Chapter IV: Part 4

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.

[44] The _Syngnathi_, or Pipefishes, swim chiefly by the undulating
movement of the dorsal fin.

[45] If the pectoral fins are to be regarded as the homologues of
the anterior extremities (which they unquestionably are), it is not
surprising that in them the spiral rotatory movements which are
traceable in the extremities of quadrupeds, and so fully developed
in the wings of bats and birds, should be clearly foreshadowed. “The
muscles of the pectoral fins,” remarks Professor Owen, “though, when
compared with those of the homologous members in higher vertebrates,
they are very small, few, and simple, yet suffice for all the
requisite movements of the fins--elevating, depressing, advancing,
and again laying them prone and flat, by an oblique stroke, upon the
sides of the body. The rays or digits of both pectorals and ventrals
(the homologues of the posterior extremities) can be divaricated
and approximated, and the intervening webs spread out or folded
up.”--_Op. cit._ vol. i. p. 252.

[46] _Vide_ “Remarks on the Swimming of the Cetaceans,” by Dr. Murie,
Proc. Zool. Soc., 1865, pp. 209, 210.

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.

[47] In a few instances the caudal fin of the fish, as has been
already stated, is more or less pressed together during the back
stroke, the compression and tilting or twisting of the tail taking
place synchronously.

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]

[48] The unusual opportunities afforded by this unrivalled collection
have enabled me to determine with considerable accuracy the movements
of the various land-animals, as well as the motions of the wings and
feet of birds, both in and out of the water. I have also studied
under the most favourable circumstances the movements of the otter,
sea-bear, seal, walrus, porpoise, turtle, triton, crocodile, frog,
lepidosiren, proteus, axolotl, and the several orders of fishes.

[49] This is the reverse of what takes place in flying, the anterior
or thick margins of the wings being invariably _directed upwards_.

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.

[50] The air-bladder is wanting in the dermopteri, plagiostomi, and
pleuronectidæ.--Owen, _op. cit._ p. 255.

_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.

[51] The frog in swimming leisurely frequently causes its extremities
to move diagonally and alternately. When, however, pursued and
alarmed, it folds its fore legs, and causes its hind ones to move
simultaneously and with great vigour by a series of sudden jerks,
similar to those made by man when swimming on his back.

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]

[52] The professional swimmer avoids bobbing, and rests the side of
his head on the water to diminish its weight and increase speed.

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.

[53] The greater power possessed by the limbs during extension, and
more especially towards the end of extension, is well illustrated by
the kick of the horse; the hind feet dealing a terrible blow when
they have reached their maximum distance from the body. Ostlers are
well aware of this fact, and in grooming a horse keep always very
close to his hind quarters, so that if he does throw up they are
forced back but not injured.

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.

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.

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.

[54] The outward direction given to the arm and hand enables them to
force away the back water from the body and limbs, and so reduce the
friction to forward motion.

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]

[55] History of British Birds, vol. i. p. 48.

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.

[56] The guillemots in diving do not use their feet; so that they
literally fly under the water. Their wings for this purpose are
reduced to the smallest possible dimensions consistent with flight.
The loons, on the other hand, while they employ their feet, rarely,
if ever, use their wings. The subaqueous progression of the grebe
resembles that of the frog.--Cuvier’s Animal Kingdom, Lond. 1840,
pp. 252, 253.

[57] In the swimming of the crocodile, turtle, triton, and frog, the
concave surfaces of the feet of the anterior extremities are likewise
turned backwards.

[58] The effective stroke is also delivered during flexion in the
shrimp, prawn, and lobster.