PROGRESSION ON AND IN THE WATER.
If we direct our attention to the water, we encounter a medium less dense than the earth, and considerably more dense than the air. As this element, in virtue of its fluidity, yields readily to external pressure, it follows that a certain relation exists between it and the shape, size, and weight of the animal progressing along or through it. Those animals make the greatest headway which are of the same specific gravity, or are a little heavier, and furnished with extensive surfaces, which, by a dexterous tilting or twisting (for the one implies the other), or by a sudden contraction and expansion, they apply wholly or in part to obtain the maximum of resistance in the one direction, and the minimum of displacement in the other. The change of shape, and the peculiar movements of the swimming surfaces, are rendered necessary by the fact, first pointed out by Sir Isaac Newton, that bodies or animals moving in water and likewise in air experience a sensible resistance, which is greater or less in proportion to the density and tenacity of the fluid and the figure, superficies, and velocity of the animal.
To obtain the degree of resistance and non-resistance necessary for progression in water, Nature, never at fault, has devised some highly ingenious expedients,—the Syringograde animals advancing by alternately sucking up and ejecting the water in which they are immersed—the Medusæ by a rhythmical contraction and dilatation of their mushroom-shaped disk—the Rotifera or wheel-animalcules by a vibratile action of their cilia, which, according to the late Professor Quekett, twist upon their pedicles so as alternately to increase and diminish the extent of surface presented to the water, as happens in the feathering of an oar. A very similar plan is adopted by the Pteropoda, found in countless multitudes in the northern seas, which, according to Eschricht, use the wing-like structures situated near the head after the manner of a double paddle, resembling in its general features that at present in use among the Greenlanders. The characteristic movement, however, and that adopted in by far the greater number of instances, is that commonly seen in the fish (figs. 29 and 30).
Fig. 29.—Skeleton of the Perch (Perca fluviatilis). Shows the jointed nature of the vertebral column, and the facilities afforded for lateral motion, particularly in the tail (d), dorsal (e, f), ventral (b, c), and pectoral (a), fins, which are principally engaged in swimming. The extent of the travelling surfaces required for water greatly exceed those required for land. Compare the tail and fins of the present figure with the feet of the ox, fig. [18], p. 37.—(After Dallas.)
Fig. 30.—The Salmon (Salmo salar) swimming leisurely. The body, it will be observed, is bent in two curves, one occurring towards the head, the other towards the tail. The shape of the salmon is admirably adapted for cleaving the water.—Original.
This, my readers are aware, consists of a lashing, curvi-linear, or flail-like movement of the broadly expanded tail, which oscillates from side to side of the body, in some instances with immense speed and power. The muscles in the fish, as has been explained, are for this purpose arranged along the spinal column, and constitute the bulk of the animal, it being a law that when the extremities are wanting, as in the water-snake, or rudimentary, as in the fish, lepidosiren,[41] proteus, and axolotl, the muscles of the trunk are largely developed. In such cases the onus of locomotion falls chiefly, if not entirely, upon the tail and lower portion of the body. The operation of this law is well seen in the metamorphosis of the tadpole, the muscles of the trunk and tail becoming modified, and the tail itself disappearing as the limbs of the perfect frog are developed. The same law prevails in certain instances where the anterior extremities are comparatively perfect, but too small for swimming purposes, as in the whale, porpoise, dugong, and manatee, and where both anterior and posterior extremities are present but dwarfed, as in the crocodile, triton, and salamander. The whale, porpoise, dugong, and manatee employ their anterior extremities in balancing and turning, the great organ of locomotion being the tail. The same may be said of the crocodile, triton, and salamander, all of which use their extremities in quite a subordinate capacity as compared with the tail. The peculiar movements of the trunk and tail evoked in swimming are seen to most advantage in the fish, and may now be briefly described.
Swimming of the Fish, Whale, Porpoise, etc.—According to Borelli,[42] and all who have written since his time, the fish in swimming causes its tail to vibrate on either side of a given line, very much as a rudder may be made to oscillate by moving its tiller. The line referred to corresponds to the axis of the fish when it is at rest and when its body is straight, and to the path pursued by the fish when it is swimming. It consequently represents the axis of the fish and the axis of motion. According to this theory the tail, when flexed or curved to make what is termed the back or non-effective stroke, is forced away from the imaginary line, its curved, concave, or biting surface being directed outwards. When, on the other hand, the tail is extended to make what is termed the effective or forward stroke, it is urged towards the imaginary line, its convex or non-biting surface being directed inwards (fig. 31).
Fig. 31.—Swimming of the Fish.—(After Borelli.)
When the tail strikes in the direction a i, the head of the fish is said to travel in the direction c h. When the tail strikes in the direction g e, the head is said to travel in the direction c b; these movements, when the tail is urged with sufficient velocity, causing the body of the fish to move in the line d c f. The explanation is apparently a satisfactory one; but a careful analysis of the swimming of the living fish induces me to believe it is incorrect. According to this, the commonly received view, the tail would experience a greater degree of resistance during the back stroke, i.e. when it is flexed and carried away from the axis of motion (d c f) than it would during the forward stroke, or when it is extended and carried towards the axis of motion. This follows, because the concave surface of the tail is applied to the water during what is termed the back or non-effective stroke, and the convex surface during what is termed the forward or effective stroke. This is just the opposite of what actually happens, and led Sir John Lubbock to declare that there was a period in which the action of the tail dragged the fish backwards, which, of course, is erroneous. There is this further difficulty. When the tail of the fish is urged in the direction g e, the head does not move in the direction c b as stated, but in the direction c h, the body of the fish describing the arc of a circle, a c h. This is a matter of observation. If a fish when resting suddenly forces its tail to one side and curves its body, the fish describes a curve in the water corresponding to that described by the body. If the concavity of the curve formed by the body is directed to the right side, the fish swims in a curve towards that side. To this there is no exception, as any one may readily satisfy himself, by watching the movements of gold fish in a vase. Observation and experiment have convinced me that when a fish swims it never throws its body into a single curve, as represented at fig. 31, p. 67, but always into a double or figure-of-8 curve, as shown at fig. 32.[43]
Fig. 32.—Swimming of the Sturgeon. From Nature. Compare with figs. [18] and [19], pp. 37 and 39; fig. [23], p. 43; and figs. [64] to 73, pp. 139, 141, and 144.—Original.
The double curve is necessary to enable the fish to present a convex or non-biting surface (c) to the water during flexion (the back stroke of authors), when the tail is being forced away from the axis of motion (a b), and a concave or biting surface (s) during extension (the forward or effective stroke of authors), when the tail is being forced with increased energy towards the axis of motion (a b); the resistance occasioned by a concave surface, when compared with a convex one, being in the ratio of two to one. The double or complementary curve into which the fish forces its body when swimming, is necessary to correct the tendency which the head of the fish has to move in the same direction, or to the same side as that towards which the tail curves. In swimming, the body of the fish describes a waved track, but this can only be done when the head and tail travel in opposite directions, and on opposite sides of a given line, as represented at fig. 32. The anterior and posterior portions of the fish alternately occupy the positions indicated at d c and w v; the fish oscillating on either side of a given line, and gliding along by a sinuous or wave movement.
I have represented the body of the fish as forced into two curves when swimming, as there are never less than two. These I designate the cephalic (d) and caudal (c) curves, from their respective positions. In the long-bodied fishes, such as the eels, the number of the curves is increased, but in every case the curves occur in pairs, and are complementary. The cephalic and caudal curves not only complement each other, but they act as fulcra for each other, the cephalic curve, with the water seized by it, forming the point d’appui for the caudal one, and vice versâ. The fish in swimming lashes its tail from side to side, precisely as an oar is lashed from side to side in sculling. It therefore describes a figure-of-8 track in the water (e f g h i j k l of fig. 32). During each sweep or lateral movement the tail is both extended and flexed. It is extended and its curve reduced when it approaches the line a b of fig. 32, and flexed, and a new curve formed, when it recedes from the line in question. The tail is effective as a propeller both during flexion and extension, so that, strictly speaking, the tail has no back or non-effective stroke. The terms effective and non-effective employed by authors are applicable only in a comparative and restricted sense; the tail always operating, but being a less effective propeller, when in the act of being flexed or curved, than when in the act of being extended or straightened. By always directing the concavity of the tail (s and t) towards the axis of motion (a b) during extension, and its convexity (c and v) away from the axis of motion (a b) during flexion, the fish exerts a maximum of propelling power with a minimum of slip. In extension of the tail the caudal curve (s) is reduced as the tail travels towards the line a b. In flexion a new curve (v) is formed as the tail travels from the line a b. While the tail travels from s in the direction t, the head travels from d in the direction w. There is therefore a period, momentary it must be, when both the cephalic and caudal curves are reduced, and the body of the fish is straight, and free to advance without impediment. The different degrees of resistance experienced by the tail in describing its figure-of-8 movements, are represented by the different-sized curves e f, g h, i j, and k l of fig. [32], p. 68. The curves e f indicate the resistance experienced by the tail during flexion, when it is being carried away from and to the right of the line a b. The curves g h indicate the resistance experienced by the tail when it is extended and carried towards the line a b. This constitutes a half vibration or oscillation of the tail. The curves i j indicate the resistance experienced by the tail when it is a second time flexed and carried away from and to the left of the line a b. The curves k l indicate the resistance experienced by the tail when it is a second time extended and carried towards the line a b. This constitutes a complete vibration. These movements are repeated in rapid succession so long as the fish continues to swim forwards. They are only varied when the fish wishes to turn round, in which case the tail gives single strokes either to the right or left, according as it wishes to go to the right or left side respectively. The resistance experienced by the tail when in the positions indicated by e f and i j is diminished by the tail being slightly compressed, by its being moved more slowly, and by the fish rotating on its long axis so as to present the tail obliquely to the water. The resistance experienced by the tail when in the positions indicated by g h, k l, is increased by the tail being divaricated, by its being moved with increased energy, and by the fish re-rotating on its long axis, so as to present the flat of the tail to the water. The movements of the tail are slowed when the tail is carried away from the line a b, and quickened when the tail is forced towards it. Nor is this all. When the tail is moved slowly away from the line a b, it draws a current after it which, being met by the tail when it is urged with increased velocity towards the line a b, enormously increases the hold which the tail takes of the water, and consequently its propelling power. The tail may be said to work without slip, and to produce the precise kind of currents which afford it the greatest leverage. In this respect the tail of the fish is infinitely superior as a propelling organ to any form of screw yet devised. The screw at present employed in navigation ceases to be effective when propelled beyond a given speed. The screw formed by the tail of the fish, in virtue of its reciprocating action, and the manner in which it alternately eludes and seizes the water, becomes more effective in proportion to the rapidity with which it is made to vibrate. The remarks now made of the tail and the water are equally apropos of the wing and the air. The tail and the wing act on a common principle. A certain analogy may therefore be traced between the water and air as media, and between the tail and extremities as instruments of locomotion. From this it follows that the water and air are acted upon by curves or wave-pressure emanating in the one instance from the tail of the fish, and in the other from the wing of the bird, the reciprocating and opposite curves into which the tail and wing are thrown in swimming and flying constituting mobile helices or screws, which, during their action, produce the precise kind and degree of pressure adapted to fluid media, and to which they respond with the greatest readiness. The whole body of the fish is thrown into action in swimming; but as the tail and lower half of the trunk are more free to move than the head and upper half, which are more rigid, and because the tendons of many of the trunk-muscles are inserted into the tail, the oscillation is greatest in the direction of the latter. The muscular movements travel in spiral waves from before backwards; and the waves of force react upon the water, and cause the fish to glide forwards in a series of curves. Since the head and tail, as has been stated, always travel in opposite directions, and the fish is constantly alternating or changing sides, it in reality describes a waved track. These remarks may be readily verified by a reference to the swimming of the sturgeon, whose movements are unusually deliberate and slow. The number of curves into which the body of the fish is thrown in swimming is increased in the long-bodied fishes, as the eels, and decreased in those whose bodies are short or are comparatively devoid of flexibility. In proportion as the curves into which the body is thrown in swimming are diminished, the degree of rotation at the tail or in the fins is augmented, some fishes, as the mackerel, using the tail very much after the manner of a screw in a steam-ship. The fish may thus be said to drill the water in two directions, viz. from behind forwards by a twisting or screwing of the body on its long axis, and from side to side by causing its anterior and posterior portions to assume opposite curves. The pectoral and other fins are also thrown into curves when in action, the movement, as in the body itself, travelling in spiral waves; and it is worthy of remark that the wing of the insect, bat, and bird obeys similar impulses, the pinion, as I shall show presently, being essentially a spiral organ.
The twisting of the pectoral fins is well seen in the common perch (Perca fluviatilis), and still better in the 15-spined Stickleback (Gasterosteus spinosus), which latter frequently progresses by their aid alone.[44] In the stickleback, the pectoral fins are so delicate, and are plied with such vigour, that the eye is apt to overlook them, particularly when in motion. The action of the fins can be reversed at pleasure, so that it is by no means an unusual thing to see the stickleback progressing tail first. The fins are rotated or twisted, and their free margins lashed about by spiral movements which closely resemble those by which the wings of insects are propelled.[45] The rotating of the fish upon its long axis is seen to advantage in the shark and sturgeon, the former of which requires to turn on its side before it can seize its prey,—and likewise in the pipefish, whose motions are unwontedly sluggish. The twisting of the tail is occasionally well marked in the swimming of the salamander. In those remarkable mammals, the whale,[46] porpoise, manatee, and dugong (figs. 33, 34, and [35]), the movements are strictly analogous to those of the fish, the only difference being that the tail acts from above downwards or vertically, instead of from side to side or laterally. The anterior extremities, which in those animals are comparatively perfect, are rotated on their long axes, and applied obliquely and non-obliquely to the water, to assist in balancing and turning. Natation is performed almost exclusively by the tail and lower half of the trunk, the tail of the whale exerting prodigious power.
Fig. 33.—The Porpoise (Phocœna communis). Here the tail is principally engaged in swimming, the anterior extremities being rudimentary, and resembling the pectoral fins of fishes. Compare with fig. [30], p. 65.—Original.
Fig. 34.—The Manatee (Manatus Americanus). In this the anterior extremities are more developed than in the porpoise, but still the tail is the great organ of natation. Compare with fig. 33, p. 73, and with fig. [30], p. 65. The shape of the manatee and porpoise is essentially that of the fish.—Original.
It is otherwise with the Rays, where the hands are principally concerned in progression, these flapping about in the water very much as the wings of a bird flap about in the air. In the beaver, the tail is flattened from above downwards, as in the foregoing mammals, but in swimming it is made to act upon the water laterally as in the fish. The tail of the bird, which is also compressed from above downwards, can be twisted obliquely, and when in this position may be made to perform the office of a rudder.
Fig. 35.—Skeleton of the Dugong. In this curious mammal the anterior extremities are more developed than in the manatee and porpoise, and resemble those found in the seal, sea-bear, and walrus. They are useful in balancing and turning, the tail being the effective instrument of propulsion. The vertebral column closely resembles that of the fish, and allows the tail to be lashed freely about in a vertical direction. Compare with fig. [29], p. 65.—(After Dallas.)
Swimming of the Seal, Sea-Bear, and Walrus.—In the seal, the anterior and posterior extremities are more perfect than in the whale, porpoise, dugong, and manatee; the general form, however, and mode of progression (if the fact of its occasionally swimming on its back be taken into account), is essentially fish-like.
Fig. 36.—The Seal (Phoca fœtida, Müll.), adapted principally for water. The extremities are larger than in the porpoise and manatee. Compare with figs. [33] and 34, p. 73.—Original.
A peculiarity is met with in the swimming of the seal, to which I think it proper to direct attention. When the lower portion of the body and posterior extremities of these creatures are flexed and tilted, as happens during the back and least effective stroke, the naturally expanded feet are more or less completely closed or pressed together, in order to diminish the extent of surface presented to the water, and, as a consequence, to reduce the resistance produced. The feet are opened to the utmost during extension, when the more effective stroke is given, in which case they present their maximum of surface. They form powerful propellers, both during flexion and extension.
The swimming apparatus of the seal is therefore more highly differentiated than that of the whale, porpoise, dugong, and manatee; the natatory tail in these animals being, from its peculiar structure, incapable of lateral compression.[47] It would appear that the swimming appliances of the seals (where the feet open and close as in swimming-birds) are to those of the sea-mammals generally, what the feathers of the bird’s wing (these also open and close in flight) are to the continuous membrane forming the wing of the insect and bat.
The anterior extremities or flippers of the seal are not engaged in swimming, but only in balancing and in changing position. When so employed the fore feet open and close, though not to the same extent as the hind ones; the resistance and non-resistance necessary being secured by a partial rotation and tilting of the flippers. By this twisting and untwisting, the narrow edges and broader portions of the flippers are applied to the water alternately. The rotating and tilting of the anterior and posterior extremities, and the opening and closing of the hands and feet in the balancing and swimming of the seal, form a series of strictly progressive and very graceful movements. They are, however, performed so rapidly, and glide into each other so perfectly, as to render an analysis of them exceedingly difficult.
In the Sea-Bear (Otaria jubata) the anterior extremities attain sufficient magnitude and power to enable the animal to progress by their aid alone; the feet and the lower portions of the body being moved only sufficiently to maintain, correct, or alter the course pursued (fig. [73]). The anterior extremities are flattened out, and greatly resemble wings, particularly those of the penguin and auk, which are rudimentary in character. Thus they have a thick and comparatively stiff anterior margin; and a thin, flexible, and more or less elastic posterior margin. They are screw structures, and when elevated and depressed in the water, twist and untwist, screw-fashion, precisely as wings do, or the tails of the fish, whale, dugong, and manatee.
Fig. 37.—The Sea-Bear (Otaria jubata), adapted principally for swimming and diving. It also walks with tolerable facility. Its extremities are larger than those of the seal, and its movements, both in and out of the water, more varied.—Original.
This remarkable creature, which I have repeatedly watched at the Zoological Gardens[48] (London), appears to fly in the water, the universal joints by which the arms are attached to the shoulders enabling it, by partially rotating and twisting them, to present the palms or flat of the hands to the water the one instant, and the edge or narrow parts the next. In swimming, the anterior or thick margins of the flippers are directed downwards, and similar remarks are to be made of the anterior extremities of the walrus, great auk, and turtle.[49]
The flippers are advanced alternately; and the twisting, screw-like movement which they exhibit in action, and which I have carefully noted on several occasions, bears considerable resemblance to the motions witnessed in the pectoral fins of fishes. It may be remarked that the twisting or spiral movements of the anterior extremities are calculated to utilize the water to the utmost—the gradual but rapid operation of the helix enabling the animal to lay hold of the water and disentangle itself with astonishing facility, and with the minimum expenditure of power. In fact, the insinuating motion of the screw is the only one which can contend successfully with the liquid element; and it appears to me that this remark holds even more true of the air. It also applies within certain limits, as has been explained, to the land. The otaria or sea-bear swims, or rather flies, under the water with remarkable address and with apparently equal ease in an upward, downward, and horizontal direction, by muscular efforts alone—an observation which may likewise be made regarding a great number of fishes, since the swimming-bladder or float is in many entirely absent.[50] Compare with figs. [33], 34, 35, and [36], pp. 73 and 74. The walrus, a living specimen of which I had an opportunity of frequently examining, is nearly allied to the seal and sea-bear, but differs from both as regards its manner of swimming. The natation of this rare and singularly interesting animal, as I have taken great pains to satisfy myself, is effected by a mixed movement—the anterior and posterior extremities participating in nearly an equal degree. The anterior extremities or flippers of the walrus, morphologically resemble those of the seal, but physiologically those of the sea-bear; while the posterior extremities possess many of the peculiarities of the hind legs of the sea-bear, but display the movements peculiar to those of the seal. In other words, the anterior extremities or flippers of the walrus are moved alternately, and reciprocate, as in the sea-bear; whereas the posterior extremities are lashed from side to side by a twisting, curvilinear motion, precisely as in the seal. The walrus may therefore, as far as the physiology of its extremities is concerned, very properly be regarded as holding an intermediate position between the seals on the one hand, and the sea-bears or sea-lions on the other.
Swimming of Man.—The swimming of man is artificial in its nature, and consequently does not, strictly speaking, fall within the scope of the present work. I refer to it principally with a view to showing that it resembles in its general features the swimming of animals.
The human body is lighter than the water, a fact of considerable practical importance, as showing that each has in himself that which will prevent his being drowned, if he will only breathe naturally, and desist from struggling.
The catastrophe of drowning is usually referrible to nervous agitation, and to spasmodic and ill-directed efforts in the extremities. All swimmers have a vivid recollection of the great difficulty experienced in keeping themselves afloat, when they first resorted to aquatic exercises and amusements. In especial they remember the short, vigorous, but flurried, misdirected, and consequently futile strokes which, instead of enabling them to skim the surface, conducted them inevitably to the bottom. Indelibly impressed too are the ineffectual attempts at respiration, the gasping and puffing and the swallowing of water, inadvertently gulped instead of air.
