in all parts, and can be wielded intelligently even to its extremity;
a circumstance which enables the insect, bat, and bird to rise upon
the air and tread it as a master--to subjugate it in fact. The wing,
no doubt, abstracts an upward and onward recoil from the air, but in
doing this it exercises a selective and controlling power; it seizes
one current, evades another, and creates a third; it feels and paws
the air as a quadruped would feel and paw a treacherous yielding
surface. It is not difficult to comprehend why this should be so.
If the flying creature is living, endowed with volition, and capable
of directing its own course, it is surely more reasonable to suppose
that it transmits to its travelling surfaces the peculiar movements
necessary to progression, than that those movements should be the
result of impact from fortuitous currents which it has no means of
regulating. That the bird, _e.g._ requires to control the wing, and
that the wing requires to be in a condition to obey the behests of the
will of the bird, is pretty evident from the fact that most of our
domestic fowls can fly for considerable distances when they are young
and when their wings are flexible; whereas when they are old and the
wings stiff, they either do not fly at all or only for short distances,
and with great difficulty. This is particularly the case with tame
swans. This remark also holds true of the steamer or race-horse duck
(_Anas brachyptera_), the younger specimens of which only are volant.
In older birds the wings become too rigid and the bodies too heavy for
flight. Who that has watched a sea-mew struggling bravely with the
storm, could doubt for an instant that the wings and feathers of the
wings are under control? The whole bird is an embodiment of animation
and power. The intelligent active eye, the easy, graceful, oscillation
of the head and neck, the folding or partial folding of one or both
wings, nay more, the slight tremor or quiver of the individual feathers
of parts of the wings so rapid, that only an experienced eye can detect
it, all confirm the belief that the living wing has not only the power
of directing, controlling, and utilizing natural currents, but of
creating and utilizing artificial ones. But for this power, what would
enable the bat and bird to rise and fly in a calm, or steer their
course in a gale? It is erroneous to suppose that anything is left to
chance where living organisms are concerned, or that animals endowed
with volition and travelling surfaces should be denied the privilege
of controlling the movements of those surfaces quite independently of
the medium on which they are destined to operate. I will never forget
the gratification afforded me on one occasion at Carlow (Ireland) by
the flight of a pair of magnificent swans. The birds flew towards and
past me, my attention having been roused by a peculiarly loud whistling
noise made by their wings. They flew about fifteen yards from the
ground, and as their pinions were urged not much faster than those of
the heron,[78] I had abundant leisure for studying their movements.
The sight was very imposing, and as novel as it was grand. I had seen
nothing before, and certainly have seen nothing since that could convey
a more elevated conception of the prowess and guiding power which birds
may exert. What particularly struck me was the perfect command they
seemed to have over themselves and the medium they navigated. They had
their wings and bodies visibly under control, and the air was attacked
in a manner and with an energy which left little doubt in my mind that
it played quite a subordinate part in the great problem before me. The
necks of the birds were stretched out, and their bodies to a great
extent rigid. They advanced with a steady, stately motion, and swept
past with a vigour and force which greatly impressed, and to a certain
extent overawed, me. Their flight was what one could imagine that of a
flying machine constructed in accordance with natural laws would be.[79]
[78] I have frequently timed the beats of the wings of the Common
Heron (_Ardea cinerea_) in a heronry at Warren Point. In March 1869
I was placed under unusually favourable circumstances for obtaining
trustworthy results. I timed one bird high up over a lake in the
vicinity of the heronry for fifty seconds, and found that in that
period it made fifty down and fifty up strokes; _i.e._ one down
and one up stroke per second. I timed another one in the heronry
itself. It was snowing at the time (March 1869), but the birds,
notwithstanding the inclemency of the weather and the early time
of the year, were actively engaged in hatching, and required to be
driven from their nests on the top of the larch trees by knocking
against the trunks thereof with large sticks. One unusually anxious
mother refused to leave the immediate neighbourhood of the tree
containing her tender charge, and circled round and round it right
overhead. I timed this bird for ten seconds, and found that she
made ten down and ten up strokes; _i.e._ one down and one up stroke
per second precisely as before. I have therefore no hesitation in
affirming that the heron, in ordinary flight, makes exactly sixty
down and sixty up strokes per minute. The heron, however, like all
other birds when pursued or agitated, has the power of greatly
augmenting the number of beats made by its wings.
[79] The above observation was made at Carlow on the Barrow in
October 1867, and the account of it is taken from my note-book.
_The Natural Wing, when elevated and depressed, must move
forwards._--It is a condition of natural wings, and of artificial wings
constructed on the principle of living wings, that when forcibly
elevated or depressed, even in a strictly vertical direction, they
inevitably dart forward. This is well shown in fig. 81.
[Illustration: FIG. 81.]
If, for example, the wing is suddenly depressed in _a vertical
direction_, as represented at _a b_, it at once darts downwards and
forwards in a curve to _c_, thus converting the vertical down stroke
into _a down oblique forward stroke_. If, again, the wing be suddenly
elevated in a strictly vertical direction, as at _c d_, the wing as
certainly darts upwards and forwards in a curve to _e_, thus converting
the vertical up stroke into an _upward oblique forward stroke_. The
same thing happens when the wing is depressed from _e_ to _f_, and
elevated from _g_ to _h_. In both cases the wing describes a waved
track, as shown at _e g_, _g i_, which clearly proves that the wing
strikes _downwards and forwards_ during the down stroke, and _upwards
and forwards_ during the up stroke. The wing, in fact, is always
advancing; its under surface attacking the air like a boy’s kite.
If, on the other hand, the wing be forcibly depressed, as indicated
by the heavy waved line _a c_, and left to itself, it will as surely
rise again and describe a waved track, as shown at _c e_. This it
does by rotating on its long axis, and in virtue of its flexibility
and elasticity, aided by the recoil obtained from the air. In other
words, it is not necessary to elevate the wing forcibly in the
direction _c d_ to obtain the upward and forward movement _c e_. One
single impulse communicated at _a_ causes the wing to travel to _e_,
and a second impulse communicated at _e_ causes it to travel to _i_.
It follows from this that a series of vigorous down impulses would,
_if a certain interval were allowed to elapse between them_, beget a
corresponding series of up impulses, in accordance with the law of
action and reaction; the wing and the air under these circumstances
being alternately active and passive. I say if a certain interval
were allowed to elapse between every two down strokes, but this is
practically impossible, as the wing is driven with such velocity
that there is positively no time to waste in waiting for the purely
mechanical ascent of the wing. That the ascent of the pinion is not,
and ought not to be entirely due to the reaction of the air, is proved
by the fact that in flying creatures (certainly in the bat and bird)
there are distinct elevator muscles and elastic ligaments delegated to
the performance of this function. The reaction of the air is therefore
only one of the forces employed in elevating the wing; the others, as
I shall show presently, are vital and vito-mechanical in their nature.
The falling downwards and forwards of the body when the wings are
ascending also contribute to this result.
[Illustration: FIG. 82. FIG. 83.
Figs. 82 and 83 show that when the wings are elevated (_e_, _f_,
_g_ of fig. 82) the body falls (_s_ of fig. 82); and that when the
wings are depressed (_h_, _i_, _j_ of fig. 83) the body is elevated
(_r_ of fig. 83). Fig. 82 shows that the wings are elevated as short
levers (_e_) until towards the termination of the up stroke, when
they are gradually expanded (_f_, _g_) to prepare them for making
the down stroke. Fig. 83 shows that the wings descend as long levers
(_h_) until towards the termination of the down stroke, when they are
gradually folded or flexed (_i_, _j_), to rob them of their momentum
and prepare them for making the up stroke. Compare with figs. 74 and
75, p. 145. By this means the air beneath the wings is vigorously
seized during the down stroke, while that above it is avoided during
the up stroke. The concavo-convex form of the wings and the forward
travel of the body contribute to this result. The wings, it will be
observed, act as a parachute both during the up and down strokes.
Compare with fig. 55, p. 112. Fig. 83 shows, in addition, the
compound rotation of the wing, how it rotates upon a as a centre,
with a radius _m b n_, and upon _a c b_ as a centre, with a radius _k
l_. Compare with fig. 80, p. 149.--_Original._]
_The Wing ascends when the Body descends, and_ vice versâ.--As the
body of the insect, bat, and bird falls forwards in a curve when the
wing ascends, and is elevated in a curve when the wing descends, it
follows that the trunk of the animal is urged along a waved line, as
represented at 1, 2, 3, 4, 5 of fig. 81, p. 157; the waved line _a c
e g i_ of the same figure giving the track made by the wing. I have
distinctly seen the alternate rise and fall of the body and wing when
watching the flight of the gull from the stern of a steam-boat.
