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THE INTERNATIONAL SCIENTIFIC SERIES.
LIST OF THE VOLUMES ALREADY PUBLISHED.
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PETTIGREW, M.D. F.R S. 130 Illustrations. Price 5s.
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☞ For List of forthcoming Volumes, see end of the book.
HENRY S. KING & CO. 65 CORNHILL, and 12 PATERNOSTER ROW.
[Illustration:
_C. Berjeau_ _W. Ballingall_
WALKING, SWIMMING, AND FLYING.]
ANIMAL LOCOMOTION
OR
WALKING, SWIMMING, AND FLYING,
WITH A DISSERTATION ON
AËRONAUTICS.
BY
J. BELL PETTIGREW, M.D. F.R.S. F.R.S.E. F.R.C.P.E.
PATHOLOGIST TO THE ROYAL INFIRMARY OF EDINBURGH; CURATOR OF THE
MUSEUM OF THE ROYAL COLLEGE OF SURGEONS OF EDINBURGH;
Extraordinary Member and late President of the Royal Medical Society
of Edinburgh; Croonian Lecturer to the Royal Society of London for
1860; Lecturer to the Royal Institution of Great Britain and Russell
Institution, 1867; Lecturer to the Royal College of Surgeons of
Edinburgh, 1872; Author of numerous Memoirs on Physiological Subjects
in the Philosophical and other Transactions, etc. etc. etc.
_ILLUSTRATED BY 130 ENGRAVINGS ON WOOD._
HENRY S. KING & CO.
65 CORNHILL, AND 12 PATERNOSTER ROW, LONDON.
1873.
[Illustration:
(_All Rights reserved._)]
PREFACE.
In the present volume I have endeavoured to explain, in simple
language, some difficult problems in “Animal Mechanics.” In order to
avoid elaborate descriptions, I have introduced a large number of
original Drawings and Diagrams, copied for the most part from my Papers
and Memoirs “On Flight,” and other forms of “Animal Progression.” I
have drawn from the same sources many of the facts to be found in
the present work. My best thanks are due to Mr. W. Ballingall, of
Edinburgh, for the highly artistic and effective manner in which he has
engraved the several subjects. The figures, I am happy to state, have
in no way deteriorated in his hands.
ROYAL COLLEGE OF SURGEONS OF EDINBURGH,
_July 1873_.
[Illustration]
CONTENTS.
ANIMAL LOCOMOTION.
INTRODUCTION.
PAGE
Motion associated with the life and well-being of animals, 1
Motion not confined to the animal kingdom; all matter in motion;
natural and artificial motion; the locomotive, steamboat, etc.
A flying machine possible, 2
Weight necessary to flight, 3
The same laws regulate natural and artificial progression, 4
Walking, swimming, and flying correlated, 5
Flight the poetry of motion, 6
Flight a more unstable movement than that of walking and
swimming; the travelling surfaces and movements of animals
adapted to the earth, the water, and the air, 7
The earth, the water, and the air furnish the fulcra for the
levers formed by the travelling surfaces of animals, 8
Weight plays an important part in walking, swimming, and flying, 9
The extremities of animals in walking act as pendulums, and
describe figure-of-8 curves, 9
In swimming, the body of the fish is thrown into figure-of-8
curves, 10
The tail of the fish made to vibrate pendulum fashion, 11
The tail of the fish, the wing of the bird, and the extremity
of the biped and quadruped are screws structurally and
functionally. They describe figure-of-8 and waved tracks, 12
The body and wing reciprocate in flight; the body rising when
the wing is falling, and _vice versâ_, 12
Flight the least fatiguing kind of motion. Aërial creatures not
stronger than terrestrial ones, 13
Fins, flippers, and wings form mobile helics or screws, 14
Artificial fins, flippers, and wings adapted for navigating the
water and air, 14
History of the figure-of-8 theory of walking, swimming, and
flying, 15
Priority of discovery on the part of the Author. Admission to
that effect on the part of Professor Marey, 16
Fundamental axioms. Of uniform motion. Motion uniformly
varied, 17
The legs move by the force of gravity. Resistance of fluids.
Mechanical effects of fluids on animals immersed in them.
Centre of gravity, 18
The three orders of lever, 19
Passive organs of locomotion. Bones, 21
Joints, 23
Ligaments. Effects of atmospheric pressure on limbs. Active
organs of locomotion. Muscles; their properties, arrangement,
modes of action, etc., 24
Muscular cycles. Centripetal and centrifugal movements of
muscles; muscular waves. Muscles arranged in longitudinal,
transverse, and oblique spiral lines, 25–27
The bones of the extremities twisted and spiral, 28
Muscles take precedence of bones in animal movements, 29
Oblique spiral muscles necessary for spiral bones and joints, 31
The spiral movements of the spine transferred to the
extremities, 33
The travelling surfaces of animals variously modified and
adapted to the media on or in which they move, 34–36
PROGRESSION ON THE LAND.
