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.
[Illustration: 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.
[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.
[Illustration:
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._]
[Illustration:
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.
[Illustration:
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.
_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.
[Illustration:
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).
[Illustration:
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._]
[Illustration:
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.
[Illustration: