Popular Science Monthly/Volume 4/March 1874/Walking, Swimming, and Flying
THROUGHOUT the realms of Nature motion is indispensable to physical stability and organic existence. It is everywhere present, and equally among molecules and masses the mind searches in vain for evidence of absolute rest. It has been declared that "organic life is a result of motion;" certain it is that motion is a condition of life. It appears in the endless manifestations of beauty and utility, in the world of living creatures of which ourselves are a part. The heavens are more beautiful when clouds are drifting, and the motions of animals give a charm to a landscape which disappears in the solitude of a desert. Stillness to the eye, like silence to the ear, becomes at last painfully oppressive. We scarcely realize, perhaps we seldom consider, how much of the joy and value of existence depends upon the movement of beings, and the marvelous perfection of the means by which it is effected. Walking, swimming, and flying, are the means by which we traverse the three great highways of Nature—the land, the water, and the air. If we change our position, it is in one or other of these. There is no more fascinating chapter of science than this. The mere fact of animal locomotion is felt to be an expression of beneficence, and of adaptation of means to ends which surpasses human ingenuity.
What laws of motion are revealed, what principles of mechanics are brought into action, when animals walk, swim, or fly, has been discussed by many writers, but by none in a more able or interesting manner than by Dr. Pettigrew, who, in a volume soon to appear in the International Series, has given the results of a long course of observations and study upon the subject.
The three modes of progression, apparently so unlike, are nevertheless essentially the same. The limbs of the quadruped, the wings of the bird, and the fins of the fish, are built upon the same general plan of structure, and are applied fundamentally to the same uses. They are traveling surfaces, and their wide range of modification is in direct relation to the media in which they are used. The one treads the solid ground, another the water, and another the yielding and elastic air. "But walking merges into swimming, and swimming into flying, by insensible gradations; and these modifications result from the fact that the earth affords a greater amount of support than the water, and the water than the air."
Most terrestrial quadrupeds can swim as well as walk, and some can fly. Many marine animals both walk and swim, and birds and insects walk, fly, and swim, indiscriminately. It is not surprising, therefore, that, between the typical foot, wing, and fin, innumerable modifications in structure and form occur; indeed, so graduated are they that it may be difficult to determine where one form ends and another begins.
In Fig. 1 we have several illustrations of the traveling surfaces of
animals. The small feet of the quadruped, the webbed feet of the ornithorhynchus, the otter, the walrus, and the triton, indicate with certainty the media to which they are adapted, and perhaps in nothing is modification of structure and form to habits more apparent than in the locomotive appendages of animals. The webbed structure between the toes of animals which live partially on the land, and of some terrestrial animals, as the water-dog, is wonderfully significant.
The wing of the penguin, Fig. 2, is scarcely more than a flipper, and the same is true of the auk.
Sir John Lubbock describes a species of insect whose wings are used as fins only. "Every variety of motion peculiar to land, air, and water-navigating animals, as such, is imitated by others which take to the elements in question, secondarily or at intervals." It is probably true, however, that no animal which lives indiscriminately in two media attains the highest development for traveling in or upon either. In such cases the maximum speed is not attained. Those animals, says the author, which swim the best, walk, as a rule, with difficulty, and vice versa, as the movements of the auk and the seal, in and out of water, amply testify. It is evident that all the supposed gaps between typical forms for locomotion are bridged by forms intermediate, and the author's position is fully sustained, that walking, swimming, and flying, are essentially the same.
Before entering upon the question of the movement and functions of specialized organs for locomotion, attention is invited to the interesting statement that, however wonderful and beautiful, in its way, the bony skeleton maybe, it is after all only an adjunct to locomotion, and of motion in general—that all the really essential movements of an animal occur in the soft parts. "The osseous system is therefore to be regarded as secondary in importance to the muscular, of which it may be considered a differentiation. Instead of regarding the muscles as adapted to the bones, the bones ought to be regarded as adapted to the muscles. Bones have no power either of originating or perpetuating motion. This begins and terminates in the muscles."
The bones are the passive organs of locomotion, in the movement of which muscular force is expended. In land animals, as a rule, the bones are harder and more elastic than in aquatic species. The cartilaginous and spongy bones of many fishes would be ill suited to bear the strains and shocks of terrestrial progression.
The velocity with which a limb may be moved will depend upon the acuteness of the angles of its several bones. Hence the fleetness of many animals, in which the angles formed by the bones are acute. This is well shown in the skeleton of a deer, of which Fig. 3 is an excellent illustration. Here we have not only the sharp angles, but lever-like adjustment of the several bones.
From these arises the power possessed by many animals to bound or leap enormous distances. The kangaroo has been known to leap twenty feet. The jerboa, when pursued, will pass over nine feet at a bound, and repeats so rapidly that a swift horse can scarcely overtake it. The greyhound and the hare will pass over sixteen feet at a stride. Animals of great weight and moderate speed have nearly straight limbs. Those of the deer are more angular than those of a horse, and those in the wing of a bird more angular than those of the fleetest quadruped.
The forms of joints which predominate in the animal kingdom are the hinge and the ball and socket. The latter gives to the extremities their extraordinary range of motion, and a power of rotation so indispensable, as we will see, to the effectiveness of all the organs of locomotion.
It has been shown that a spiral configuration occurs in the bones and joints of the wing of the bat and the bird, and in the extremities of most quadrupeds. "The bones of animals are, as a rule, twisted levers, and act after the manner of screws." Thus it is that their traveling surfaces in progression may be turned at almost any angle, getting from the resisting media in which they move as much propelling power as possible, with a minimum of slip or waste.
It is because the traveling surfaces of animals "are screws structurally and functionally" that they can seize and let go the fulcra on which they act; particularly is this the case in the water and in the air, the form and movement being such that the greatest results are secured with the least expenditure of force. The muscles cover the bones, in layers or strata, and run longitudinally with them, and with each other, but also at every degree of obliquity. The spiral structure and movement of bones of animals have been carefully analyzed by Dr. Pettigrew, and some of his conclusions are illustrated in Fig. 4.
The voluntary muscles of the wing, he finds, are upon the same pattern as are those of the involuntary muscles of the heart. He compared the bones removed from the forelimb of a quadruped or bird, with a cast obtained from the cavity of a hollow muscle, the left ventricle of the heart of a mammal, and found that the bones and the cast are twisted upon themselves, and form elegant screws, the threads of which run in the same direction.
The movement of the limbs of a quadruped is in curves, which, continued, form a figure-of-8, or a series of them; a fact in progression first pointed out by Dr. Pettigrew. Quadrupeds, says the author, walk; fishes swim; insects, bats, and birds, fly by figure-of-8 movements; and, in human locomotion, the same phenomenon is observed. The diagram, Fig. 5, shows the curved track made by man walking. The accuracy of this is easily verified by observation. As the limbs swing forward, they move in the arc of an ellipse; that is, in a slight curve outward, and with the arms form the double curves, as shown in the figure. In the movement of the horse, walking or trotting, the same phenomenon appears, as the figure shows.
The wings of birds, bats, and insects, describe similar curves. They are produced by the rotation of the wing, as it rises and falls, so that it twists, screw-like, on its long axis, one-half of the figure being formed in the ascent, the other in the descent of the wing.
The double curves or figure-of-8 lines which thus occur are not mere coincidences, nor in any sense accidental, but the expression of a law of movement of vertebrated animals, and, from a most extended series of observations, Dr. Pettigrew concludes:
"That quadrupeds walk, and fishes swim, and insects, bats, and birds fly, by figure-of-8 movements.
"That the flipper of the sea-bear, the swimming wing of the penguin, and the wing of the insect, bat, and bird, are screws structurally, and resemble the blade of an ordinary screw-propeller.
"That those organs are screws functionally, from their twisting and untwisting, and from their rotating in the direction of their length, when they are made to oscillate.
"That they have a reciprocating action, and reverse their planes more or less completely at every stroke.
"That the wing describes a figure-of-8 track in space when the flying animal is artificially fixed.
"That the wing, when the flying animal is progressing at a high rate of speed in an horizontal direction, describes a looped and then a wave track, from the fact that the figure-of-8 is gradually opened out and unraveled as the animal advances."
He constructed artificial fish-tails, fins, flippers, and wings—flexible and elastic—slightly twisted upon themselves, and applied them respectively to the water and air by a sculling or figure-of-8 motion. The curved surfaces and movements peculiar to the living organs were reproduced. The purely mechanical movement shown in this application of traveling structures to their environment scarcely admits of doubt.
Man is enabled to travel in two of the three great highways of Nature. He can progress upon the land, swim in the water, but fly he cannot; nor has he yet invented a means by which flying is possible. By his applications of natural laws he has "outraced the quadruped on the land and the fish in the sea," and the conclusion from the analogy and nature of things is, that the "tramways of the air will yet be traversed by man's ingenuity."
A balloon floats, it does not fly. It floats because it is lighter than the air; a bird is enabled to fly because it is heavier than the air, and weight is an important element in all, but especially in aërial and land locomotion. It is that by which the extremities of animals seize and hold their position in the media in which they move. If a man were no heavier than the air, the movement of his limbs would avail him nothing. The earth is his fulcrum, as the air is that of the bird, and water that of the fish. Progression, therefore, implies gravity and the power of resistance, which gravity affords. A body which floats is carried along with the media in which it is; having lost its weight, it has lost its power of self-control. A man who cannot swim is at the mercy of the slightest current or wave, if in depth at which the lifting power of the water makes his foothold insecure.
A man standing still commences to progress by throwing his body slightly forward. Some momentum is thus obtained, the limb being simultaneously advanced. "The throwing forward of the body may be said to inaugurate the movement of walking." The same occurs with a horse, but, if attached to a load, great impetus is attained by the body before either of the limbs is lifted. Momentum thus relieves muscular strain in the limbs and economizes force. How completely this principle is applied in swimming and flying will be presently noticed.
In ordinary walking of man or quadruped, the limbs swing forward without muscular effort. According to Prof. Weber, they swing by the force of gravity as a pendulum, and obey the same laws. If suspended they oscillate freely, and gravity brings them to a position of rest. How much the muscles are saved from exhaustion in ordinary locomotion, by gravity, becomes obvious when we attempt to overcome it by climbing or leaping. The foot being upon the ground, the limb rotates upon it as an axis, carrying the body forward and slightly elevating it, but the elevation is in the arc of a circle, and when the other foot reaches the ground the body is slightly lowered. Thus in progression the trunk continually rises and falls; it really undulates along a given line.
But other motions than those referred to are developed in the act of walking. The movements of the arms and feet are complementary; the right foot and left arm advance together, and vice versa. This begets a diagonal movement which produces oscillation or twisting of the trunk, which is excessive in awkward walkers. To repress this oscillatory swing is indispensable, if great velocity is to be attained. Trained runners flex their arms and hold them steadily at their greatest speed, and every school-boy does the same instinctively, without considering why the act is important. The swiftest-running birds have small wings; those of the ostrich are scarcely more than rudimental.
The diagonal twist or movement referred to is expressed in a spiral wave of motion which traverses the trunk in the direction of its length. This motion is obvious in fishes in the act of swimming. It is a resultant of motions, which are in all vertebrates essentially the same. In the walking of a cat or panther, this wave of curvature is continuous along the spine. It is really a lateral undulation. We will follow this curved motion in the swimming of aquatic animals. The illustration of Borelli (Fig. 8), shows only a single curve of the body.
Prof. Owen, commenting on this figure, says the tail of the animal moving from a to i causes the centre of gravity to move forward, and turns the head of the fish in the direction c to h. This implies but a single curve of the body, whereas there are two, the one complementary of the other, as is shown to be the case by Dr. Pettigrew. He says: "Observation and experiment have convinced me that when a fish swims it never throws its body into a single curve, as represented in Borelli's figure, but always in a double or figure-of-8 curve, as shown in Fig. 9.
In swimming, the body of the fish describes a waved track along a medial line. The two curves of the body act as fulcra for each other, as occurs in the movement of a snake, by its usual lateral undulations when swimming. In fishes and swimming reptiles of great length, there may occur more than two curves, as four, but never three, each curve having its complementary one. "The fish lashes its tail from side to side, and a figure-of-8 track is formed by the movement," both strokes propelling, but with unequal power during the course of the stroke. There is a feature in this movement which is equally obvious in the movement of the wings of birds in the air, that is, a current is formed, as the tail is carried to one side, against which the return-stroke is given; thus, as the author observes, "the tail may be said to work without slip, and to produce the precise kind of currents which afford it the greatest leverage."
And this is true, whether the swing of the tail in swimming is sideways, as with the salmon, or vertically, as occurs with the whale or the porpoise. The fins act as balancers of the body, but the equipoise is impaired if they are injured; and the removal of the tail, as Owen observes, destroys the power of locomotion.
The specific gravity of aquatic animals is nearly that of the water they inhabit, or is made so in many cases by the gaseous contents of their air-bladders. Nevertheless, momentum is an important element in swimming. It originates in the movement of the fins and tail, and not from throwing the body forward, as occurs in the initial movement of walking. The momentum and velocity attained by some fishes are astonishingly great. A blow from the head of a sperm-whale may endanger a strong ship, and the sword of a sword-fish has been driven through the oak-planks of a vessel more than twelve inches in thickness.
When the flying-fish rises in the air, it is by the momentum it attains in the water by the lashing of its fins and tail. Fairly in the air, its wings give it support, and, in the opinion of Dr. Pettigrew, act as true pinions within certain limits, but are too small to sustain the creature indefinitely.
The transition from swimming to progression m the air is natural and easy. The method by which the flying-fish rises from the water is similar to that of the albatross, that prince of flying-birds, and, indeed, to perhaps all other birds, when in the act of taking flight upon the water. Momentum is obtained by rushing forward with both feet and wings. The albatross frequently goes in this way many rods before it is fairly launched upon the air. Then, with powerful strokes, it rises above the waves. Its expenditure of force is chiefly in rising, when, without further effort, except to screw and unscrew its pinions upon the wind, it floats facing the gale. For more than an hour it will sail with wings apparently motionless, and it seems most at rest when the winds are highest. In this case it is sustained by the momentum it attained, and the wonderful kite-like position and adjustment of its wings. But, it manifestly could not maintain its position in this way, if moving before the wind, or in a perfectly calm atmosphere. The wings must then be called into play to afford lifting as well as propelling power. The momentum must be supplied.