Walking of the Quadruped, Biped, etc., 37
Locomotion of the Horse, 39
Locomotion of the Ostrich, 45
Locomotion of Man, 51
PROGRESSION ON AND IN THE WATER.
Swimming of the Fish, Whale, Porpoise, etc., 66
Swimming of the Seal, Sea-Bear, and Walrus, 74
Swimming of Man, 78
Swimming of the Turtle, Triton, Crocodile, etc., 89
Flight under water, 90
Difference between sub-aquatic and aërial flight, 92
Flight of the Flying-fish; the kite-like action of the wings, 98
PROGRESSION IN OR THROUGH THE AIR.
The wing a lever of the third order, 103
Weight necessary to flight, 110
Weight contributes to horizontal flight, 112
Weight, momentum and power to a certain extent synonymous
in flight, 114
Air-cells in insects and birds not necessary to flight, 115
How balancing is effected in flight, 118
Rapidity of wing movements partly accounted for, 120
The wing area variable and in excess, 124
The wing area decreases as the size and weight of the volant
animal increases, 132
Wings, their form, etc. All wings screws, structurally and
functionally, 136
The wing, during its action, reverses its planes, and describes
a figure-of-8 track in space, 140
The wing, when advancing with the body, describes a looped
and waved track, 143
The margins of the wing, thrown into opposite curves during
extension and flexion, 146
The tip of the bat and bird’s wing describes an ellipse, 147
The wing capable of change of form in all its parts, 147
The wing during its vibration produces a cross pulsation, 148
Compound rotation of the wing, 149
The wing vibrates unequally with reference to a given line, 150
Points wherein the screws formed by the wings differ from
those in common use, 151
The wing at all times thoroughly under control, 154
The natural wing when elevated and depressed must move
forwards, 156
The wing ascends when the body descends, and _vice versâ_, 159
The wing acts upon yielding fulcra, 165
The wing acts as a true kite both during the down and up
strokes, 165
Where the kite formed by the wing differs from the boy’s kite, 166
The angles formed by the wing during its vibrations, 167
The body and wings move in opposite curves, 168
THE WINGS OF INSECTS, BATS, AND BIRDS.
Elytra or wing cases and membranous wings; their shape and
uses, 170
THE WINGS OF BATS.
The bones of the wing of the bat; the spiral configuration of
their articular surfaces, 176
THE WINGS OF BIRDS.
The bones of the wing of the bird; their articular surfaces,
movements, etc., 178
Traces of design in the wing of the bird; the arrangement of
the primary, secondary, and tertiary feathers, etc., 18O
The wing of the bird not always opened up to the same extent
in the up stroke, 182
Flexion of the wing necessary to the flight of birds, 183
Consideration of the forces which propel the wings of insects, 186
Speed attained by insects, 188
Consideration of the forces which propel the wings of bats and
birds, 189
Lax condition of the shoulder-joint in bats and birds, 190
The wing flexed and partly elevated by the action of elastic
ligaments; the nature and position of said ligaments in
the Pheasant, Snipe, Crested Crane, Swan, etc., 191
The elastic ligaments more highly differentiated in wings which
vibrate rapidly, 193
Power of the wing, to what owing, 194
Reasons why the effective stroke should be delivered downwards
and forwards, 195
The wing acts as an elevator, propeller, and sustainer, both
during extension and flexion, 197
Flight divisible into four kinds, 197
The flight of the Albatross compared to the movements of a
compass set upon gimbals, 199
The regular and irregular in flight, 201
Mode of ascending, descending, turning, etc., 201
The flight of birds referable to muscular exertion and weight, 204
Lifting capacity of birds, 205
AËRONAUTICS.
The balloon, 210
The inclined plane, 211
The aërial screw, 215
Artificial wings (Borelli’s views), 219
Marey’s views, 226
Chabrier’s views, 233
Straus-Durckheim’s views, 233
The Author’s views; his method of constructing and applying
artificial wings, as contra-distinguished from that of Borelli,
Chabrier, Durckheim, and Marey, 235
The wave wing of the Author, 236
How to construct an artificial wave wing on the insect type, 240
How to construct a wave wing which shall evade the superimposed
air during the up stroke, 241
Compound wave wing of the Author, 242
How to apply artificial wings to the air, 245
As to the nature of the forces required for propelling
artificial wings, 246
Necessity for supplying the roots of artificial wings with
elastic structures in imitation of the muscles and elastic
ligaments of flying animals, 247
The artificial wave wing can be driven at any speed--it can
make its own currents or utilize existing ones, 251
Compound rotation of the artificial wave wing. The different
parts of the wing travel at different speeds, 252
How the wave wing creates currents and rises upon them, and
how the air assists in elevating the wing, 253
The artificial wing propelled at various degrees of speed
during the down and up strokes, 255
The artificial wave wing as a propeller, 256
A new form of aërial screw, 256
The aërial wave screw operates upon water, 257
The sculling action of the wing, 231
CONCLUDING REMARKS, 258
[Illustration]
LIST OF ILLUSTRATIONS.
The Engravings are, with few exceptions, from Photographs, Drawings,
and Designs by Mr. Charles Berjeau and the Author. Such as are not
original are duly acknowledged.
FRONTISPIECE. PAGE
In the clutch of the enemy--(_The Graphic_).
The three orders of lever--(_Bishop_), 19, 20
The skeleton of a Deer--(_Pander and D’Alton_), 21
Muscular cycle in the act of flexing the arm, 25
Screws formed by the bones of the wing of the bird, the bones
of the anterior extremity of the Elephant, and the cast of the
interior of the left ventricle of the heart, 28
The muscular system of the Horse--(_Bagg_), 30
The feet of the Deer, Ornithorhynchus, Otter, Frog, and Seal, 34
The Red-throated Dragon, 35
The Flying Lemur, 35
The Bat, 36
Chillingham Bull with extremities describing figure-of-8
movements, 37
Double waved tracks described by Man in walking, 39
Horse in the act of trotting, 41
Footprints of the Horse in the walk, trot, and gallop--
(_Gamgee_), 43
Skeleton of the Ostrich--(_Dallas_), 47
Ostriches pursued by a hunter, 48
Skeleton of Man, 55
The positions assumed by the extremities and feet in walking--
(_Weber_), 59
Preparing to run--(_Flaxman_), 62
The skeleton of a Perch--(_Dallas_), 65
The Salmon swimming leisurely, 65
Swimming of the fish according to Borelli, 67
Swimming of the fish according to the Author, 68
The Porpoise and Manatee, 73
The skeleton of the Dugong--(_Dallas_), 74
The Seal, 74
The Sea-Bear, 76
The elliptical, looped, and spiral tracks made in swimming, 81
The several attitudes assumed by the extremities in swimming
in the prone position, 82
Overhand swimming, 85
Side swimming, 86
Swimming of the Turtle and Triton, 89
Swimming of the Little Penguin, 91
Sub-aquatic flight or diving, 94
The feet of the Swan as seen in the open and closed condition, 96
The foot of the Grebe with swimming membrane--(_Dallas_), 97
Double waved track described by the feet of swimming birds, 97
The flight of the Flying-fish, 98
The wing a lever of the third order, 105
Figure-of-8 vertical track made by the wing in flight, 107
Do. horizontal track, 108
Feathers and cork flying forward, 112
Diagram illustrating how wings obtain their high speed, 120
Butterfly with large wings, 124
Beetle with small wings, 125
Partridge with small wings; Heron with large wings, 126
The wings of the Hawk and Albatross, 136, 137
The Green Plover with one wing flexed and the other extended, 138
Blur or impression produced on the eye by the rapidly
oscillating wing of the insect, 139
Diagram in which the down and up strokes of the wing of the
insect are analysed, 141
Diagrams illustrating the looped and waved tracks described
by the wing of the insect, bat, and bird, 144
Figures showing the positions assumed by the wing of the bird
during the up and down strokes (side view), 145
The positions assumed by the wing of the insect as it hastens
to and fro and describes a figure-of-8 track, 147
The figure-of-8 curves made by the wing of the bird in flexion
and extension, 147
The long and short axes of the wing, 149
The waved tracks described by the wing and body of the
bird as they alternately rise and fall, 157, 163
The positions assumed by the wing of the bird during the
down and up strokes (front view), 158
Analysis of the movements of the wing, 160, 161
The kite-like action and waved movements of the wing, 166
The Centaur Beetle and Water Bug, 171
The Dragon Fly, 172
The screws formed by the wing of the insect, bat,
and bird, 174, 175, 176
The muscles, elastic ligaments, and feathers of the wing
of the bird, 181
The flight of the King-fisher, 183
The flight of the Gull, 186
The flight of the Owl, 198
The flight of the Albatross, 200
Pigeon and Duck alighting, 203, 204
Hawk and quarry--(_The Graphic_), 206
The Vauxhall Balloon of Mr. Green, 208
Mr. Henson’s Flying Machine, 212
Mr. Stringfellow’s Flying Machine, 213
Sir George Cayley’s Flying Apparatus, 215
Flying Machine designed by De la Landelle, 217
Borelli’s Artificial Bird, 220
Diagrams illustrating the true and false action of the wing, 228
The sculling action of the wing as seen in the bird, 231
The artificial wave wing of the Author, 237
Do. do. with driving apparatus, 239
Various forms of artificial wings by the Author, 241
The compound wave wing of the Author, 243
Diagrams illustrative of artificial wing movements, 250
Diagram illustrating the currents produced by the movements
of artificial wings, 253
The aërial wave screw of the Author, 256
Swallow in pursuit of insects, 260
[Illustration]
[Illustration]
ANIMAL LOCOMOTION.