The direction of the stroke in the insect, as has been already
explained, is much more horizontal than in the bat or bird (compare
figs. 82 and 83 with figs. 64, 65, and 66, p. 139). In either case,
however, the down stroke must be delivered in a more or less forward
direction. This is necessary for support and propulsion. A horizontal
to-and-fro movement will elevate, and an up-and-down vertical movement
propel, but an oblique forward motion is requisite for progressive
flight.
In all wings, whatever their position during the intervals of rest,
and whether in one piece or in many, this feature is to be observed
in flight. The wings are slewed downwards and forwards, _i.e._ they
are carried more or less in the direction of the head during their
descent, and reversed or carried in an opposite direction during their
ascent. In stating that the wings are carried away from the head
during the back stroke, I wish it to be understood that they do not
therefore necessarily travel backwards in space when the insect is
flying forwards. On the contrary, the wings, as a rule, move forward
in curves, both during the down and up strokes. The fact is, that the
wings at their roots are hinged and geared to the trunk so loosely,
that the body is free to oscillate in a forward or backward direction,
or in an up, down, or oblique direction. As a consequence of this
freedom of movement, and as a consequence likewise of the speed at
which the insect is travelling, the wings during the back stroke are
for the most part actually travelling forwards. This is accounted for
by the fact, that the body falls downwards and forwards in a curve
during the up or return stroke of the wings, and because the horizontal
speed attained by the body is as a rule so much greater than that
attained by the wings, that the latter are never allowed time to travel
backward, the lesser movement being as it were swallowed up by the
greater. For a similar reason, the passenger of a steam-ship may travel
rapidly in the direction of the stern of the vessel, and yet be carried
forward in space,--the ship sailing much quicker than he can walk.
While the wing is descending, it is rotating upon its root as a centre
(short axis). It is also, and this is a most important point, rotating
upon its anterior margin (long axis), in such a manner as to cause the
several parts of the wing to assume various angles of inclination with
the horizon.
Figs. 84 and 85 supply the necessary illustration.
[Illustration: FIG. 84.]
[Illustration: FIG. 85.]
In flexion, as a rule, the under surface of the wing (fig. 84 _a_) is
arranged in the same plane with the body, both being in a line with or
making a slight angle with the horizon (_x x_).[80] When the wing is
made to descend, it gradually, in virtue of its simultaneously rotating
upon its long and short axes, makes a certain angle with the horizon
as represented at _b_. The angle is increased at the termination of
the down stroke as shown at _c_, so that the wing, particularly its
posterior margin, during its descent (_A_), is screwed or crushed down
upon the air with its concave or biting surface directed forwards and
towards the earth. The same phenomena are indicated at _a b c_ of
fig. 85, but in this figure the wing is represented as travelling more
decidedly forwards during its descent, and this is characteristic of
the down stroke of the insect’s wing--the stroke in the insect being
delivered in a very oblique and more or less horizontal direction
(figs. 64, 65, and 66, p. 139; fig. 71, p. 144). The forward travel of
the wing during its descent has the effect of diminishing the angles
made by the under surface of the wing with the horizon. Compare _b c d_
of fig. 85 with the same letters of fig. 84. At fig. 88 (p. 166) the
angles for a similar reason are still further diminished. This figure
(88) gives a very accurate idea of the kite-like action of the wing
both during its descent and ascent.
[80] It happens occasionally in insects that the posterior margin
of the wing is on a higher level than the anterior one towards the
termination of the up stroke. In such cases the posterior margin is
suddenly rotated in a downward and forward direction at the beginning
of the down stroke--the downward and forward rotation securing
additional elevating power for the wing. The posterior margin of the
wing in bats and birds, unless they are flying downwards, never rises
above the anterior one, either during the up or down stroke.
[Illustration: FIG. 86.]
The downward screwing of the posterior margin of the wing during the
down stroke is well seen in the dragon-fly, represented at fig. 86,
p. 161.
