are exactly reproduced, the only difference being that in the present
figure the wing is applied to the air in a more or less horizontal
direction, whereas in fig. 53 it is applied in a more or less vertical
direction. The letters in both figures are the same. The insects whose
wings tack upon the air in a more or less horizontal direction, have an
extensive range, each wing describing nearly half a circle, these half
circles corresponding to the area of support. The body of the insect
is consequently the centre of a circle of motion. It corresponds to
_x_ of the present figure (fig. 54). When the wing is seized by the
hand at _a_, and the root made to travel in the direction _n e_, the
body of the wing travels in the direction _j f_. While so travelling,
it flies upwards in a double curve, kite-fashion, and elevates the
weight _l_. When it reaches the point _f_, it reverses suddenly to
prepare for a return stroke, which is produced by causing the root of
the wing to travel in the direction _m a_, the body and tip travelling
in the direction _i b_. During the reverse stroke, the wing flies
upwards in a double curve, kite-fashion, and elevates the weight _k_.
The more rapidly these movements are repeated, the more powerful the
wing becomes, and the greater the weight it elevates. This follows
because of the reciprocating action of the wing,--the wing, as already
explained, always drawing a current of air after it during the one
stroke, which is met and utilized by it during the next stroke. The
reciprocating action of the wing here referred to is analogous in
all respects to that observed in the flippers of the seal, sea-bear,
walrus, and turtle; the swimming wing of the penguin; and the tail
of the whale, dugong, manatee, porpoise, and fish. If the muscles
of the insect were made to act at the points _a e_, the body of the
insect would be elevated as at _k l_, by the reciprocating action of
the wings. The amount of tractile power developed in the arrangement
represented at fig. 53 (p. 107), can be readily ascertained by fixing a
spring or a weight acting over a pulley to the anterior margin (_a b_
or _e f_) of the wing; weights acting over pulleys being attached to
the root of the wing (_a_ or _e_).
The amount of elevating power developed in the arrangement represented
at fig. 54, can also be estimated by causing weights acting over
pulleys to operate upon the root of the wing (_a_ or _e_), and watching
how far the weights (_k_ or _l_) are raised. In these calculations
allowance is of course to be made for friction. The object of the two
sets of experiments described and figured, is to show that the wing can
exert a tractile power either in a nearly horizontal direction or in a
nearly vertical one, flight being produced in both cases. I wish now to
show that a body not supplied with wings or inclined surfaces will, if
left to itself, fall vertically downwards; whereas, if it be furnished
with wings, its vertical fall is converted into oblique downward
flight. These are very interesting points. Experiment has shown me that
a wing when made to vibrate vertically produces horizontal traction;
when made to vibrate horizontally, vertical traction; the vertical fall
of a body armed with wings producing oblique traction. The descent
of weights can also be made to propel the wings either in a vertical
or horizontal direction; the vibration of the wings upon the air in
natural flight causing the weights (body of flying creature) to move
forward. This shows the very important part performed by weight in all
kinds of flight.
_Weight necessary to Flight._--However paradoxical it may seem, a
certain amount of weight is indispensable in flight.
In the first place, it gives peculiar efficacy and energy to the up
stroke, by acting upon the inclined planes formed by the wings in the
direction of the plane of progression. The power and the weight may
thus be said to reciprocate, the two sitting, as it were, side by side,
and blending their peculiar influences to produce a common result.
Secondly, it adds momentum,--a heavy body, when once fairly under
weigh, meeting with little resistance from the air, through which it
sweeps like a heavy pendulum.
Thirdly, the mere act of rotating the wings on and off the wind during
extension and flexion, with a slight downward stroke, apparently
represents the entire exertion on the part of the volant animal, the
rest being performed by weight alone.
This last circumstance is deserving of attention, the more especially
as it seems to constitute the principal difference between a living
flying thing and an aërial machine. If a flying-machine was constructed
in accordance with the principles which we behold in nature, the weight
and the propelling power of the machine would be made to act upon the
sustaining and propelling surfaces, whatever shape they assumed, and
these in turn would be made to operate upon the air, and _vice versâ_.