[Illustration:
_Engraved by W. Ballingall._
IN THE CLUTCH OF THE ENEMY.]
ANIMAL LOCOMOTION.
INTRODUCTION.
The locomotion of animals, as exemplified in walking, swimming, and
flying, is a subject of permanent interest to all who seek to trace
in the creature proofs of beneficence and design in the Creator.
All animals, however insignificant, have a mission to perform--a
destiny to fulfil; and their manner of doing it cannot be a matter of
indifference, even to a careless observer. The most exquisite form
loses much of its grace if bereft of motion, and the most ungainly
animal conceals its want of symmetry in the co-adaptation and exercise
of its several parts. The rigidity and stillness of death alone are
unnatural. So long as things “live, move, and have a being,” they are
agreeable objects in the landscape. They are part and parcel of the
great problem of life, and as we are all hastening towards a common
goal, it is but natural we should take an interest in the movements
of our fellow-travellers. As the locomotion of animals is intimately
associated with their habits and modes of life, a wide field is opened
up, teeming with incident, instruction, and amusement. No one can see a
bee steering its course with admirable precision from flower to flower
in search of nectar; or a swallow darting like a flash of light along
the lanes in pursuit of insects; or a wolf panting in breathless haste
after a deer; or a dolphin rolling like a mill-wheel after a shoal of
flying fish, without feeling his interest keenly awakened.
Nor is this love of motion confined to the animal kingdom. We admire a
cataract more than a canal; the sea is grander in a hurricane than in
a calm; and the fleecy clouds which constantly flit overhead are more
agreeable to the eye than a horizon of tranquil blue, however deep
and beautiful. We never tire of sunshine and shadow when together: we
readily tire of either by itself. Inorganic changes and movements are
scarcely less interesting than organic ones. The disaffected growl of
the thunder, and the ghastly lightning flash, scorching and withering
whatever it touches, forcibly remind us that everything above, below,
and around is in motion. Of absolute rest, as Mr. Grove eloquently
puts it, nature gives us no evidence. All matter, whether living or
dead, whether solid, liquid, or gaseous, is constantly changing form:
in other words, is constantly moving. It is well it is so; for those
incessant changes in inorganic matter and living organisms introduce
that fascinating variety which palls not upon the eye, the ear, the
touch, the taste, or the smell. If an absolute repose everywhere
prevailed, and plants and animals ceased to grow; if day ceased to
alternate with night and the fountains were dried up or frozen; if the
shadows refused to creep, the air and rocks to reverberate, the clouds
to drift, and the great race of created beings to move, the world would
be no fitting habitation for man. In change he finds his present solace
and future hope. The great panorama of life is interesting because it
moves. One change involves another, and everything which co-exists,
co-depends. This co-existence and inter-dependence causes us not only
to study ourselves, but everything around us. By discovering natural
laws we are permitted in God’s good providence to harness and yoke
natural powers, and already the giant Steam drags along at incredible
speed the rumbling car and swiftly gliding boat; the quadruped has been
literally outraced on the land, and the fish in the sea; each has been,
so to speak, beaten in its own domain. That the tramway of the air may
and will be traversed by man’s ingenuity at some period or other, is,
reasoning from analogy and the nature of things, equally certain. If
there were no flying things--if there were no insects, bats, or birds
as models, artificial flight (such are the difficulties attending its
realization) might well be regarded an impossibility. As, however, the
flying creatures are legion, both as regards number, size, and pattern,
and as the bodies of all are not only manifestly heavier than the
air, but are composed of hard and soft parts, similar in all respects
to those composing the bodies of the other members of the animal
kingdom, we are challenged to imitate the movements of the insect,
bat, and bird in the air, as we have already imitated the movements
of the quadruped on the land and the fish in the water. We have made
two successful steps, and have only to make a third to complete that
wonderfully perfect and very comprehensive system of locomotion which
we behold in nature. Until this third step is taken, our artificial
appliances for transit can only be considered imperfect and partial.
Those authors who regard artificial flight as impracticable sagely
remark that the land supports the quadruped and the water the fish.
This is quite true, but it is equally true that the air supports the
bird, and that the evolutions of the bird on the wing are quite as safe
and infinitely more rapid and beautiful than the movements of either
the quadruped on the land or the fish in the water. What, in fact,
secures the position of the quadruped on the land, the fish in the
water, and the bird in the air, is the life; and by this I mean that
prime moving or self-governing power which co-ordinates the movements
of the _travelling surfaces_ (whether feet, fins, or wings) of all
animals, and adapts them to the medium on which they are destined to
operate, whether this be the comparatively unyielding earth, the mobile
water, or the still more mobile air. Take away this life suddenly--the
quadruped falls downwards, the fish (if it be not specially provided
with a swimming bladder) sinks, and the bird gravitates of necessity.