Birds rise from the ground most readily facing the wind, but usually run or leap, and the wings, by vigorous strokes, continue the impulse secured. With the first down-stroke of the wings the body is lifted, and some velocity attained; when the wings rise, the body falls somewhat, but is at the same time advancing. This rise and fall of the body in flying continue, and the body, in progressing, undulates above and below a given line. In the flight of birds with large wings and slow stroke, it is easily observed. The illustration, Fig. 11, shows the positions of the wings of the gull in the course of a complete oscillation:
When the down-stroke is completed the bird has been raised, but is lowered again when the wings have attained their maximum elevation. Thus it is seen how directly gravity aids in flight. The body is the weight; the wings are long levers attached to it at one end; the air is the fulcrum. Fig. 12 shows the undulatory track of a flying-bird:
The instant the descent of the wing begins, the body moves upward and forward; but it is shown by the author that some forward motion results also from the up-stroke. Certain it is that the upward movement must not counteract the other. There is no provision for
waste of energy. The form of the upper surface of the wing is convex, the under surface being concave. The value of this will be apparent, as the Duke of Argyle suggests, if we attempt to move the concave or convex side of an umbrella against the wind; one side holds the air, the other discharges it. The wing of the albatross shows how completely the feathers are adjusted, on the upper side, to avoid any* hold upon the air.
This arrangement, with the flexibility and screw-like motion of the wings of the gull, shown in Fig. 14, explains the exceedingly small resistance experienced in the upward movement, and also the forward impetus which it communicates.
It is in the down-stroke, or, as Dr. Pettigrew insists, in the beginning of the down-stroke, that force is chiefly expended. This movement is essentially a muscular act, and by this force alone no bird could sustain long-continued flight. The lark, whose flight is upward, soon descends to the earth. It lifts itself against gravity, simply by expenditure of vital force. But, the moment forward motion is attained, other forces relieve the strain upon the pinions, and their inclined surfaces convert gravity into a propelling power. It is obvious, however, that flight is attended with considerable muscular exertion. Migrating birds alight in unsuitable positions for rest, but the swallow will fly 1,000 miles in a single journey, and the condor attains an altitude of six miles.
The heron will strike the air 60 times in a minute, which, with 60 up-strokes, gives 120 movements, and this is continued through long
flights; and the same is true of many ducks and land-birds which strike the air with extreme and apparently exhausting rapidity. So swift are the motions of the wings of the humming-bird that they produce only a blurred spot before the eye.
That wings act as true kites, when in motion, is a familiar observation, but they are kites which continually change their surfaces and position in respect to the air, which artificial kites do not. An important difference between them is the rigidity of the one and the wonderful flexibility of the other. The kite rises as its oblique surface is pressed against the elastic air; the same is true of the wing. But the wing rotates, so that the proper obliquity of its parts is continually maintained; it rolls on and off the wind; it rotates not only throughout its length, but in each of its parts. The quills, which are convex, rotate, and present closed or oblique surfaces, which hold or discharge the air.
We have space for but one more of the numerous diagrams and figures which Dr. Pettigrew has prepared, illustrating the phenomena of flight. Fig. 16 is of the extended wing of a partridge, seen from beneath and from behind.
The wings, when flexed and extended in flight, assume curved surfaces, which change at each instant and carve the air, as does the tail of the fish the water, into complex wave-lines; and such is the structure of the wing that these results are inevitable when it is put in motion. "The muscles, bones, ligaments, and feathers, are so adjusted with reference to each other that, if the wing is moved at all, it must move in the proper direction." The bird no more thinks of its motions in flying than we do of ours in walking; the actions are mechanical and instinctive. An opinion long prevailed that heated air in the hollow bones of the bird gave it buoyancy and power of flight. This is shown to be a fallacy.
Three principal forces are expressed in flight: muscular and elastic force of the wing, weight of the body, and recoil of the air. By the mechanical structure of the wing, these forces act, react, and combine. Thus birds traverse the aerial ocean; the wild-goose drives his train along invisible tracks; the albatross and petrel are at home in the gale, undisturbed by its clamor; and the condor, with easy motion, treads with his pinions the elastic floors of the upper air.
The more rapid the strokes of the wing the greater the achievement. Not so with one of the most ingenious of human contrivances for progression. The screw, if urged beyond a certain velocity, holds and carries with it the water, and its propelling power is lost. It wants the flexibility of the wing and the fin—the adaptation is not complete.
- "Animal Locomotion; or, Walking, Swimming, and Flying." By J. Bell Pettigrew, F. R. S. International Scientific Series. New York: D. Appleton & Co.