In the aërial machine, as far as yet devised, there is no sympathy
between the weight to be elevated and the lifting power, whilst in
natural flight the wings and the weight of the flying creature act in
concert and reciprocate; the wings elevating the body the one instant,
the body by its fall elevating the wings the next. When the wings
elevate the body they are active, the body being passive. When the body
elevates the wings it is active, the wings being passive. The force
residing in the wings, and the force residing in the body (weight is a
force when launched in space and free to fall in a vertical direction)
cause the mass of the volant animal to oscillate vertically on either
side of an imaginary line--this line corresponding to the path of the
insect, bat, or bird in the air. While the wings and body act and
react upon each other, the wings, body, and air likewise act and react
upon each other. In the flight of insects, bats, and birds, _weight_
is to be regarded as an independent moving power, this being made to
act upon the oblique surfaces presented by the wings in conjunction
with the power expended by the animal--the latter being, by this
arrangement, conserved to a remarkable extent. Weight, assisted by
the elastic ligaments or springs, which recover all wings in flexion,
is to be regarded as the mechanical expedient resorted to by nature
in supplementing the efforts of all flying things.[63] Without this,
flight would be of short duration, laboured, and uncertain, and the
almost miraculous journeys at present performed by the denizens of the
air impossible.
[63] Weight, as is well known, is the sole moving power in the
clock--the pendulum being used merely to regulate the movements
produced by the descent of the leads. In watches, the onus of motion
is thrown upon a _spiral spring_; and it is worthy of remark that the
mechanician has seized upon, and ingeniously utilized, two forces
largely employed in the animal kingdom.
_Weight contributes to Horizontal Flight._--That the weight of the body
plays an important part in the production of flight may be proved by a
very simple experiment.
[Illustration: FIG. 55.]
If I take two primary feathers and fix them in an ordinary cork, as
represented at fig. 55, and allow the apparatus to drop from a height,
I find the cork does not fall vertically downwards, but _downwards_
and _forwards_ in a curve. This follows, because the feathers _a_,
_b_ are twisted flexible inclined planes, which arch in an upward
direction. They are in fact true wings in the sense that an insect
wing in one piece is a true wing. (Compare _a_, _b_, _c_ of fig. 55,
with _g_, _g´_, _s_ of fig. 82, p. 158.) When dragged downwards by
the cork (_c_), which would, if left to itself, fall vertically, they
have what is virtually a down stroke communicated to them. Under these
circumstances a struggle ensues between the cork tending to fall
vertically and the feathers tending to travel in an upward direction,
and, as a consequence, the apparatus describes the curve _d e f g_
before reaching the earth _h_, _i_. This is due to the action and
reaction of the feathers and air upon each other, and to the influence
which gravity exerts upon the cork. The forward travel of the cork and
feathers, as compared with the space through which they fall, is very
great. Thus, in some instances, I found they advanced as much as a yard
and a half in a descent of three yards. Here, then, is an example of
flight produced by purely mechanical appliances. The winged seeds fly
in precisely the same manner. The seeds of the plane-tree have, _e.g._
two wings which exactly resemble the wings employed for flying; thus
they taper from the root towards the tip, and from the anterior margin
towards the posterior margin, the margins being twisted and disposed
in different planes to form true screws. This arrangement prevents the
seed from falling rapidly or vertically, and if a breeze is blowing
it is wafted to a considerable distance before it reaches the ground.
Nature is uniform and consistent throughout. She employs the same
principle, and very nearly the same means, for flying a heavy, solid
seed which she employs for flying an insect, a bat, or a bird.
When artificial wings constructed on the plan of natural ones, with
stiff roots, tapering semi-rigid anterior margins, and thin yielding
posterior margins, are allowed to drop from a height, they describe
double curves in falling, the roots of the wings reaching the ground
first. This circumstance proves the greater buoying power of the tips
of the wings as compared with the roots. I might refer to many other
experiments made in this direction, but these are sufficient to show
that weight, when acting upon wings, or, what is the same thing, upon
elastic twisted inclined planes, must be regarded as an independent
moving power. But for this circumstance flight would be at once the
most awkward and laborious form of locomotion, whereas in reality
it is incomparably the easiest and most graceful. The power which
rapidly vibrating wings have in sustaining a body which tends to
fall vertically downwards, is much greater than one would naturally
imagine, from the fact that the body, which is always beginning to
fall, is never permitted actually to do so. Thus, when it has fallen
sufficiently far to assist in elevating the wings, it is at once
elevated by the vigorous descent of those organs. The body consequently
never acquires the downward momentum which it would do if permitted
to fall through a considerable space uninterruptedly. It is easy to
restrain even a heavy body when beginning to fall, while it is next to
impossible to check its progress when it is once fairly launched in
space and travelling rapidly in a downward direction.