There is a sudden subsiding and cessation of motion in either case,
but the quadruped and fish have no advantage over the bird in this
respect. The _savans_ who oppose this view exclaim not unnaturally
that there is no great difficulty in propelling a machine either
along the land or the water, seeing that both these media support it.
There is, I admit, no great difficulty now, but there were apparently
insuperable difficulties before the locomotive and steam-boat were
invented. _Weight_, moreover, instead of being a barrier to artificial
flight is absolutely necessary to it. This statement is quite opposed
to the commonly received opinion, but is nevertheless true. No bird is
lighter than the air, and no machine constructed to navigate it should
aim at being specifically lighter. What is wanted is a reasonable but
not cumbrous amount of weight, and a duplicate (in principle if not
in practice) of those structures and movements which enable insects,
bats, and birds to fly. Until the structure and uses of wings are
understood, the way of “an eagle in the air” must of necessity remain
a mystery. The subject of flight has never, until quite recently, been
investigated systematically or rationally, and, as a result, very
little is known of the laws which regulate it. If these laws were
understood, and we were in possession of trustworthy data for our
guidance in devising artificial pinions, the formidable Gordian knot of
flight, there is reason to believe, could be readily untied.
That artificial flight is a possible thing is proved beyond
doubt--_1st_, by the fact that flight is a natural movement; and _2d_,
because the natural movements of walking and swimming have already been
successfully imitated.
The very obvious bearing which natural movements have upon artificial
ones, and the relation which exists between organic and inorganic
movements, invest our subject with a peculiar interest.
It is the blending of natural and artificial progression in theory
and practice which gives to the one and the other its chief charm.
The history of artificial progression is essentially that of natural
progression. The same laws regulate and determine both. The wheel of
the locomotive and the screw of the steam-ship apparently greatly
differ from the limb of the quadruped, the fin of the fish, and the
wing of the bird; but, as I shall show in the sequel, the curves which
go to form the wheel and the screw are found in the travelling surfaces
of all animals, whether they be limbs (furnished with feet), or fins,
or wings.
It is a remarkable circumstance that the undulation or wave made by
the wing of an insect, bat, or bird, when those animals are fixed
or hovering before an object, and when they are flying, corresponds
in a marked manner with the track described by the stationary and
progressive waves in fluids, and likewise with the waves of sound.
This coincidence would seem to argue an intimate relation between the
instrument and the medium on which it is destined to operate--the wing
acting in those very curves into which the atmosphere is naturally
thrown in the transmission of sound. Can it be that the animate and
inanimate world reciprocate, and that animal bodies are made to impress
the inanimate in precisely the same manner as the inanimate impress
each other? This much seems certain:--The wind communicates to the
water similar impulses to those communicated to it by the fish in
swimming; and the wing in its vibrations impinges upon the air as an
ordinary sound does. The extremities of quadrupeds, moreover, describe
waved tracks on the land when walking and running; so that one great
law apparently determines the course of the insect in the air, the fish
in the water, and the quadruped on the land.
We are, unfortunately, not taught to regard the travelling surfaces and
movements of animals as correlated in any way to surrounding media,
and, as a consequence, are apt to consider walking as distinct from
swimming, and walking and swimming as distinct from flying, than which
there can be no greater mistake. Walking, swimming, and flying are in
reality only modifications of each other. Walking merges into swimming,
and swimming into flying, by insensible gradations. The modifications
which result in walking, swimming, and flying are necessitated by the
fact that the earth affords a greater amount of support than the water,
and the water than the air.
That walking, swimming, and flying represent integral parts of the
same problem is proved by the fact that most quadrupeds swim as well
as walk, and some even fly; while many marine animals walk as well as
swim, and birds and insects walk, swim, and fly indiscriminately. When
the land animals, properly so called, are in the habit of taking to
the water or the air; or the inhabitants of the water are constantly
taking to the land or the air; or the insects and birds which are
more peculiarly organized for flight, spend much of their time on
the land and in the water; their organs of locomotion must possess
those peculiarities of structure which characterize, as a class,
those animals which live on the land, in the water, or in the air
respectively.
In this we have an explanation of the gossamer wing of the insect,--the
curiously modified hand of the bat and bird,--the webbed hands and
feet of the Otter, Ornithorhynchus, Seal, and Walrus,--the expanded
tail of the Whale, Porpoise, Dugong, and Manatee,--the feet of the
Ostrich, Apteryx, and Dodo, exclusively designed for running,--the feet
of the Ducks, Gulls, and Petrels, specially adapted for swimming,--and
the wings and feet of the Penguins, Auks, and Guillemots, especially
designed for diving. Other and intermediate modifications occur in the
Flying-fish, Flying Lizard, and Flying Squirrel; and some animals, as
the Frog, Newt, and several of the aquatic insects (the Ephemera or
May-fly for example[1]) which begin their career by swimming, come
ultimately to walk, leap, and even fly.[2]
[1] The Ephemeræ in the larva and pupa state reside in the water
concealed during the day under stones or in horizontal burrows which
they form in the banks. Although resembling the perfect insect in
several respects, they differ materially in having longer antennæ, in
wanting ocelli, and in possessing horn-like mandibles; the abdomen
has, moreover, on each side a row of plates, mostly in pairs, which
are a kind of false branchiæ, and which are employed not only in
respiration, _but also as paddles_.--Cuvier’s Animal Kingdom, p. 576.