Here the arrows _r s_ indicate the range of the wing. At the beginning
of the down stroke the upper or dorsal surface of the wing (_i d f_)
is inclined slightly upwards and forwards. As the wing descends the
posterior margin (_i f_) twists and rotates round the anterior margin
(_i d_), and greatly increases the angle of inclination as seen at
_i j_, _g h_. This rotation of the posterior margin (_i j_) round
the anterior margin (_g h_) has the effect of causing the different
portions of the under surface of the wing to assume various angles
of inclination with the horizon, the wing attacking the air like a
boy’s kite. The angles are greatest towards the root of the wing and
least towards the tip. They accommodate themselves to the speed at
which the different parts of the wing travel--a small angle with a
high speed giving the same amount of buoying power as a larger angle
with a diminished speed. The screwing of the under surface of the wing
(particularly the posterior margin) in a downward direction during the
down stroke is necessary to insure the necessary upward recoil; the
wing being made to swing downwards and forwards pendulum fashion, for
the purpose of elevating the body, which it does by acting upon the
air as a long lever, and after the manner of a kite. During the down
stroke the wing is active, the air passive. In other words, the wing is
depressed by a purely vital act.
The down stroke is readily explained, and its results upon the body
obvious. The real difficulty begins with the up or return stroke. If
the wing was simply to travel in an upward and backward direction from
_c_ to _a_ of fig. 84, p. 160, it is evident that it would experience
much resistance from the superimposed air, and thus the advantages
secured by the descent of the wing would be lost. What really happens
is this. The wing does not travel upwards and _backwards_ in the
direction _c b a_ of fig. 84 (the body, be it remembered, is advancing)
but upwards and _forwards_ in the direction _c d e f g_. This is
brought about in the following manner. The wing is at right angles
to the horizon (_x x´_) at _c_. It is therefore caught by the air at
the point (2) because of the more or less horizontal travel of the
body; the elastic ligaments and other structures combined with the
resistance experienced from the air rotating the posterior or thin
margin of the pinion in an upward direction, as shown at _d e f g_
and _d f g_ of figs. 84 and 85, p. 160. The wing by this partly vital
and partly mechanical arrangement is rotated off the wind in such a
manner as to keep its dorsal or non-biting surface directed upwards,
while its concave or biting surface is directed downwards. The wing,
in short, has its planes so arranged, and its angles so adjusted to
the speed at which it is travelling, that it darts up a gradient like
a true kite, as shown at _c d e f g_ of figs. 84 and 85, p. 160, or _g
h i_ of fig. 88, p. 166. The wing consequently elevates and propels
during its _ascent_ as well as during its _descent_. It is, in fact,
a kite during both the down and up strokes. The ascent of the wing is
greatly assisted by the _forward travel_, and _downward and forward
fall_ of the body. This view will be readily understood by supposing,
what is really the case, that the wing is more or less fixed by the air
in space at the point indicated by 2 of figs. 84 and 85, p. 160; the
body, the instant the wing is fixed, falling downwards and forwards
in a curve, which, of course, is equivalent to placing the wing
above, and, so to speak, behind the volant animal--in other words, to
elevating the wing preparatory to a second down stroke, as seen at _g_
of the figures referred to (figs. 84 and 85). The ascent and descent
of the wing is always very much greater than that of the body, from
the fact of the pinion acting as a long lever. The peculiarity of the
wing consists in its being a flexible lever which acts upon yielding
fulcra (the air), the body participating in, and to a certain extent
perpetuating, the movements originally produced by the pinion. The
part which the body performs in flight is indicated at fig. 87. At _a_
the body is depressed, the wing being elevated and ready to make the
down stroke at _b_. The wing descends in the direction _c d_, but the
moment it begins to descend the body moves _upwards and forwards_ (see
arrows) in a curved line to _e_. As the wing is attached to the body
the wing is made gradually to assume the position _f_. The body (_e_),
it will be observed, is now on a higher level than the wing (_f_); the
under surface of the latter being so adjusted that it strikes upwards
and forwards as a kite. It is thus that the wing sustains and propels
during the up stroke. The body (_e_) now falls _downwards and forwards_
in a curved line to _g_, and in doing this it elevates or assists in
elevating the wing to _j_. The pinion is a second time depressed in
the direction _k l_, which has the effect of forcing the body along a
waved track and in _an upward direction_ until it reaches the point
_m_. The ascent of the body and the descent of the wing take place
simultaneously (_m n_). The body and wing, are alternately above and
beneath a given line _x x´_.
