the feet of swimming birds. Compare with figs. 18 and 19, pp. 37 and
39, and with fig. 32, p. 68.--_Original._]
In swimming birds, each foot describes one side of an ellipse when it
is extended and thrust from the body, the other side of the ellipse
being described when the foot is flexed and drawn towards the body.
The curve described by the right foot when pushed from the body is
seen at the arrow _r_ of fig. 50; that formed by the left foot when
drawn towards the body, at the arrow _s_ of the same figure. The curves
formed by the feet during extension and flexion produce, when united
in the act of swimming, waved lines, these constituting a chart for the
movements of the extremities of swimming birds.
There is consequently an obvious analogy between the swimming of birds
and the walking of man (compare fig. 50, p. 97, with fig. 19, p. 39);
between the walking of man and the walking of the quadruped (compare
figs. 18 and 19, pp. 37 and 39); between the walking of the quadruped
and the swimming of the walrus, sea-bear, and seal; between the
swimming of the seal, whale, dugong, manatee, and porpoise, and that of
the fish (compare fig. 32, p. 68, with figs. 18 and 19, pp. 37 and 39);
and between the swimming of the fish and the flying of the insect, bat,
and bird (compare all the foregoing figures with figs. 71, 73, and 81,
pp. 144 and 157).
[Illustration:
FIG. 51.--The Flying-fish (_Exocœtus exsiliens_, Linn.), with wings
expanded and elevated in the act of flight (_vide_ arrows). This
anomalous and interesting creature is adapted both for swimming
and flying. The swimming-tail is consequently retained, and the
pectoral fins, which act as wings, are enormously increased in
size.--_Original._]
_Flight of the Flying-fish; the kite-like action of the Wings,
etc._--Whether the flying-fish uses its greatly expanded pectoral fins
as a bird its wings, or only as parachutes, has not, so far as I am
aware, been determined by actual observation. Most observers are of
opinion that these singular creatures glide up the wind, and do not
beat it after the manner of birds; so that their flight (or rather
leap) is indicated by the arc of a circle, the sea supplying the
chord. I have carefully examined the structure, relations, and action
of those fins, and am satisfied in my own mind that they act as true
pinions within certain limits, their inadequate dimensions and limited
range alone preventing them from sustaining the fish in the air for
indefinite periods. When the fins are fully flexed, as happens when
the fish is swimming, they are arranged along the sides of the body;
but when it takes to the air, they are raised above the body and make
a certain angle with it. In being raised they are likewise inclined
forwards and outwards, the fins rotating on their long axes until they
make an angle of something like 30° with _the horizon_--this being, as
nearly as I can determine, the greatest angle made by the wings during
the down stroke in the flight of insects and birds.
The pectoral fins, or pseudo-wings of the flying-fish, like all other
wings, act after the manner of kites--the angles of inclination
which their under surfaces make with the horizon varying according
to the degree of extension, the speed acquired, and the pressure to
which they are subjected by being carried against the air. When the
flying-fish, after a preliminary rush through the water (in which it
acquires initial velocity), throws itself into the air, it is supported
and carried forwards by the kite-like action of its pinions;--this
action being identical with that of the boy’s kite when the boy runs,
and by pulling upon the string causes the kite to glide upwards and
forwards. In the case of the boy’s kite _a pulling force_ is applied
to the kite in front. In the case of the flying-fish (and everything
which flies) _a similar force_ is applied to the kites formed by the
wings by the weight of the flying mass, which always tends to fall
vertically downwards. Weight supplies a motor power in flight similar
to that supplied by the leads in a clock. In the case of the boy’s
kite, the hand of the operator furnishes the power; in flight, a large
proportion of the power is furnished by the weight of the body of the
flying creature. It is a matter of indifference how a kite is flown, so
long as its under surface is made to impinge upon the air over which
it passes.[59] A kite will fly effectually when it is neither acted
upon by the hand nor a weight, provided always there is a stiff breeze
blowing. In flight one of two things is necessary. Either the under
surface of the wings must be carried rapidly against still air, or
the air must rush violently against the under surface of the expanded
but motionless wings. Either the wings, the body bearing them, or the
air, must be in rapid motion; one or other must be active. To this
there is no exception. To fly a kite in still air the operator must
run. If a breeze is blowing the operator does not require to alter
his position, the breeze doing the entire work. It is the same with
wings. In still air a bird, or whatever attempts to fly, must flap
its wings energetically until it acquires initial velocity, when the
flapping may be discontinued; or it must throw itself from a height,
in which case the initial velocity is acquired by the weight of the
body acting upon the inclined planes formed by the motionless wings.
The flapping and gliding action of the wings constitute the difference
between ordinary flight and that known as skimming or sailing flight.
The flight of the flying-fish is to be regarded rather as an example
of the latter than the former, the fish transferring the velocity
acquired by the vigorous lashing of its tail in the water to the
air,--an arrangement which enables it to dispense in a great measure
with the flapping of the wings, which act by a combined parachute and
wedge action. In the flying-fish the flying-fin or wing attacks the
air _from beneath_, whilst it is being raised above the body. It has
no downward stroke, the position and attachments of the fin preventing
it from descending beneath the level of the body of the fish. In this
respect the flying-fin of the fish differs slightly from the wing of
the insect, bat, and bird. The gradual expansion and raising of the
fins of the fish, coupled with the fact that the fins never descend
below the body, account for the admitted absence of beating, and have
no doubt originated the belief that the pectoral fins are merely
passive organs. If, however, they do not act as true pinions within
the limits prescribed, it is difficult, and indeed impossible, to
understand how such small creatures can obtain the momentum necessary
to project them a distance of 200 or more yards, and to attain, as they
sometimes do, an elevation of twenty or more feet above the water.
Mr. Swainson, in crossing the line in 1816, zealously attempted to
discover the true action of the fins in question, but the flight of the
fish is so rapid that he utterly failed. He gives it as his opinion
that flight is performed in two ways,--first by a spring or leap, and
second by the spreading of the pectoral fins, which are employed in
propelling the fish in a forward direction, either by flapping or by a
motion analogous to the skimming of swallows. He records the important
fact, that the flying-fish can change its course after leaving the
water, which satisfactorily proves that the fins are not simply passive
structures. Mr. Lord, of the Royal Artillery,[60] thus writes of those
remarkable specimens of the finny tribe:--“There is no sight more
charming than the flight of a shoal of flying-fish, as they shoot forth
from the dark green wave in a glittering throng, like silver birds in
some gay fairy tale, gleaming brightly in the sunshine, and then, with
a mere touch on the crest of the heaving billow, again flitting onward
reinvigorated and refreshed.”
[59] “On the Various Modes of Flight in relation to Aëronautics.” By
the Author.--Proceedings of the Royal Institution of Great Britain,
March 1867.
[60] Nature and Art, November 1866, p. 173.
Before proceeding to a consideration of the graceful and, in some
respects, mysterious evolutions of the denizens of the air, and
the far-stretching pinions by which they are produced, it may not
be out of place to say a few words in recapitulation regarding the
extent and nature of the surfaces by which progression is secured
on land and on or in the water. This is the more necessary, as the
travelling-surfaces employed by animals in walking and swimming bear
a certain, if not a fixed, relation to those employed by insects,
bats, and birds in flying. On looking back, we are at once struck with
the fact, remarkable in some respects, that the travelling-surfaces,
whether feet, flippers, fins, or pinions, are, as a rule, increased
in proportion to the tenuity of the medium on which they are destined
to operate. In the ox (fig. 18, p. 37) we behold a ponderous body,
slender extremities, and unusually small feet. The feet are slightly
expanded in the otter (fig. 12, p. 34), and considerably so in the
ornithorhynchus (fig. 11, p. 34). The travelling-area is augmented
in the seal (fig. 14, p. 34; fig. 36, p. 74), penguin (figs. 46 and
47, pp. 91 and 94), sea-bear (fig. 37, p. 76), and turtle (fig. 44,
p. 89). In the triton (fig. 45, p. 89) a huge swimming-tail is added
to the feet--the tail becoming larger, and the extremities (anterior)
diminishing, in the manatee (fig. 34, p. 73) and porpoise (fig. 33,
p. 73), until we arrive at the fish (fig. 30, p. 65), where not only
the tail but _the lower half of the body_ is actively engaged in
natation. Turning from the water to the air, we observe a remarkable
modification in the huge pectoral fins of the flying-fish (fig. 51,
p. 98), these enabling the creature to take enormous leaps, and serving
as pseudo-pinions. Turning in like manner from the earth to the air,
we encounter the immense tegumentary expansions of the flying-dragon
(fig. 15, p. 35) and galeopithecus (fig. 16, p. 35), the floating
or buoying area of which greatly exceeds that of some of the flying
beetles.
In those animals which fly, as bats (fig. 17, p. 36), insects (figs. 57
and 58, p. 124 and 125), and birds (figs. 59 and 60, p. 126), the
travelling surfaces, because of the extreme tenuity of the air, are
prodigiously augmented; these in many instances greatly exceeding the
actual area of the body. While, therefore, the movements involved in
walking, swimming, and flying are to be traced in the first instance
to the shortening and lengthening of the muscular, elastic, and other
tissues operating on the bones, and their peculiar articular surfaces;
they are to be referred in the second instance to the extent and
configuration of the travelling areas--these on all occasions being
accurately adapted to the capacity and strength of the animal and the
density of the medium on or in which it is intended to progress. Thus
the land supplies the resistance, and affords the support necessary
to prevent the small feet of land animals from sinking to dangerous
depths, while the water, immensely less resisting, furnishes the
peculiar medium requisite for buoying the fish, and for exposing,
without danger and to most advantage, the large surface contained in
its ponderous lashing tail,--the air, unseen and unfelt, furnishing
that quickly yielding and subtle element in which the greatly expanded
pinions of the insect, bat, and bird are made to vibrate with lightning
rapidity, discoursing, as they do so, a soft and stirring music very
delightful to the lover of nature.