London, 1840.
[2] Kirby and Spence observe that some insects which are not
naturally aquatic, do, nevertheless, swim very well if they fall
into the water. They instance a kind of grasshopper (_Acrydium_),
which can paddle itself across a stream with great rapidity by the
powerful strokes of its hind legs.--(Introduction to Entomology, 5th
edit., 1828, p. 360.) Nor should the remarkable discovery by Sir John
Lubbock of a swimming insect (_Polynema natans_), which uses its
wings _exclusively as fins_, be overlooked.--Linn. Trans. vol. xxiv.
p. 135.
Every degree and variety of motion, which is peculiar to the land, and
to the water- and air-navigating animals as such, is imitated by others
which take to the elements in question secondarily or at intervals.
Of all animal movements, flight is indisputably the finest. It may be
regarded as the poetry of motion. The fact that a creature as heavy,
bulk for bulk, as many solid substances, can by the unaided movements
of its wings urge itself through the air with a speed little short of
a cannon-ball, fills the mind with wonder. Flight (if I may be allowed
the expression) is a more unstable movement than that of walking and
swimming; the instability increasing as the medium to be traversed
becomes less dense. It, however, does not essentially differ from the
other two, and I shall be able to show in the following pages, that the
materials and forces employed in flight are literally the same as those
employed in walking and swimming. This is an encouraging circumstance
as far as artificial flight is concerned, as the same elements and
forces employed in constructing locomotives and steamboats may, and
probably will at no distant period, be successfully employed in
constructing flying machines. Flight is a purely mechanical problem. It
is warped in and out with the other animal movements, and forms a link
of a great chain of motion which drags its weary length over the land,
through the water, and, notwithstanding its weight, through the air. To
understand flight, it is necessary to understand walking and swimming,
and it is with a view to simplifying our conceptions of this most
delightful form of locomotion that the present work is mainly written.
The chapters on walking and swimming naturally lead up to those on
flying.
In the animal kingdom the movements are adapted either to the land,
the water, or the air; these constituting the three great highways of
nature. As a result, the instruments by which locomotion is effected
are specially modified. This is necessary because of the different
densities and the different degrees of resistance furnished by the
land, water, and air respectively. On the land the extremities of
animals encounter the _maximum_ of _resistance_, and occasion the
_minimum_ of _displacement_. In the air, the pinions experience the
_minimum_ of _resistance_, and effect the _maximum_ of _displacement_;
the water being intermediate both as regards the degree of resistance
offered and the amount of displacement produced. The speed of an animal
is determined by its shape, mass, power, and the density of the medium
on or in which it moves. It is more difficult to walk on sand or snow
than on a macadamized road. In like manner (unless the travelling
surfaces are specially modified), it is more troublesome to swim than
to walk, and to fly than to swim. This arises from the displacement
produced, and the consequent want of support. The land supplies the
fulcrum for the levers formed by the extremities or travelling surfaces
of animals with terrestrial habits; the water furnishes the fulcrum for
the levers formed by the tail and fins of fishes, sea mammals, etc.;
and the air the fulcrum for the levers formed by the wings of insects,
bats, and birds. The fulcrum supplied by the land is immovable; that
supplied by the water and air movable. The mobility and immobility
of the fulcrum constitute the principal difference between walking,
swimming, and flying; the travelling surfaces of animals increasing in
size as the medium to be traversed becomes less dense and the fulcrum
more movable. Thus terrestrial animals have smaller travelling surfaces
than amphibia, amphibia than fishes, and fishes than insects, bats, and
birds. Another point to be studied in connexion with unyielding and
yielding fulcra, is the resistance offered to forward motion. A land
animal is supported by the earth, and experiences little resistance
from the air through which it moves, unless the speed attained is
high. Its principal friction is that occasioned by the contact of
its travelling surfaces with the earth. If these are few, the speed
is generally great, as in quadrupeds. A fish, or sea mammal, is of
nearly the same specific gravity as the water it inhabits; in other
words, it is supported with as little or less effort than a land
animal. As, however, the fluid in which it moves is more dense than
air, the resistance it experiences in forward motion is greater than
that experienced by land animals, and by insects, bats, and birds.