[Illustration: FIG. 87.]
A careful study of figs. 84, 85, 86, and 87, pp. 160, 161, and 163,
shows the great importance of the twisted configuration and curves
peculiar to the natural wing. If the wing was not curved in every
direction it could not be rolled on and off the wind during the down
and up strokes, as seen more particularly at fig. 87, p. 163. This,
however, is a vital point in progressive flight. The wing (_b_) is
rolled on to the wind in the direction _b a_, its under concave or
biting surface being crushed hard down with the effect of elevating
the body to _e_. The body falls to _g_, and the wing (_f_) is rolled
off the wind in the direction _f j_, and elevated until it assumes the
position _j_. The elevation of the wing is effected partly by the fall
of the body, partly by the action of the elevator muscles and elastic
ligaments, and partly by the reaction of the air, operating on its
under or concave biting surface. The wing is therefore to a certain
extent resting during the up stroke.
The concavo-convex form of the wing is admirably adapted for the
purposes of flight. In fact, the power which the wing possesses of
always keeping its concave or under surface directed _downwards_ and
_forwards_ enables it to seize the air at every stage of both the up
and down strokes so as to supply a persistent buoyancy. The action
of the natural wing is accompanied by remarkably little slip--the
elasticity of the organ, the resiliency of the air, and the shortening
and elongating of the elastic ligaments and muscles all co-operating
and reciprocating in such a manner that the descent of the wing
elevates the body; the descent of the body, aided by the reaction
of the air and the shortening of the elastic ligaments and muscles,
elevating the wing. The wing during the up stroke _arches above the
body_ after the manner of a parachute, and prevents the body from
falling. The sympathy which exists between the parts of a flying
animal and the air on which it depends for support and progress is
consequently of the most intimate character.
The up stroke (_B_, _D_ of figs. 84 and 85, p. 160), as will be seen
from the foregoing account, is a compound movement due in some measure
to recoil or resistance on the part of the air; to the shortening of
the muscles, elastic ligaments, and other vital structures; to the
elasticity of the wing; and to the falling of the body in a downward
and forward direction. The wing may be regarded as rotating during the
down stroke upon 1 of figs. 84 and 85, p. 160, which may be taken to
represent the long and short axes of the wing; and during the up stroke
upon 2, which may be taken to represent the yielding fulcrum furnished
by the air. A second pulsation is indicated by the numbers 3 and 4 of
the same figures (84, 85).
_The Wing acts upon yielding Fulcra._--The chief peculiarity of the
wing, as has been stated, consists in its being a twisted flexible
lever specially constructed to act upon yielding fulcra (the air). The
points of contact of the wing with the air are represented at _a b c
d e f g h i j k l_ respectively of figs. 84 and 85, p. 160; and the
imaginary points of rotation of the wing upon its long and short axes
at 1, 2, 3, and 4 of the same figures. The assumed points of rotation
advance from 1 to 3 and from 2 to 4 (_vide_ arrows marked _r_ and _s_,
fig. 85); these constituting the steps or pulsations of the wing. The
actual points of rotation correspond to the little loops _a b c d f g h
i j l_ of fig. 85. The wing descends at _A_ and _C_, and ascends at _B_
and _D_.
_The Wing acts as a true Kite both during the Down and Up
Strokes._--If, as I have endeavoured to explain, the wing, even when
elevated and depressed in a strictly vertical direction, inevitably
and invariably darts forward, it follows as a consequence that the
wing, as already partly explained, flies forward as a true kite, both
during the down and up strokes, as shown at _c d e f g h i j k l m_ of
fig. 88; and that its under concave or biting surface, in virtue of the
forward travel communicated to it by the body in motion, is closely
applied to the air, both during its ascent and descent--a fact hitherto
overlooked, but one of considerable importance, as showing how the wing
furnishes a persistent buoyancy, alike when it rises and falls.
[Illustration: FIG. 88.]