In order to swim well, the operator must be perfectly calm. He must, moreover, know how to apply his extremities to the water with a view to propulsion. As already stated, the body will float if left to itself; the support obtained is, however, greatly increased by projecting it along the surface of the water. This, as all swimmers are aware, may be proved by experiment. It is the same principle which prevents a thin flat stone from sinking when projected with force against the surface of water. A precisely similar result is obtained if the body be placed slantingly in a strong current, and the hands made to grasp a stone or branch. In this case the body is raised to the surface of the stream by the action of the running water, the body remaining motionless. The quantity of water which, under the circumstances, impinges against the body in a given time is much greater than if the body was simply immersed in still water. To increase the area of support, either the supporting medium or the body supported must move. The body is supported in water very much as the kite is supported in air. In both cases the body and the kite are made to strike the water and the air at a slight upward angle. When the extremities are made to move in a horizontal or slightly downward direction, they at once propel and support the body. When, however, they are made to act in an upward direction, as in diving, they submerge the body. This shows that the movements of the swimming surfaces may, according to their direction, either augment or destroy buoyancy. The swimming surfaces enable the seal, sea-bear, otter, ornithorhynchus, bird, etc., to disappear from and regain the surface of the water. Similar remarks may be made of the whale, dugong, manatee, and fish.
Man, in order to swim, must learn the art of swimming. He must serve a longer or shorter apprenticeship to a new form of locomotion, and acquire a new order of movements. It is otherwise with the majority of animals. Almost all quadrupeds can swim the first time they are immersed, as may readily be ascertained by throwing a newly born kitten or puppy into the water. The same may be said of the greater number of birds. This is accounted for by the fact that quadrupeds and birds are lighter, bulk for bulk, than water, but more especially, because in walking and running the movements made by their extremities are precisely those required in swimming. They have nothing to learn, as it were. They are buoyant naturally, and if they move their limbs at all, which they do instinctively, they swim of necessity. It is different with man. The movements made by him in walking and running are not those made by him in swimming; neither is the position resorted to in swimming that which characterizes him on land. The vertical position is not adapted for water, and, as a consequence, he requires to abandon it and assume a horizontal one; he requires, in fact, to throw himself flat upon the water, either upon his side, or upon his dorsal or ventral aspect. This position assimilates him to the quadruped and bird, the fish, and everything that swims; the trunks of all swimming animals, being placed in a prone position. Whenever the horizontal position is assumed, the swimmer can advance in any direction he pleases. His extremities are quite free, and only require to be moved in definite directions to produce definite results. The body can be propelled by the two arms, or the two legs; or by the right arm and leg, or the left arm and leg; or by the right arm and left leg, or the left arm and right leg. Most progress is made when the two arms and the two legs are employed. An expert swimmer can do whatever he chooses in water. Thus he can throw himself upon his back, and by extending his arms obliquely above his head until they are in the same plane with his body, can float without any exertion whatever; or, maintaining the floating position, he can fold his arms upon his chest and by alternately flexing and extending his lower extremities, can propel himself with ease and at considerable speed; or, keeping his legs in the extended position and motionless, he can propel himself by keeping his arms close to his body, and causing his hands to work like sculls, so as to make figure-of-8 loops in the water. This motion greatly resembles that made by the swimming wings of the penguin. It is most effective when the hands are turned slightly upwards, and a greater or less backward thrust given each time the hands reciprocate. The progress made at first is slow, but latterly very rapid, the rapidity increasing according to the momentum acquired. The swimmer, in addition to the foregoing methods, can throw himself upon his face, and by alternately flexing and extending his arms and legs, can float and propel himself for long periods with perfect safety and with comparatively little exertion. He can also assume the vertical position, and by remaining perfectly motionless, or by treading the water with his feet, can prevent himself from sinking; nay more, he can turn a somersault in the water either in a forward or backward direction. The position most commonly assumed in swimming is the prone one, where the ventral surface of the body is directed towards the water. In this case the anterior and posterior extremities are simultaneously flexed and drawn towards the body slowly, after which they are simultaneously and rapidly extended. The swimming of the frog conveys an idea of the movement.[51] In ordinary swimming, when the anterior and posterior extremities are simultaneously flexed, and afterwards simultaneously extended, the hands and feet describe four ellipses; an arrangement which, as explained, increases the area of support furnished by the moving parts. The ellipses are shown at fig. 38; the continuous lines representing extension, the dotted lines flexion.
Fig. 38.
Fig. 39.
Fig. 40.
Thus when the arms and legs are pushed away from the body, the arms describe the inner sides of the ellipses (fig. 38, a a), the legs describing the outer sides (c c). When the arms and legs are drawn towards the body, the arms describe the outer sides of the ellipses (b b), the legs describing the inner sides (d d). As the body advances, the ellipses are opened out and loops formed, as at e e, f f of fig. 39. If the speed attained is sufficiently high, the loops are converted into waved lines, as in walking and flying.—(Vide g g, h h of fig. 40, p. 81, and compare with fig. [18], p. 37, and figs. [71] and 73, p. 144.) The swimming of man, like the walking, swimming, and flying of animals, is effected by alternately flexing and extending the limbs, as shown more particularly at fig. 41, A, B, C.
Fig. 41.—A shows the arms and legs folded or flexed and drawn towards the mesial line of the body.—Original.
B shows the arms and legs opened out or extended and carried away from the mesial line of the body.—Original.
C shows the arms and legs in an intermediate position, i.e. when they are neither flexed nor extended. The arms and legs require to be in the position shown at A before they can assume that represented at B, and they require to be in the position shown at B before they can assume that represented at C. When the arms and legs are successively assuming the positions indicated at A, B, and C, they move in ellipses, as explained.—Original.
By alternately flexing and extending the limbs, the angles made by their several parts with each other are decreased and increased,—an arrangement which diminishes and augments the degree of resistance experienced by the swimming surfaces, which by this means are made to elude and seize the water by turns. This result is further secured by the limbs being made to move more slowly in flexion than in extension, and by the limbs being made to rotate in the direction of their length in such a manner as to diminish the resistance experienced during the former movement, and increase it during the latter. When the arms are extended, the palms of the hands and the inner surfaces of the arms are directed downwards, and assist in buoying up the anterior portion of the body. The hands are screwed slightly round towards the end of extension, the palms acting in an outward and backward direction (fig. 41, B). In this movement the posterior surfaces of the arms take part; the palms and posterior portions of the arms contributing to the propulsion of the body. When the arms are flexed, the flat of the hands is directed downwards (fig. 41, C). Towards the end of flexion the hands are slightly depressed, which has the effect of forcing the body upwards, and hence the bobbing or vertical wave-movement observed in the majority of swimmers.[52]
During flexion the posterior surfaces of the arms act powerfully as propellers, from the fact of their striking the water obliquely in a backward direction. I avoid the terms back and forward strokes, because the arms and hands, so long as they move, support and propel. There is no period either in extension or flexion in which they are not effective. When the legs are pushed away from the body, or extended (a movement which is effected rapidly and with great energy, as shown at fig. 41, B), the soles of the feet, the anterior surfaces of the legs, and the posterior surfaces of the thighs, are directed outwards and backwards. This enables them to seize the water with great avidity, and to propel the body forward. The efficiency of the legs and feet as propelling organs during extension is increased by their becoming more or less straight, and by their being moved with greater rapidity than in flexion; there being a general back-thrust of the limbs as a whole, and a particular back-thrust of their several parts.[53] In this movement the inner surfaces of the legs and thighs act as sustaining organs and assist in floating the posterior part of the body. The slightly inclined position of the body in the water, and the forward motion acquired in swimming, contribute to this result. When the legs and feet are drawn towards the body or flexed, as seen at fig. 41, C, A, their movements are slowed, an arrangement which reduces the degree of friction experienced by the several parts of the limbs when they are, as it were, being drawn off the water preparatory to a second extension.