As a consequence fishes are for the most part elliptical in shape;
this being the form calculated to cleave the water with the greatest
ease. A flying animal is immensely heavier than the air. The support
which it receives, and the resistance experienced by it in forward
motion, are reduced to a minimum. Flight, because of the rarity of
the air, is infinitely more rapid than either walking, running, or
swimming. The flying animal receives support from the air by increasing
the size of its travelling surfaces, which act after the manner of
twisted inclined planes or kites. When an insect, a bat, or a bird is
launched in space, its weight (from the tendency of all bodies to fall
vertically downwards) presses upon the inclined planes or kites formed
by the wings in such a manner as to become converted directly into a
_propelling_, and indirectly into a _buoying_ or supporting power. This
can be proved by experiment, as I shall show subsequently. But for the
share which the weight or mass of the flying creature takes in flight,
the protracted journeys of birds of passage would be impossible. Some
authorities are of opinion that birds even sleep upon the wing. Certain
it is that the albatross, that prince of the feathered tribe, can sail
about for a whole hour without once flapping his pinions. This can only
be done in virtue of the weight of the bird acting upon the inclined
planes or kites formed by the wings as stated.
The weight of the body plays an important part in walking and swimming,
as well as in flying. A biped which advances by steps and not by leaps
may be said to roll over its extremities,[3] the foot of the extremity
which happens to be upon the ground for the time forming the centre
of a circle, the radius of which is described by the trunk in forward
motion. In like manner the foot which is off the ground and swinging
forward pendulum fashion in space, may be said to roll or rotate upon
the trunk, the head of the femur forming the centre of a circle the
radius of which is described by the advancing foot. A double rolling
movement is thus established, the body rolling on the extremity the
one instant, the extremity rolling on the trunk the next. During these
movements the body rises and falls. The double rolling movement is
necessary not only to the progression of bipeds, but also to that of
quadrupeds. As the body cannot advance without the extremities, so
the extremities cannot advance without the body. The double rolling
movement is necessary to continuity of motion. If there was only one
movement there would be dead points or halts in walking and running,
similar to what occur in leaping. The continuity of movement necessary
to progression in some bipeds (man for instance) is further secured
by a pendulum movement in the arms as well as in the legs, the right
arm swinging before the body when the right leg swings behind it, and
the converse. The right leg and left arm advance simultaneously, and
alternate with the left leg and right arm, which likewise advance
together. This gives rise to a double twisting of the body at the
shoulders and loins. The legs and arms when advancing move in curves,
the convexities of the curves made by the right leg and left arm, which
advance together when a step is being made, being directed outwards,
and forming, when placed together, a more or less symmetrical ellipse.
If the curves formed by the legs and arms respectively be united,
they form waved lines which intersect at every step. This arises from
the fact that the curves formed by the right and left legs are found
alternately on either side of a given line, the same holding true
of the right and left arms. Walking is consequently to be regarded
as the result of a twisting diagonal movement in the trunk and in
the extremities. Without this movement, the momentum acquired by the
different portions of the moving mass could not be utilized. As the
momentum acquired by animals in walking, swimming, and flying forms an
important factor in those movements, it is necessary that we should
have a just conception of the value to be attached to weight when in
motion. In the horse when walking, the stride is something like five
feet, in trotting ten feet, but in galloping eighteen or more feet. The
stride is in fact determined by the speed acquired by the mass of the
body of the horse; the momentum at which the mass is moving carrying
the limbs forward.[4]
[3] This is also true of quadrupeds. It is the posterior part of the
feet which is set down first.
[4] “According to Sainbell, the celebrated horse Eclipse, when
galloping at liberty, and with its greatest speed, passed over the
space of twenty-five feet at each stride, which he repeated 2-1/3
times in a second, being nearly four miles in six minutes and two
seconds. The race-horse Flying Childers was computed to have passed
over eighty-two feet and a half in a second, or nearly a mile in a
minute.”
In the swimming of the fish, the body is thrown into double or
figure-of-8 curves, as in the walking of the biped. The twisting of
the body, and the continuity of movement which that twisting begets,
reappear. The curves formed in the swimming of the fish are never
less than two, a caudal and a cephalic one. They may and do exceed
this number in the long-bodied fishes. The tail of the fish is made to
vibrate pendulum fashion on either side of the spine, when it is lashed
to and fro in the act of swimming. It is made to rotate upon one or
more of the vertebræ of the spine, the vertebra or vertebræ forming the
centre of a lemniscate, which is described by the caudal fin. There
is, therefore, an obvious analogy between the tail of the fish and the
extremity of the biped. This is proved by the conformation and swimming
of the seal,--an animal in which the posterior extremities are modified
to resemble the tail of the fish. In the swimming of the seal the hind
legs are applied to the water by a sculling figure-of-8 motion, in the
same manner as the tail of the fish. Similar remarks might be made with
regard to the swimming of the whale, dugong, manatee, and porpoise,
sea mammals, which still more closely resemble the fish in shape. The
double curve into which the fish throws its body in swimming, and
which gives continuity of motion, also supplies the requisite degree
of steadiness. When the tail is lashed from side to side there is a
tendency to produce a corresponding movement in the head, which is
at once corrected by the complementary curve. Nor is this all; the
cephalic curve, in conjunction with the water contained within it,
forms the _point d’appui_ for the caudal curve, and _vice versa_.