_Weight, Momentum, and Power, to a certain extent, synonymous in
Flight._--When a bird rises it has little or no momentum, so that if
it comes in contact with a solid resisting surface it does not injure
itself. When, however, it has acquired all the momentum of which it
is capable, and is in full and rapid flight, such contact results in
destruction. My friend Mr. A. D. Bartlett informed me of an instance
where a wild duck terminated its career by coming violently in contact
with one of the glasses of the Eddystone Lighthouse. The glass, which
was fully an inch in thickness, was completely smashed. Advantage is
taken of this circumstance in killing sea-birds, a bait being placed
on a board and set afloat with a view to breaking the neck of the
bird when it stoops to seize the carrion. The additional power due to
momentum in heavy bodies in motion is well illustrated in the start
and progress of steamboats. In these the _slip_, as it is technically
called, decreases as the speed of the vessel increases; the strength of
a man, if applied by a hawser attached to the stern of a moderate-sized
vessel, being sufficient to retard, and, in some instances prevent, its
starting. In such a case the power of the engine is almost entirely
devoted to “slip” or in giving motion to the fluid in which the screw
or paddle is immersed. It is consequently not the power residing in the
paddle or screw which is cumulative, but the momentum inhering in the
mass. In the bird, the momentum, _alias_ weight, is made to act upon
the inclined planes formed by the wings, these adroitly converting it
into sustaining and propelling power. It is to this circumstance, more
than any other, that the prolonged flight of birds is mainly due, the
inertia or dead weight of the trunk aiding and abetting the action
of the wings, and so relieving the excess of exertion which would
necessarily devolve on the bird. It is thus that the power which in
living structures resides in the mass is conserved, and the mass itself
turned to account. But for this reciprocity, no bird could retain its
position in the air for more than a few minutes at a time. This is
proved by the comparatively brief upward flight of the lark and the
hovering of the hawk when hunting. In both these cases the body is
exclusively sustained by the action of the wings, the weight of the
trunk taking no part in it; in other words, the weight of the body does
not contribute to flight by adding its momentum and the impulse which
momentum begets. In the flight of the albatross, on the other hand,
the momentum acquired by the moving mass does the principal portion
of the work, the wings for the most part being simply rotated on and
off the wind to supply the proper angles necessary for the inertia or
mass to operate upon. It appears to me that in this blending of active
and passive power the mystery of flight is concealed, and that no
arrangement will succeed in producing flight artificially which does
not recognise and apply the principle here pointed out.
_Air-cells in Insects and Birds not necessary to Flight._--The boasted
levity of insects, bats, and birds, concerning which so much has been
written by authors in their attempts to explain flight, is delusive in
the highest degree.
Insects, bats, and birds are as heavy, bulk for bulk, as most other
living creatures, and flight can be performed perfectly by animals
which have neither air-sacs nor hollow bones; air-sacs being found in
animals never designed to fly. Those who subscribe to the heated-air
theory are of opinion that the air contained in the cavities of
insects and birds is so much lighter than the surrounding atmosphere,
that it must of necessity contribute materially to flight. I may
mention, however, that the quantity of air imprisoned is, to begin
with, so infinitesimally small, and the difference in weight which
it experiences by increase of temperature so inappreciable, that it
ought not to be taken into account by any one endeavouring to solve
the difficult and important problem of flight. The Montgolfier or
fire-balloons were constructed on the heated-air principle; but as
these have no analogue in nature, and are apparently incapable of
improvement, they are mentioned here rather to expose what I regard a
false theory than as tending to elucidate the true principles of flight.
When we have said that cylinders and hollow chambers increase the area
of the insect and bird, and that an insect and bird so constructed
is stronger, weight for weight, than one composed of solid matter, we
may dismiss the subject; flight being, as I shall endeavour to show
by-and-by, not so much a question of levity as one of weight and power
intelligently directed, upon properly constructed flying surfaces.