There are several grave objections to the ordinary or old method of swimming just described. 1st, The body is laid prone on the water, which exposes a large resisting surface (fig. 41, A, B, C, p. 82). 2d, The arms and legs are spread out on either side of the trunk, so that they are applied very indirectly as propelling organs (fig. 41, B, C). 3d, The most effective part of the stroke of the arms and legs corresponds to something like a quarter of an ellipse, the remaining three quarters being dedicated to getting the arms and legs into position. This arrangement wastes power and greatly increases friction; the attitudes assumed by the body at B and C of fig. 41 being the worst possible for getting through the water. 4th, The arms and legs are drawn towards the trunk the one instant (fig. 41, A), and pushed away from it the next (fig. 41, B). This gives rise to dead points, there being a period when neither of the extremities are moving. The body is consequently impelled by a series of jerks, the swimming mass getting up and losing momentum between the strokes.
In order to remedy these defects, scientific swimmers have of late years adopted quite another method. Instead of working the arms and legs together, they move first the arm and leg of one side of the body, and then the arm and leg of the opposite side. This is known as the overhand movement, and corresponds exactly with the natural walk of the giraffe, the amble of the horse, and the swimming of the sea-bear. It is that adopted by the Indians. In this mode of swimming the body is thrown more or less on its side at each stroke, the body twisting and rolling in the direction of its length, as shown at fig. 42, an arrangement calculated greatly to reduce the amount of friction experienced in forward motion.
Fig. 42.—Overhand Swimming.—Original.
The overhand movement enables the swimmer to throw himself forward on the water, and to move his arms and legs in a nearly vertical instead of a horizontal plane; the extremities working, as it were, above and beneath the trunk, rather than on either side of it. The extremities are consequently employed in the best manner possible for developing their power and reducing the friction to forward motion caused by their action. This arrangement greatly increases the length of the effective stroke, both of the arms and legs, this being equal to nearly half an ellipse. Thus when the left arm and leg are thrust forward, the arm describes the curve a b (fig. 42), the leg e describing a similar curve. As the right side of the body virtually recedes when the left side advances, the right arm describes the curve c d, while the left arm is describing the curve a b; the right leg f describing a curve the opposite of that described by e (compare arrows). The advancing of the right and left sides of the body alternately, in a nearly straight line, greatly contributes to continuity of motion, the impulse being applied now to the right side and now to the left, and the limbs being disposed and worked in such a manner as in a great measure to reduce friction and prevent dead points or halts. When the left arm and leg are being thrust forward (a b, e of fig. 42), the right arm and leg strike very nearly directly backward (c d, f of fig. 42). The right arm and leg, and the resistance which they experience from the water consequently form a point d’appui for the left arm and leg; the two sides of the body twisting and screwing upon a moveable fulcrum (the water)—an arrangement which secures a maximum of propulsion with a minimum of resistance and a minimum of slip. The propulsive power is increased by the concave surfaces of the hands and feet being directed backwards during the back stroke, and by the arms being made to throw their back water in a slightly outward direction, so as not to impede the advance of the legs. The overhand method of swimming is the most expeditious yet discovered, but it is fatiguing, and can only be indulged in for short distances.
Fig. 43.—Side-stroke Swimming.—Original.
An improvement on the foregoing for long distances is that known as the side stroke. In this method, as the term indicates, the body is thrown more decidedly upon the side. Either side may be employed, some preferring to swim on the right side, and some on the left; others swimming alternately on the right and left sides. In swimming by the side stroke (say on the left side), the left arm is advanced in a curve, and made to describe the upper side of an ellipse, as represented at a b of fig. 43. This done, the right arm and legs are employed as propellers, the right arm and legs making a powerful backward stroke, in which the concavity of the hand is directed backwards and outwards, as shown at c d of the same figure.[54] The right arm in this movement describes the under side of an ellipse, and acts in a nearly vertical plane. When the right arm and legs are advanced, some swimmers lift the right arm out of the water, in order to diminish friction—the air being more easily penetrated than the water. The lifting of the arm out of the water increases the speed, but the movement is neither graceful nor comfortable, as it immerses the head of the swimmer at each stroke. Others keep the right arm in the water and extend the arm and hand in such a manner as to cause it to cut straight forward. In the side stroke the left arm (if the operator swims on the left side) acts as a cutwater (fig. 43, b). It is made to advance when the right arm and legs are forced backwards (fig. 43, c d). The right arm and legs move together, and alternate with the left arm, which moves by itself. The right arm and legs are flexed and carried forwards, while the left arm is extended and forced backwards, and vice versâ. The left arm always moves in an opposite direction to the right arm and legs. We have thus in the side stroke three limbs moving together in the same direction and keeping time, the fourth limb always moving in an opposite direction and out of time with the other three. The limb which moves out of time is the left one if the operator swims on the left side, and the right one if he swims on the right side. In swimming on the left side, the right arm and legs are advanced slowly the one instant, and forced in a backward direction with great energy and rapidity the next. Similar remarks are to be made regarding the left arm. When the right arm and legs strike backwards they communicate to the body a powerful forward impulse, which, seeing the body is tilted upon its side and advancing as on a keel, transmits it to a considerable distance. This arrangement reduces the amount of resistance to forward motion, conserves the energy of the swimmer, and secures in a great measure continuity of movement, the body being in the best possible position for gliding forward between the strokes.
In good side swimming the legs are made to diverge widely when they are extended or pushed away from the body, so as to include within them a fluid wedge, the apex of which is directed forwards. When fully extended, the legs are made to converge in such a manner that they force the body away from the wedge, and so contribute to its propulsion. By this means the legs in extension are made to give what may be regarded a double stroke, viz. an outward and inward one. When the double move has been made, the legs are flexed or drawn towards the body preparatory to a new stroke. In swimming on the left side, the left or cutwater arm is extended or pushed away from the body in such a manner that the concavity of the left hand is directed forwards, and describes the upper half of a vertical ellipse. It thus meets with comparatively little resistance from the water. When, however, the left arm is flexed and drawn towards the body, the concavity of the left hand is directed backwards and made to describe the under half of the ellipse, so as to scoop and seize the water, and thus contribute to the propulsion of the body. The left or cutwater arm materially assists in floating the anterior portions of the body. The stroke made by the left arm is equal to a quarter of a circle, that made by the right arm to half a circle. The right arm, when the operator swims upon the left side, is consequently the more powerful propeller. The right arm, like the left, assists in supporting the anterior portion of the body. In swimming on the left side the major propelling factors are the right arm and hand and the right and left legs and feet. Swimming by the side stroke is, on the whole, the most useful, graceful, and effective yet devised. It enables the swimmer to make headway against wind, wave, and tide in quite a remarkable manner. Indeed, a dexterous side-stroke swimmer can progress when a powerful breast-swimmer would be driven back. In still water an expert non-professional swimmer ought to make a mile in from thirty to thirty-five minutes. A professional swimmer may greatly exceed this. Thus, Mr. J. B. Johnson, when swimming against time, August 5th, 1872, in the fresh-water lake at Hendon, near London, did the full mile in twenty-six minutes. The first half-mile was done in twelve minutes. Cæteris paribus, the shorter the distance, the greater the speed. In August 1868, Mr. Harry Parker, a well-known professional swimmer, swam 500 yards in the Serpentine in seven minutes fifty seconds. Among non-professional swimmers the performance of Mr. J. B. Booth is very creditable. This gentleman, in June 1871, swam 440 yards in seven minutes fourteen seconds in the fresh-water lake at Hendon, already referred to. I am indebted for the details regarding time to Mr. J. A. Cowan of Edinburgh, himself acknowledged to be one of the fastest swimmers in Scotland. The speed attained by man in the water is not great when his size and power are taken into account. It certainly contrasts very unfavourably with that of seals, and still more unfavourably with that of fishes. This is due to his small hands and feet, the slow movements of his arms and legs, and the awkward manner in which they are applied to and withdrawn from the water.
Fig. 44.—The Turtle (Chelonia imbricata), adapted for swimming and diving, the extremities being relatively larger than in the seal, sea-bear, and walrus. The anterior extremities have a thick anterior margin and a thin posterior one, and in this respect resemble wings. Compare with figs. [36] and 37, pp. 74 and 76.—Original.
Fig. 45.—The Crested Newt (Triton cristatus, Laur.) In the newt a tail is superadded to the extremities, the tail and the extremities both acting in swimming.—Original.
Swimming of the Turtle, Triton, Crocodile, etc.—The swimming of the turtle differs in some respects from all the other forms of swimming. While the anterior extremities of this quaint animal move alternately, and tilt or partially rotate during their action, as in the sea-bear and walrus, the posterior extremities likewise move by turns. As, moreover, the right anterior and left posterior extremities move together, and reciprocate with the left anterior and right posterior ones, the creature has the appearance of walking in the water (fig. 44).
The same remarks apply to the movements of the extremities of the triton (fig. 45, p. 89) and crocodile, when swimming, and to the feebly developed corresponding members in the lepidosiren, proteus, and axolotl, specimens of all of which are to be seen in the Zoological Society’s Gardens, London. In the latter, natation is effected principally, if not altogether, by the tail and lower half of the body, which is largely developed and flattened laterally for this purpose, as in the fish.
The muscular power exercised by the fishes, the cetaceans, and the seals in swimming, is conserved to a remarkable extent by the momentum which the body rapidly acquires—the velocity attained by the mass diminishing the degree of exertion required in the individual or integral parts. This holds true of all animals, whether they move on the land or on or in the water or air.
The animals which furnish the connecting link between the water and the air are the diving-birds on the one hand, and the flying-fishes on the other,—the former using their wings for flying above and through the water, as occasion demands; the latter sustaining themselves for considerable intervals in the air by means of their enormous pectoral fins.
Flight under water, etc.—Mr. Macgillivray thus describes a flock of red mergansers which he observed pursuing sand-eels in one of the shallow sandy bays of the Outer Hebrides:—“The birds seemed to move under the water with almost as much velocity as in the air, and often rose to breathe at a distance of 200 yards from the spot at which they had dived.”[55]
Fig. 46.—The Little Penguin (Aptenodytes minor, Linn.), adapted exclusively for swimming and diving. In this quaint bird the wing forms a perfect screw, and is employed as such in swimming and diving. Compare with fig. [37], p. 76, and fig. [44], p. 89.—Original.
In birds which fly indiscriminately above and beneath the water, the wing is provided with stiff feathers, and reduced to a minimum as regards size. In subaqueous flight the wings may act by themselves, as in the guillemots, or in conjunction with the feet, as in the grebes .[56] To convert the wing into a powerful oar for swimming, it is only necessary to extend and flex it in a slightly backward direction, the mere act of extension causing the feathers to roll down, and giving to the back of the wing, which in this case communicates the more effective stroke, the angle or obliquity necessary for sending the animal forward. This angle, I may observe, corresponds with that made by the foot during extension, so that, if the feet and wings are both employed, they act in harmony. If proof were wanting that it is the back or convex surface of the wing which gives the more effective stroke in subaquatic flight, it would be found in the fact that in the penguin and great auk, which are totally incapable of flying out of the water, the wing is actually twisted round in order that the concave surface, which takes a better hold of the water, may be directed backwards (fig. 46).[57] The thick margin of the wing when giving the effective stroke is turned downwards, as happens in the flippers of the sea-bear, walrus, and turtle. This, I need scarcely remark, is precisely the reverse of what occurs in the ordinary wing in aërial flight. In those extraordinary birds (great auk and penguin) the wing is covered with short, bristly-looking feathers, and is a mere rudiment and exceedingly rigid, the movement which wields it emanating, for the most part, from the shoulder, where the articulation partakes of the nature of a universal joint. The wing is beautifully twisted upon itself, and when it is elevated and advanced, it rolls up from the side of the bird at varying degrees of obliquity, till it makes a right angle with the body, when it presents a narrow or cutting edge to the water. The wing when fully extended, as in ordinary flight, makes, on the contrary, an angle of something like 30° with the horizon. When the wing is depressed and carried backwards,[58] the angles which its under surface make with the surface of the water are gradually increased. The wing of the penguin and auk propels both when it is elevated and depressed. It acts very much after the manner of a screw; and this, as I shall endeavour to show, holds true likewise of the wing adapted for aërial flight.
Difference between Subaquatic and Aërial Flight.—The difference between subaquatic flight or diving, and flight proper, may be briefly stated. In aërial flight, the most effective stroke is delivered downwards and forwards by the under, concave, or biting surface of the wing which is turned in this direction; the less effective stroke being delivered in an upward and forward direction by the upper, convex, or non-biting surface of the wing. In subaquatic flight, on the contrary, the most effective stroke is delivered downwards and backwards, the least effective one upwards and forwards. In aërial flight the long axis of the body of the bird and the short axis of the wings are inclined slightly upwards, and make a forward angle with the horizon. In subaquatic flight the long axis of the body of the bird, and the short axis of the wings are inclined slightly downwards and make a backward angle with the surface of the water. The wing acts more or less efficiently in every direction, as the tail of the fish does. The difference noted in the direction of the down stroke in flying and diving, is rendered imperative by the fact that a bird which flies in the air is heavier than the medium it navigates, and must be supported by the wings; whereas a bird which flies under the water or dives, is lighter than the water, and must force itself into it to prevent its being buoyed up to the surface. However paradoxical it may seem, weight is necessary to aërial flight, and levity to subaquatic flight. A bird destined to fly above the water is provided with travelling surfaces, so fashioned and so applied (they strike from above, downwards and forwards), that if it was lighter than the air, they would carry it off into space without the possibility of a return; in other words, the action of the wings would carry the bird obliquely upwards, and render it quite incapable of flying either in a horizontal or downward direction. In the same way, if a bird destined to fly under the water (auk and penguin) was not lighter than the water, such is the configuration and mode of applying its travelling surfaces (they strike from above, downwards and backwards), they would carry it in the direction of the bottom without any chance of return to the surface. In aërial flight, weight is the power which nature has placed at the disposal of the bird for regulating its altitude and horizontal movements, a cessation of the play of its wings, aided by the inertia of its trunk, enabling the bird to approach the earth. In subaquatic flight, levity is a power furnished for a similar but opposite purpose; this, combined with the partial slowing or stopping of the wings and feet, enabling the diving bird to regain the surface at any moment. Levity and weight are auxiliary forces, but they are necessary forces when the habits of the aërial and aquatic birds and the form and mode of applying their travelling surfaces are taken into account. If the aërial flying bird was lighter than the air, its wings would require to be twisted round to resemble the diving wings of the penguin and auk. If, on the other hand, the diving bird (penguin or auk) was heavier than the water, its wings would require to resemble aërial wings, and they would require to strike in an opposite direction to that in which they strike normally. From this it follows that weight is necessary to the bird (as at present constructed) destined to navigate the air, and levity to that destined to navigate the water. If a bird was made very large and very light, it is obvious that the diving force at its disposal would be inadequate to submerge it. If, again, it was made very small and very heavy, it is equally plain that it could not fly. Nature, however, has struck the just balance; she has made the diving bird, which flies under the water, relatively much heavier than the bird which flies in the air, and has curtailed the travelling surfaces of the former, while she has increased those of the latter. For the same reason, she has furnished the diving bird with a certain degree of buoyancy, and the flying bird with a certain amount of weight—levity tending to bring the one to the surface of the water, weight the other to the surface of the earth, which is the normal position of rest for both. The action of the subaquatic or diving wing of the king penguin is well seen at p. 94, fig. 47.