PROGRESSION IN OR THROUGH THE AIR.
The atmosphere, because of its great tenuity, mobility, and comparative
imponderability, presents little resistance to bodies passing through
it at low velocities. If, however, the speed be greatly accelerated,
the passage of even an ordinary cane is sensibly impeded.
This comes of the action and reaction of matter, the resistance
experienced varying according to the density of the atmosphere and
the shape, extent, and velocity of the body acting upon it. While,
therefore, scarcely any impediment is offered to the progress of an
animal in motion, it is often exceedingly difficult to compress the
air with sufficient rapidity and energy to convert it into a suitable
fulcrum for securing the onward impetus. This arises from the fact
that bodies moving in the air experience the _minimum of resistance_
and occasion the _maximum of displacement_. Another and very obvious
difficulty is traceable to the great disparity in the weight of air as
compared with any known solid, this in the case of water being nearly
as 1000 to 1. According to the density of the medium so is its buoying
or sustaining power.
_The Wing a Lever of the Third Order._--To meet the peculiarities
stated above, the insect, bat, and bird are furnished with extensive
surfaces in the shape of pinions or wings, which they can apply
with singular velocity and power, as levers of the third order
(fig. 3, p. 20),[61] at various angles, or by alternate slow and
sudden movements, to obtain the necessary degree of resistance and
non-resistance. Although the third order of lever is particularly
inefficient when the fulcrum is _rigid_ and _immobile_, it possesses
singular advantages when these conditions are reversed, _i.e._ when the
fulcrum, as happens with the air, is _elastic_ and _yielding_. In this
case a very slight movement at the root of the pinion, or that end of
the lever directed towards the body, is succeeded by an immense sweep
of the extremity of the wing, where its elevating and propelling power
is greatest. This arrangement insures that the large quantity of air
necessary for propulsion and support shall be compressed under the most
favourable conditions.
[61] In this form of lever the power is applied between the fulcrum
and the weight to be raised. The mass to be elevated is the body of
the insect, bat, or bird,--the force which resides in the living
pinion (aided by the inertia of the trunk) representing the power,
and the air the fulcrum.
It follows from this that those insects and birds are endowed with the
greatest powers of flight whose wings are the longest. The dragon-fly
and albatross furnish examples. The former on some occasions dashes
along with amazing velocity and wheels with incredible rapidity; at
other times it suddenly checks its headlong career and hovers or fixes
itself in the air after the manner of the kestrel and humming-birds.
The flight of the albatross is also remarkable. This magnificent
bird, I am informed on reliable authority, sails about with apparent
unconcern for hours together, and rarely deigns to flap its enormous
pinions, which stream from its body like ribbons to the extent, in some
cases, of seven feet on either side.
The manner in which the wing levers the body upwards and forwards in
flight is shown at fig. 52.
[Illustration: FIG. 52.]
In this fig. _f f´_ represent the moveable fulcra furnished by the
air; _p p´_ the power residing in the wing, and _b_ the body to be
flown. In order to make the problem of flight more intelligible, I
have prolonged the lever formed by the wing beyond the body (_b_), and
have applied to the root of the wing so extended the weight _w w´_.
_x_ represents the universal joint by which the wing is attached to
the body. When the wing ascends, as shown at _p_, the air (= fulcrum
_f_) resists its upward passage, and forces the body (_b_), or its
representative (_w_), slightly downwards. When the wing descends, as
shown at _p´_, the air (= fulcrum _f´_) resists its downward passage,
and forces the body (_b_), or its representative (_w´_), slightly
upwards. From this it follows, that when the wing rises the body
falls, and _vice versâ_; the wing describing the arc of a large circle
(_f f´_), the body (_b_), or the weights representing it (_w w´_)
describing the arc of a much smaller circle. The body, therefore, as
well as the wing, rises and falls in flight. When the wing descends it
elevates the body, the wing being active and the body passive; when
the body descends it elevates the wing, the body being active and the
wing passive. The elevator muscles, and the reaction of the air on
the under surface of the wing, contribute to its elevation. It is in
this manner that weight forms a factor in flight, the wing and the
weight of the body reciprocating and mutually assisting and relieving
each other. This is an argument for employing four wings in artificial
flight, the wings being so arranged that the two which are up shall
always by their fall mechanically elevate the two which are down. Such
an arrangement is calculated greatly to conserve the driving power,
and, as a consequence, to reduce the weight. It is the upper or dorsal
surface of the wing which more especially operates upon the air during
the up stroke, and the under or ventral surface which operates during
the down stroke. The wing, which at the beginning of the down stroke
has its surfaces and margins (anterior and posterior) arranged in
nearly the same plane with the horizon,[62] rotates upon its anterior
margin as an axis during its descent and causes its under surface to
make a gradually increasing angle with the horizon, the posterior
margin (fig. 53, _c_) in this movement descending beneath the anterior
one. A similar but opposite rotation takes place during the up stroke.