The bodies of insects, bats, and birds are constructed on strictly
mechanical principles,--lightness, strength, and durability of frame
being combined with power, rapidity, and precision of action. The
cylindrical method of construction is in them carried to an extreme,
the bodies and legs of insects displaying numerous unoccupied spaces,
while the muscles and solid parts are tunnelled by innumerable
air-tubes, which communicate with the surrounding medium by a series of
apertures termed spiracles.
A somewhat similar disposition of parts is met with in birds, these
being in many cases furnished not only with hollow bones, but also
(especially the aquatic ones) with a liberal supply of air-sacs. They
are likewise provided with a dense covering of feathers or down,
which adds greatly to their bulk without materially increasing their
weight. Their bodies, moreover, in not a few instances, particularly
in birds of prey, are more or less flattened. The air-sacs are well
seen in the swan, goose, and duck; and I have on several occasions
minutely examined them with a view to determine their extent and
function. In two of the specimens which I injected, the material
employed had found its way not only into those usually described,
but also into others which ramify in the substance of the muscles,
particularly the pectorals. No satisfactory explanation of the purpose
served by these air-sacs has, I regret to say, been yet tendered.
According to Sappey,[64] who has devoted a large share of attention to
the subject, they consist of a membrane which is neither serous nor
mucous, but partly the one and partly the other; and as blood-vessels
in considerable numbers, as my preparations show, ramify in their
substance, and they are in many cases covered with muscular fibres
which confer on them a rhythmic movement, some recent observers (Mr.
Drosier[65] of Cambridge, for example) have endeavoured to prove that
they are adjuncts of the lungs, and therefore assist in aërating the
blood. This opinion was advocated by John Hunter as early as 1774,[66]
and is probably correct, since the temperature of birds is higher
than that of any other class of animals, and because they are obliged
occasionally to make great muscular exertions both in swimming and
flying. Others have viewed the air-sacs in connexion with the hollow
bones frequently, though not always, found in birds,[67] and have come
to look upon the heated air which they contain as being more or less
essential to flight. That the air-cells have absolutely nothing to do
with flight is proved by the fact that some excellent fliers (take the
bats, _e.g._) are destitute of them, while birds such as the ostrich
and apteryx, which are incapable of flying, are provided with them.
Analogous air-sacs, moreover, are met with in animals never intended
to fly; and of these I may instance the great air-sac occupying the
cervical and axillary regions of the orang-outang, the float or
swimming-bladder in fishes, and the pouch communicating with the
trachea of the emu.[68]
[64] Sappey enumerates fifteen air-sacs,--the _thoracic_, situated
at the lower part of the neck, behind the sternum; _two cervical_,
which run the whole length of the neck to the head, which they supply
with air; _two pairs of anterior_, and _two pairs of posterior
diaphragmatic_; and _two pairs of abdominal_.
[65] “On the Functions of the Air-cells and the Mechanism of
Respiration in Birds,” by W. H. Drosier, M.D., Caius College.--Proc.
Camb. Phil. Soc., Feb. 12, 1866.
[66] “An Account of certain Receptacles of Air in Birds which
communicate with the Lungs, and are lodged among the Fleshy Parts and
in the Hollow Bones of these Animals.”--Phil. Trans., Lond. 1774.
[67] According to Dr. Crisp the swallow, martin, snipe, and many
birds of passage have no air in their bones (Proc. Zool. Soc., Lond.
part xxv. 1857, p. 13). The same author, in a second communication
(pp. 215 and 216), adds that the glossy starling, spotted flycatcher,
whin-chat, wood-wren, willow-wren, black-headed bunting, and canary,
five of which are birds of passage, have likewise no air in their
bones. The following is Dr. Crisp’s summary:--Out of ninety-two birds
examined he found “air in many of the bones, five (_Falconidæ_);
air in the humeri and not in the inferior extremities, thirty-nine;
no air in the extremities and probably none in the other bones,
forty-eight.”
[68] Nearly allied to this is the great gular pouch of the bustard.
Specimens of the air-sac in the orang, emu, and bustard, and likewise
of the air-sacs of the swan and goose, as prepared by me, may be seen
in the Museum of the Royal College of Surgeons of England.
The same may be said of the hollow bones,--some really admirable
fliers, as the swifts, martins, and snipes, having their bones filled
with marrow, while those of the wingless running birds alluded to have
air. Furthermore and finally, a living bird weighing 10 lbs. weighs the
same when dead, plus a very few grains; and all know what effect a few
grains of heated air would have in raising a weight of 10 lbs. from the
ground.