Fig. 47.—At A the penguin is in the act of diving, and it will be observed that the anterior or thick margin of the wing is directed downwards and forwards, while the posterior margin is directed upwards and backwards. This has the effect of directing the under or ventral concave surface of the wing upwards and backwards, the most effective stroke being delivered in a downward and backward direction. The efficacy of the wing in counteracting levity is thus obvious. At B the penguin is in the act of regaining the surface of the water, and in this case the wing is maintained in one position, or made to strike downwards and forwards like the aërial wing, the margins and under surface of the pinion being reversed for this purpose. The object now is not to depress but to elevate the body. Those movements are facilitated by the alternate play of the feet. (Compare fig. 47 with fig. [37], p. 76.)
From what has been stated it will be evident that the wing acts very differently in and out of the water; and this is a point deserving of attention, the more especially as it seems to have hitherto escaped observation. In the water the wing, when most effective, strikes downwards and backwards, and acts as an auxiliary of the foot; whereas in the air it strikes downwards and forwards. The oblique surfaces, spiral or otherwise, presented by animals to the water and air are therefore made to act in opposite directions, as far as the down strokes are concerned. This is owing to the greater density of the water as compared with the air,—the former supporting or nearly supporting the animal moving upon or in it; the latter permitting the creature to fall through it in a downward direction during the ascent of the wing. To counteract the tendency of the bird in motion to fall downwards and forwards, the down stroke is delivered in this direction; the kite-like action of the wing, and the rapidity with which it is moved causing the mass of the bird to pursue a more or less horizontal course. I offer this explanation of the action of the wing in and out of the water after repeated and careful observation in tame and wild birds, and, as I am aware, in opposition to all previous writers on the subject.
The rudimentary wings or paddles of the penguin (the movements of which I had an opportunity of studying in a tame specimen) are principally employed in swimming and diving. The feet, which are of moderate size and strongly webbed, are occasionally used as auxiliaries. There is this difference between the movements of the wings and feet of this most curious bird, and it is worthy of attention. The wings act together, or synchronously, as in flying birds; the feet, on the other hand, are moved alternately. The wings are wielded with great energy, and, because of their semi-rigid condition, are incapable of expansion. They therefore present their maximum and minimum of surface by a partial rotation or tilting of the pinion, as in the walrus, sea-bear, and turtle. The feet, which are moved with less vigour, are, on the contrary, rotated or tilted to a very slight extent, the increase and diminution of surface being secured by the opening and closing of the membranous expansion or web between the toes. In this latter respect they bear a certain analogy to the feet of the seal, the toes of which, as has been explained, spread out or divaricate during extension, and the reverse. The feet of the penguin entirely differ from those of the seal, in being worked separately, the foot of one side being flexed or drawn towards the body, while its fellow is being extended or pushed away from it. The feet, moreover, describe definite curves in opposite directions, the right foot proceeding from within outwards, and from above downwards during extension, or when it is fully expanded and giving the effective stroke; the left one, which is moving at the same time, proceeding from without inwards and from below upwards during flexion, or when it is folded up, as happens during the back stroke. In the acts of extension and flexion the legs are slightly rotated, and the feet more or less tilted. The same movements are seen in the feet of the swan, and in those of swimming birds generally (fig. 48).
Fig. 48.—Swan, in the act of swimming, the right foot being fully expanded, and about to give the effective stroke, which is delivered outwards, downwards, and backwards, as represented at r of fig. 50; the left foot being closed, and about to make the return stroke, which is delivered in an inward, upward, and forward direction, as shown at s of fig. 50. In rapid swimming the swan flexes its legs simultaneously and somewhat slowly; it then vigorously extends them.—Original.
Fig. 49.—Foot of Grebe (Podiceps). In this foot each toe is provided with its swimming membrane; the membrane being closed when the foot is flexed, and expanded when the foot is extended. Compare with foot of swan (fig. 48), where the swimming membrane is continued from the one toe to the other.—(After Dallas.)
One of the most exquisitely constructed feet for swimming and diving purposes is that of the grebe (fig. 49). This foot consists of three swimming toes, each of which is provided with a membranous expansion, which closes when the foot is being drawn towards the body during the back stroke, and opens out when it is being forced away from the body during the effective stroke.
Fig. 50.—Diagram representing the double waved track described by the feet of swimming birds. Compare with figs. [18] and [19], pp. 37 and 39, and with fig. [32], p. 68.—Original.
In swimming birds, each foot describes one side of an ellipse when it is extended and thrust from the body, the other side of the ellipse being described when the foot is flexed and drawn towards the body. The curve described by the right foot when pushed from the body is seen at the arrow r of fig. 50; that formed by the left foot when drawn towards the body, at the arrow s of the same figure. The curves formed by the feet during extension and flexion produce, when united in the act of swimming, waved lines, these constituting a chart for the movements of the extremities of swimming birds.
There is consequently an obvious analogy between the swimming of birds and the walking of man (compare fig. 50, p. 97, with fig. [19], p. 39); between the walking of man and the walking of the quadruped (compare figs. [18] and 19, pp. 37 and 39); between the walking of the quadruped and the swimming of the walrus, sea-bear, and seal; between the swimming of the seal, whale, dugong, manatee, and porpoise, and that of the fish (compare fig. [32], p. 68, with figs. [18] and 19, pp. 37 and 39); and between the swimming of the fish and the flying of the insect, bat, and bird (compare all the foregoing figures with figs. [71], [73], and [81], pp. 144 and 157).
Fig. 51.—The Flying-fish (Exocœtus exsiliens, Linn.), with wings expanded and elevated in the act of flight (vide arrows). This anomalous and interesting creature is adapted both for swimming and flying. The swimming-tail is consequently retained, and the pectoral fins, which act as wings, are enormously increased in size.—Original.