The rotation referred to causes the wing to twist on its long axis
screw-fashion, and to describe a figure-of-8 track in space, one-half
of the figure being described during the ascent of the wing, the other
half during its descent. The twisting of the wing and the figure-of-8
track described by it when made to vibrate, are represented at fig. 53.
The rotation of the wing on its long axis as it ascends and descends
causes the under surface of the wing to act as a kite, both during
the up and down strokes, provided always the body bearing the wing
is in forward motion. But the upper surface of the wing, as has been
explained, acts when the wing is being elevated, so that both the upper
and under surfaces of the wing are efficient during the up stroke.
When the wing ascends, the upper surface impinges against the air;
the under surface impinging at the same time from its being carried
obliquely forward, after the manner of a kite, by the body, which is in
motion. During the down stroke, the under surface only acts. The wing
is consequently effective both during its ascent and descent, its slip
being nominal in amount. The wing acts as a kite, both when it ascends
and descends. It acts more as a propeller than an elevator during its
ascent; and more as an elevator than a propeller during its descent.
It is, however, effective both in an upward and downward direction.
The efficiency of the wing is greatly increased by the fact that when
it ascends it draws a current of air up after it, which current being
met by the wing during its descent, greatly augments the power of the
down stroke. In like manner, when the wing descends it draws a current
of air down after it, which being met by the wing during its ascent,
greatly augments the power of the up stroke. These induced currents
are to the wing what a stiff autumn breeze is to the boy’s kite. The
wing is endowed with this very remarkable property, that it creates
the current on which it rises and progresses. It literally flies on a
whirlwind of its own forming.
[62] In some cases the posterior margin is slightly elevated above
the horizon (fig. 53, _g_).
These remarks apply more especially to the wings of bats and birds,
and those insects whose wings are made to vibrate in a more or less
vertical direction. The action of the wing is readily imitated, as a
reference to fig. 53 will show.
[Illustration: FIG. 53.]
If, for example, I take a tapering elastic reed, as represented at
_a b_, and supply it with a flexible elastic sail (_c d_), and a
ball-and-socket joint (_x_), I have only to seize the reed at _a_
and cause it to oscillate upon _x_ to elicit all the wing movements.
By depressing the root of the reed in the direction _n e_, the wing
flies up as a kite in the direction _j f_. During the upward movement
the wing flies upwards and forwards, and describes a double curve. By
elevating the root of the reed in the direction _m a_, the wing flies
down as a kite in the direction _i b_. During the downward movement the
wing flies downwards and forwards, and describes a double curve. These
curves, when united, form a waved track, which represents progressive
flight. During the rise and fall of the wing a large amount of tractile
force is evolved, and if the wings and the body of the flying creature
are inclined slightly upwards, kite-fashion, as they invariably are
in ordinary flight, the whole mass of necessity moves upwards and
forwards. To this there is no exception. A sheet of paper or a card
will float along if its anterior margin is slightly raised, and if
it be projected with sufficient velocity. The wings of all flying
creatures when made to vibrate, twist and untwist, the posterior thin
margin of each wing twisting round the anterior thick one, like the
blade of a screw. The artificial wing represented at fig. 53 (p. 107)
does the same, _c d_ twisting round _a b_, and _g h_ round _e f_. The
natural and artificial wings, when elevated and depressed, describe a
figure-of-8 track in space when the bodies to which they are attached
are stationary. When the bodies advance, the figure-of-8 is opened out
to form first a looped and then a waved track. I have shown how those
insects, bats, and birds which flap their wings in a more or less
vertical direction evolve tractile or propelling power, and how this,
operating on properly constructed inclined surfaces, results in flight.
I wish now to show that flight may also be produced by a very oblique
and almost horizontal stroke of the wing, as in some insects, _e.g._
the wasp, blue-bottle, and other flies. In those insects the wing is
made to vibrate with a figure-of-8 sculling motion in a very oblique
direction, and with immense energy. This form of flight differs in no
respect from the other, unless in the direction of the stroke, and can
be readily imitated, as a reference to fig. 54 will show.
[Illustration: FIG. 54.]