
in the act of dropping upon the water; the head and body being
inclined upwards and forwards, the feet expanded, and the wings
delivering vigorous short strokes in a downward and forward
direction.--_Original._]
_The Flight of Birds referable to Muscular Exertion and Weight._--The
various movements involved in ascending, descending, wheeling, gliding,
and progressing horizontally, are all the result of muscular power and
weight, properly directed and acting upon appropriate surfaces--that
apparent buoyancy in birds which we so highly esteem, arising not from
superior lightness, but from their possessing that degree of solidity
which enables them to subjugate the air,--weight and independent
motion, _i.e._ motion associated with animal life, or what is
equivalent thereto, being the two things indispensable in successful
aërial progression. The weight in insects and birds is in great measure
owing to their greatly developed muscular system, this being in that
delicate state of tonicity which enables them to act through its
instrumentality with marvellous dexterity and power, and to expend or
reserve their energies, which they can do with the utmost exactitude,
in their apparently interminable flights.
_Lifting-capacity of Birds._--The muscular power in birds is usually
greatly in excess, particularly in birds of prey, as, _e.g._ the
condors, eagles, hawks, and owls. The eagles are remarkable in this
respect--these having been known to carry off young deer, lambs,
rabbits, hares, and, it is averred, even young children. Many of
the fishing birds, as the pelicans and herons, can likewise carry
considerable loads of fish;[101] and even the smaller birds, as the
records of spring show, are capable of transporting comparatively large
twigs for building purposes. I myself have seen an owl, which weighed
a little over 10 ounces, lift 2-1/2 ounces, or a quarter of its own
weight, without effort, after having fasted twenty-four hours; and a
friend informs me that a short time ago a splendid osprey was shot at
Littlehampton, on the coast of Sussex, with a fish 5 lbs. weight in its
mouth.
[101] The heron is in the habit, when pursued by the falcon, of
disgorging the contents of his crop in order to reduce his weight.
There are many points in the history and economy of birds which crave
our sympathy while they elicit our admiration. Their indubitable
courage and miraculous powers of flight invest them with a superior
dignity, and secure for their order almost a duality of existence. The
swallow, tiny and inconsiderable as it may appear, can traverse 1000
miles at a single journey; and the albatross, despising compass and
landmark, trusts himself boldly for weeks together to the mercy or
fury of the mighty ocean. The huge condor of the Andes lifts himself
by his sovereign will to a height where no sound is heard, save the
airy tread of his vast pinions, and, from an unseen point, surveys in
solitary grandeur the wide range of plain and pasture-land;[102] while
the bald eagle, nothing daunted by the din and indescribable confusion
of the queen of waterfalls, the stupendous Niagara, sits composedly on
his giddy perch, until inclination or desire prompts him to plunge
into or soar above the drenching mists which, shapeless and ubiquitous,
perpetually rise from the hissing waters of the nether caldron.
[102] The condor, on some occasions, attains an altitude of six miles.
[Illustration: FIG. 108.--Hawk and quarry.--_After The Graphic_.]
AËRONAUTICS.
[Illustration: THE VAUXHALL BALLOON OF MR. GREEN.]
AËRONAUTICS.
The subject of artificial flight, notwithstanding the large share of
attention bestowed upon it, has been particularly barren of results.
This is the more to be regretted, as the interest which has been
taken in it from early Greek and Roman times has been universal. The
unsatisfactory state of the question is to be traced to a variety of
causes, the most prominent of which are--
_1st_, The extreme difficulty of the problem.
_2d_, The incapacity or theoretical tendencies of those who have
devoted themselves to its elucidation.
_3d_, The great rapidity with which wings, especially insect wings,
are made to vibrate, and the difficulty experienced in analysing their
movements.
_4th_, The great weight of all flying things when compared with a
corresponding volume of air.
_5th_, The discovery of the balloon, which has retarded the science of
aërostation, by misleading men’s minds and causing them to look for a
solution of the problem by the aid of a machine lighter than the air,
and which has no analogue in nature.
Flight has been unusually unfortunate in its votaries. It has been
cultivated, on the one hand, by profound thinkers, especially
mathematicians, who have worked out innumerable theorems, but have
never submitted them to the test of experiment; and on the other, by
uneducated charlatans who, despising the abstractions of science,
have made the most ridiculous attempts at a practical solution of the
problem.
Flight, as the matter stands at present, may be divided into two
principal varieties which represent two great sects or schools--
_1st_, The Balloonists, or those who advocate the employment of a
machine specifically lighter than the air.
_2d_, Those who believe that weight is necessary to flight. The second
school may be subdivided into
(_a_) Those who advocate the employment of rigid inclined planes driven
forward in a straight line, or revolving planes (aërial screws); and
(_b_) Such as trust for elevation and propulsion to the vertical
flapping of wings.
_Balloon._--The balloon, as my readers are aware, is constructed on the
obvious principle that a machine lighter than the air must necessarily
rise through it. The Montgolfier brothers invented such a machine in
1782. Their balloon consisted of a paper globe or cylinder, the motor
power being super-heated air supplied by the burning of vine twigs
under it. The Montgolfier or fire balloon, as it was called, was
superseded by the hydrogen gas balloon of MM. Charles and Robert, this
being in turn supplanted by the ordinary gas balloon of Mr. Green.
Since the introduction of coal gas in the place of hydrogen gas, no
radical improvement has been effected, all attempts at guiding the
balloon having signally failed. This arises from the vast extent of
surface which it necessarily presents, rendering it a fair conquest
to every breeze that blows; and because the power which animates it
is a mere lifting power which, in the absence of wind, must act in
a vertical line. The balloon consequently rises through the air in
opposition to the law of gravity, very much as a dead bird falls in a
downward direction in accordance with it. Having no hold upon the air,
this cannot be employed as a fulcrum for regulating its movements, and
hence the cardinal difficulty of ballooning as an art.