Flight of the Flying-fish; the kite-like action of the Wings, etc.—Whether the flying-fish uses its greatly expanded pectoral fins as a bird its wings, or only as parachutes, has not, so far as I am aware, been determined by actual observation. Most observers are of opinion that these singular creatures glide up the wind, and do not beat it after the manner of birds; so that their flight (or rather leap) is indicated by the arc of a circle, the sea supplying the chord. I have carefully examined the structure, relations, and action of those fins, and am satisfied in my own mind that they act as true pinions within certain limits, their inadequate dimensions and limited range alone preventing them from sustaining the fish in the air for indefinite periods. When the fins are fully flexed, as happens when the fish is swimming, they are arranged along the sides of the body; but when it takes to the air, they are raised above the body and make a certain angle with it. In being raised they are likewise inclined forwards and outwards, the fins rotating on their long axes until they make an angle of something like 30° with the horizon—this being, as nearly as I can determine, the greatest angle made by the wings during the down stroke in the flight of insects and birds.
The pectoral fins, or pseudo-wings of the flying-fish, like all other wings, act after the manner of kites—the angles of inclination which their under surfaces make with the horizon varying according to the degree of extension, the speed acquired, and the pressure to which they are subjected by being carried against the air. When the flying-fish, after a preliminary rush through the water (in which it acquires initial velocity), throws itself into the air, it is supported and carried forwards by the kite-like action of its pinions;—this action being identical with that of the boy’s kite when the boy runs, and by pulling upon the string causes the kite to glide upwards and forwards. In the case of the boy’s kite a pulling force is applied to the kite in front. In the case of the flying-fish (and everything which flies) a similar force is applied to the kites formed by the wings by the weight of the flying mass, which always tends to fall vertically downwards. Weight supplies a motor power in flight similar to that supplied by the leads in a clock. In the case of the boy’s kite, the hand of the operator furnishes the power; in flight, a large proportion of the power is furnished by the weight of the body of the flying creature. It is a matter of indifference how a kite is flown, so long as its under surface is made to impinge upon the air over which it passes.[59] A kite will fly effectually when it is neither acted upon by the hand nor a weight, provided always there is a stiff breeze blowing. In flight one of two things is necessary. Either the under surface of the wings must be carried rapidly against still air, or the air must rush violently against the under surface of the expanded but motionless wings. Either the wings, the body bearing them, or the air, must be in rapid motion; one or other must be active. To this there is no exception. To fly a kite in still air the operator must run. If a breeze is blowing the operator does not require to alter his position, the breeze doing the entire work. It is the same with wings. In still air a bird, or whatever attempts to fly, must flap its wings energetically until it acquires initial velocity, when the flapping may be discontinued; or it must throw itself from a height, in which case the initial velocity is acquired by the weight of the body acting upon the inclined planes formed by the motionless wings. The flapping and gliding action of the wings constitute the difference between ordinary flight and that known as skimming or sailing flight. The flight of the flying-fish is to be regarded rather as an example of the latter than the former, the fish transferring the velocity acquired by the vigorous lashing of its tail in the water to the air,—an arrangement which enables it to dispense in a great measure with the flapping of the wings, which act by a combined parachute and wedge action. In the flying-fish the flying-fin or wing attacks the air from beneath, whilst it is being raised above the body. It has no downward stroke, the position and attachments of the fin preventing it from descending beneath the level of the body of the fish. In this respect the flying-fin of the fish differs slightly from the wing of the insect, bat, and bird. The gradual expansion and raising of the fins of the fish, coupled with the fact that the fins never descend below the body, account for the admitted absence of beating, and have no doubt originated the belief that the pectoral fins are merely passive organs. If, however, they do not act as true pinions within the limits prescribed, it is difficult, and indeed impossible, to understand how such small creatures can obtain the momentum necessary to project them a distance of 200 or more yards, and to attain, as they sometimes do, an elevation of twenty or more feet above the water. Mr. Swainson, in crossing the line in 1816, zealously attempted to discover the true action of the fins in question, but the flight of the fish is so rapid that he utterly failed. He gives it as his opinion that flight is performed in two ways,—first by a spring or leap, and second by the spreading of the pectoral fins, which are employed in propelling the fish in a forward direction, either by flapping or by a motion analogous to the skimming of swallows. He records the important fact, that the flying-fish can change its course after leaving the water, which satisfactorily proves that the fins are not simply passive structures. Mr. Lord, of the Royal Artillery,[60] thus writes of those remarkable specimens of the finny tribe:—“There is no sight more charming than the flight of a shoal of flying-fish, as they shoot forth from the dark green wave in a glittering throng, like silver birds in some gay fairy tale, gleaming brightly in the sunshine, and then, with a mere touch on the crest of the heaving billow, again flitting onward reinvigorated and refreshed.”
Before proceeding to a consideration of the graceful and, in some respects, mysterious evolutions of the denizens of the air, and the far-stretching pinions by which they are produced, it may not be out of place to say a few words in recapitulation regarding the extent and nature of the surfaces by which progression is secured on land and on or in the water. This is the more necessary, as the travelling-surfaces employed by animals in walking and swimming bear a certain, if not a fixed, relation to those employed by insects, bats, and birds in flying. On looking back, we are at once struck with the fact, remarkable in some respects, that the travelling-surfaces, whether feet, flippers, fins, or pinions, are, as a rule, increased in proportion to the tenuity of the medium on which they are destined to operate. In the ox (fig. [18], p. 37) we behold a ponderous body, slender extremities, and unusually small feet. The feet are slightly expanded in the otter (fig. [12], p. 34), and considerably so in the ornithorhynchus (fig. [11], p. 34). The travelling-area is augmented in the seal (fig. [14], p. 34; fig. [36], p. 74), penguin (figs. [46] and 47, pp. 91 and 94), sea-bear (fig. [37], p. 76), and turtle (fig. [44], p. 89). In the triton (fig. [45], p. 89) a huge swimming-tail is added to the feet—the tail becoming larger, and the extremities (anterior) diminishing, in the manatee (fig. [34], p. 73) and porpoise (fig. 33, p. 73), until we arrive at the fish (fig. [30], p. 65), where not only the tail but the lower half of the body is actively engaged in natation. Turning from the water to the air, we observe a remarkable modification in the huge pectoral fins of the flying-fish (fig. [51], p. 98), these enabling the creature to take enormous leaps, and serving as pseudo-pinions. Turning in like manner from the earth to the air, we encounter the immense tegumentary expansions of the flying-dragon (fig. [15], p. 35) and galeopithecus (fig. [16], p. 35), the floating or buoying area of which greatly exceeds that of some of the flying beetles.
In those animals which fly, as bats (fig. [17], p. 36), insects (figs. [57] and 58, p. 124 and 125), and birds (figs. [59] and 60, p. 126), the travelling surfaces, because of the extreme tenuity of the air, are prodigiously augmented; these in many instances greatly exceeding the actual area of the body. While, therefore, the movements involved in walking, swimming, and flying are to be traced in the first instance to the shortening and lengthening of the muscular, elastic, and other tissues operating on the bones, and their peculiar articular surfaces; they are to be referred in the second instance to the extent and configuration of the travelling areas—these on all occasions being accurately adapted to the capacity and strength of the animal and the density of the medium on or in which it is intended to progress. Thus the land supplies the resistance, and affords the support necessary to prevent the small feet of land animals from sinking to dangerous depths, while the water, immensely less resisting, furnishes the peculiar medium requisite for buoying the fish, and for exposing, without danger and to most advantage, the large surface contained in its ponderous lashing tail,—the air, unseen and unfelt, furnishing that quickly yielding and subtle element in which the greatly expanded pinions of the insect, bat, and bird are made to vibrate with lightning rapidity, discoursing, as they do so, a soft and stirring music very delightful to the lover of nature.