When a fish swims, the anterior and posterior portions of its body
(supposing it to be a short-bodied fish) form curves, the convexities
of which are directed on opposite sides of a given line, as is the case
in the extremities of the biped when walking. The mass of the fish,
like the mass of the biped, when once set in motion, contributes to
progression by augmenting the rate of speed. The velocity acquired by
certain fishes is very great. A shark can gambol around the bows of a
ship in full sail; and a sword-fish (such is the momentum acquired by
it) has been known to thrust its tusk through the copper sheathing of
a vessel, a layer of felt, four inches of deal, and fourteen inches of
oaken plank.[5]
[5] A portion of the timbers, etc., of one of Her Majesty’s ships,
having the tusk of a sword-fish imbedded in it, is to be seen in the
Hunterian Museum of the Royal College of Surgeons of England.
The wing of the bird does not materially differ from the extremity
of the biped or the tail of the fish. It is constructed on a similar
plan, and acts on the same principle. The tail of the fish, the wing
of the bird, and the extremity of the biped and quadruped, are screws
structurally and functionally. In proof of this, compare the bones of
the wing of a bird with the bones of the arm of a man, or those of
the fore-leg of an elephant, or any other quadruped. In either case
the bones are twisted upon themselves like the screw of an augur. The
tail of the fish, the extremities of the biped and quadruped, and the
wing of the bird, when moving, describe waved tracks. Thus the wing
of the bird, when it is made to oscillate, is thrown into double or
figure-of-8 curves, like the body of the fish. When, moreover, the wing
ascends and descends to make the up and down strokes, it rotates within
the _facettes_ or depressions situated on the scapula and coracoid
bones, precisely in the same way that the arm of a man rotates in the
glenoid cavity, or the leg in the acetabular cavity in the act of
walking. The ascent and descent of the wing in flying correspond to the
steps made by the extremities in walking; the wing rotating upon the
body of the bird during the down stroke, the body of the bird rotating
on the wing during the up stroke. When the wing descends it describes
a downward and forward curve, and elevates the body in an upward and
forward curve. When the body descends, it describes a downward and
forward curve, the wing being elevated in an upward and forward curve.
The curves made by the wing and body in flight form, when united, waved
lines, which intersect each other at every beat of the wing. The wing
and the body act upon each other alternately (the one being active
when the other is passive), and the descent of the wing is not more
necessary to the elevation of the body than the descent of the body is
to the elevation of the wing. It is thus that the weight of the flying
animal is utilized, slip avoided, and continuity of movement secured.
As to the actual waste of tissue involved in walking, swimming, and
flying, there is much discrepancy of opinion. It is commonly believed
that a bird exerts quite an enormous amount of power as compared with
a fish; a fish exerting a much greater power than a land animal. This,
there can be no doubt, is a popular delusion. A bird can fly for a
whole day, a fish can swim for a whole day, and a man can walk for a
whole day. If so, the bird requires no greater power than the fish, and
the fish than the man. The speed of the bird as compared with that of
the fish, or the speed of the fish as compared with that of the man, is
no criterion of the power exerted. The speed is only partly traceable
to the power. As has just been stated, it is due in a principal
measure to the shape and size of the travelling surfaces, the density
of the medium traversed, the resistance experienced to forward motion,
and the part performed by the mass of the animal, when moving and
acting upon its travelling surfaces. It is erroneous to suppose that a
bird is stronger, weight for weight, than a fish, or a fish than a man.
It is equally erroneous to assume that the exertions of a flying animal
are herculean as compared with those of a walking or swimming animal.
Observation and experiment incline me to believe just the opposite. A
flying creature, when fairly launched in space (because of the part
which weight plays in flight, and the little resistance experienced in
forward motion), sweeps through the air with almost no exertion.[6]
This is proved by the sailing flight of the albatross, and by the fact
that some insects can fly when two-thirds of their wing area have been
removed. (This experiment is detailed further on.) These observations
are calculated to show the grave necessity for studying the media to
be traversed; the fulcra which the media furnish, and the size, shape,
and movements of the travelling surfaces. The travelling surfaces of
animals, as has been already explained, furnish the levers by whose
instrumentality the movements of walking, swimming, and flying are
effected.
[6] A flying creature exerts its greatest power when rising. The
effort is of short duration, and inaugurates rather than perpetuates
flight. If the volant animal can launch into space from a height, the
preliminary effort may be dispensed with as in this case, the weight
of the animal acting upon the inclined planes formed by the wings
gets up the initial velocity.