Finding that no marked improvement has been made in the balloon since
its introduction in 1782, the more advanced thinkers have within
the last quarter of a century turned their attention in an opposite
direction, and have come to regard flying creatures, all of which are
much heavier than the air, as the true models for flying machines. An
old doctrine is more readily assailed than uprooted, and accordingly
we find the followers of the new faith met by the assertion that
insects and birds have large air cavities in their interior; that
those cavities contain heated air, and that this heated air in some
mysterious manner contributes to, if it does not actually produce,
flight. No argument could be more fallacious. Many admirable fliers,
such as the bats, have no air-cells; while many birds, the apteryx
for example, and several animals never intended to fly, such as the
orang-outang and a large number of fishes, are provided with them.
It may therefore be reasonably concluded that flight is in no way
connected with air-cells, and the best proof that can be adduced is to
be found in the fact that it can be performed to perfection in their
absence.
_The Inclined Plane._--The modern school of flying is in some respects
quite as irrational as the ballooning school.
The favourite idea with most is the wedging forward of a rigid
_inclined plane_ upon the air by means of a “_vis a tergo_.”
The inclined plane may be made to advance in a _horizontal line_,
or made _to rotate_ in the form of a screw. Both plans have their
adherents. The one recommends a large supporting area extending on
either side of the weight to be elevated; the surface of the supporting
area making a very slight angle with the horizon, and the whole being
wedged forward by the action of vertical screw propellers. This was the
plan suggested by Henson and Stringfellow.
Mr. Henson designed his aërostat in 1843. “The chief feature of the
invention was the very great expanse of its sustaining planes, which
were larger in proportion to the weight it had to carry than those of
many birds. The machine advanced _with its front edge a little raised_,
the effect of which was to present its under surface to the air over
which it passed, the resistance of which, acting upon it like a strong
wind on the sails of a windmill, prevented the descent of the machine
and its burden. The sustaining of the whole, therefore, depended upon
_the speed at which it travelled through the air, and the angle at
which its under surface impinged on the air in its front_.... The
machine, fully prepared for flight, was started from the top of an
inclined plane, in descending which it attained a velocity necessary to
sustain it in its further progress. That velocity would be gradually
destroyed by the resistance of the air to forward flight; it was,
therefore, the office of the steam-engine and the vanes it actuated
simply to repair the loss of velocity; it was made therefore only of
the power and weight necessary for that small effect” (fig. 109). The
editor of Newton’s Journal of Arts and Science speaks of it thus:--“The
apparatus consists of a car containing the goods, passengers, engines,
fuel, etc., to which a rectangular frame, made of wood or bamboo cane,
and covered with canvas or oiled silk, is attached. This frame extends
on either side of the car in a similar manner to the outstretched wings
of a bird; but with this difference, that _the frame is immovable_.
Behind the wings are two vertical fan wheels, furnished with oblique
vanes, which are intended to propel the apparatus through the air.
The rainbow-like circular wheels are the propellers, answering to the
wheels of a steam-boat, and acting upon the air after the manner of a
windmill. These wheels receive motion from bands and pulleys from a
steam or other engine contained in the car. To an axis at the stern of
the car a triangular frame is attached, resembling the tail of a bird,
which is also covered with canvas or oiled silk. This may be expanded
or contracted at pleasure, and is moved up and down for the purpose of
causing the machine to ascend or descend. Beneath the tail is a rudder
for directing the course of the machine to the right or to the left;
and to facilitate the steering a sail is stretched between two masts
which rise from the car. The amount of canvas or oiled silk necessary
for buoying up the machine is stated to be equal to one square foot for
each half pound of weight.”
[Illustration: FIG. 109.--Mr. Henson’s Flying Machine.]
Wenham[103] has advocated the employment of _superimposed planes_,
with a view to augmenting the support furnished while it diminishes
the horizontal space occupied by the planes. These planes Wenham
designates _Aëroplanes_. They are inclined at a very slight angle to
the horizon, and are wedged forward either by the weight to be elevated
or by the employment of vertical screws. Wenham’s plan was adopted
by Stringfellow in a model which he exhibited at the Aëronautical
Society’s Exhibition, held at the Crystal Palace in the summer of 1868.
[103] “Aërial Locomotion,” by F. H. Wenham.--_World of Science_, June
1867.
The subjoined woodcut (fig. 110), taken from a photograph of Mr.
Stringfellow’s model, gives a very good idea of the arrangement; _a b
c_ representing the superimposed planes, _d_ the tail, and _e f_ the
vertical screw propellers.
[Illustration: FIG. 110.--Mr. Stringfellow’s Flying Machine.]
The superimposed planes (_a b c_) in this machine contained a
sustaining area of twenty-eight square feet in addition to the tail
(_d_).
Its engine represented a third of a horse power, and the weight of
the whole (engine, boiler, water, fuel, superimposed planes, and
propellers) was under 12 lbs. Its sustaining area, if that of the tail
(_d_) be included, was something like thirty-six square feet, _i.e._
three square feet for every pound--the sustaining area of the gannet,
it will be remembered (p. 134), being less than one square foot of wing
for every two pounds of body.
The model was forced by its propellers along a wire at a great speed,
but, so far as I could determine from observation, failed to lift
itself notwithstanding its extreme lightness and the comparatively very
great power employed.[104]
[104] Mr. Stringfellow stated that his machine occasionally left the
wire, and was sustained by its superimposed planes alone.