_How Balancing is effected in Flight, the Sound produced by the Wing,
etc._--The manner in which insects, bats, and birds balance themselves
in the air has hitherto, and with reason, been regarded a mystery,
for it is difficult to understand how they maintain their equilibrium
when the wings are beneath their bodies. Figs. 67 and 68, p. 141,
throw considerable light on the subject in the case of the insect. In
those figures the space (_a_, _g_) mapped out by the wing during its
vibrations is entirely occupied by it; _i.e._ the wing (such is its
speed) is in every portion of the space at nearly the same instant, the
space representing what is practically a solid basis of support. As,
moreover, the wing is jointed to the upper part of the body (thorax)
by a universal joint, which admits of every variety of motion, the
insect is always suspended (very much as a compass set upon gimbals is
suspended); the wings, when on a level with the body, vibrating in such
a manner as to occupy a circular area (_vide_ _r d b f_ of fig. 56,
p. 120), in the centre of which the body (_a e c_) is placed. The
wings, when vibrating above and beneath the body occupy a conical area;
the apex of the cone being directed upwards when the wings are below
the body, and downwards when they are above the body. Those points
are well seen in the bird at figs. 82 and 83, p. 158. In fig. 82 the
inverted cone formed by the wings when above the body is represented,
and in fig. 83 that formed by the wings when below the body is given.
In these figures it will be observed that the body, from the insertion
of the roots of the wings into its upper portion, is always suspended,
and this, of course, is equivalent to suspending the centre of gravity.
In the bird and bat, where the stroke is delivered more vertically
than in the insect, the _basis of support_ is increased by the tip of
the wing folding inwards and backwards in a more or less horizontal
direction at the end of the down stroke; and outwards and forwards at
the end of the up stroke. This is accompanied by the rotation of the
outer portion of the wing upon the wrist as a centre, the tip of the
wing, because of the ever varying position of the wrist, describing
an ellipse. In insects whose wings are broad and large (butterfly),
and which are driven at a comparatively low speed, the balancing power
is diminished. In insects whose wings, on the contrary, are long and
narrow (blow-fly), and which are driven at a high speed, the balancing
power is increased. It is the same with short and long winged birds, so
that the function of balancing is in some measure due to the form of
the wing, and the speed with which it is driven; the long wing and the
wing vibrated with great energy increasing the capacity for balancing.
When the body is light and the wings very ample (butterfly and heron),
the reaction elicited by the ascent and descent of the wing displaces
the body to a marked extent. When, on the other hand, the wings are
small and the body large, the reaction produced by the vibration of
the wing is scarcely perceptible. Apart, however, from the shape and
dimensions of the wing, and the rapidity with which it is urged, it
must never be overlooked that all wings (as has been pointed out) are
attached to the bodies of the animals bearing them by some form of
universal joint, and in such a manner that the bodies, whatever the
position of the wings, are accurately balanced, and swim about in a
more or less horizontal position, like a compass set upon gimbals. To
such an extent is this true, that the position of the wing is a matter
of indifference. Thus the pinion may be above, beneath, or on a level
with the body; or it may be directed forwards, backwards, or at right
angles to the body. In either case the body is balanced mechanically
and without effort. To prove this point I made an artificial wing and
body, and united the one to the other by a universal joint. I found,
as I had anticipated, that in whatever position the wing was placed,
whether above, beneath, or on a level with the body, or on either side
of it, the body almost instantly attained a position of rest. The body
was, in fact, equally suspended and balanced from all points.
[Illustration: FIG. 56.[69]]
[69] In this diagram I have purposely represented the right wing
by a straight _rigid_ rod. The natural wing, however, is curved,
_flexible_, and _elastic_. It likewise _moves in curves_, the curves
being most marked towards the end of the up and down strokes, as
shown at _m n_, _o p_. The curves, which are double figure-of-8
curves, are obliterated towards the middle of the strokes (_a r_).
This remark holds true of all natural wings, and of all artificial
wings properly constructed. The curves and the reversal thereof
are necessary to give continuity of motion to the wing during its
vibrations, and what is not less important, to enable the wing
alternately to seize and dismiss the air.