resembling the keel of a boat. The feathers 
are furnished with an oleaginous secretion, 
which prevents the water from penetrating to 
the skin ; they also enlarge the bulk of the bird 
without very sensibly increasing its weight. In 
the Palmipedes, the osseous system, though 
more dense and less permeated with air than 
in birds destined for long and continued flight, 
is yet so light as to render their specific gravity 
considerably less than water, so that a large 
_ proportion of the body is sustained above its 
ay by hydrostatic pressure alone. The in- 
_ terdigital membranes which give an expanded 
surface to the feet of these birds, acting at the 
end of the long lever, formed by the metatarsal 
bone, enable them to strike the water with 
_ considerable force.* In the effective stroke pro- 
_ duced by the extension of the legs, the flat 
surface of the feet is presented to the water in 
the direction of motion, whilst in the back 
_ stroke they are drawn forwards very obliquely 
and with less force. In the former action, the 
centre of gravity is accelerated, but during the 
latter it is retarded, so that there results a 
succession of impulses and a variable motion. 
The Swan and other Palmipedes sometimes 
pend out their wings as a sail, upon which 
_ the wind acts with sutlicient force to propel 
them along without the expenditure of any 
muscular power. The specific gravity of birds, 
being much less than unity, enables them to 
glide upon the surface of the water without any 
_ expenditure of muscular action in the vertical, 
_ consequently it is required only in the horizontal 
direction. 
__ Quadrupeds—Many quadrupeds have their 
feet palmated to afford a larger surface for 
“striking the water in swimming. Many of the 
_ Saurian, Batrachian, and Chelonian tribes have 
their feet thus organized, though the Caymans 
_ are semipalmated. The lateral direction of the 
locomotive organs of the three former orders 
_ enables them to give an oblique stroke down- 
_ wards and backwards, so as to communicate 
__ an ascending as well as a horizontal impulse to 
_ the centre of gravity, and thus to prevent their 
sinking whilst they are urged forwards. In the 
‘common Otter the feet are also palmated, a 
construction which enables them to move in 
the water with surprising agility, and with suf- 
_ ficient velocity to overtake and capture the fish 
on which they prey. In a similar manner 
the feet of the Newfoundland Dog are also 
furnished with interdigital membranes, but 
__ Owing to the number of their respiratory move- 
id ‘ments in a minute they are incapable of re- 
maining below the surface of the water for 
lengthened periods. The Ruminantia, Carni- 
-vora, and Pachydermata, being all of less spe- 
cific gravity than water, can swim with facility, 
and their locomotive organs, acting as in ter- 
-restrial progression, render swimming a task of 
__€asy accomplishment. Quadrupeds swim by 
__ the alternate extension and flexion of their 
____ legs; the effective stroke is performed during 
_ €xtension, and the back stroke during flexion, 
* 
es 
‘ 
7 
Ss 
* See Principles of the Resistance of Fluids. 
MOTION. 
439 
presenting in the former a larger area to the 
water than in the latter. In consequence of 
the difference of their specific gravities, the 
Horse is capable of swimming even when 
loaded with the weight of a man, with a large 
proportion of its body above the surface of 
the water. The feet of the Solidunguli are well 
formed for striking the water, the flat portions 
of which are employed in the effective and the 
convex in the back stroke, so that the propor- 
tion of the resistance of the water in these 
two strokes, owing to the figure of the foot, 
are to each other nearly as two to one.* 
Man.—The figure of the human body, 
the position of the respiratory apertures, the 
number of respiratory movements made in a 
a minute, the different plane in which the loco- 
motive organs usually act in terrestrial progres- 
sion, and the small surfaces which the hand and 
feet present tothe water, contribute to render man 
the least adapted of almost all animals for swim- 
ming. The specific gravity varies in different indi- 
viduals ; it is rather greater than water when the 
chest is nearly exhausted, and less when well 
expanded with air; hence a man has alwaysan 
hydrostatic apparatus which will keep him 
floating, if he has the knowledge of this fact 
and sufficient presence of mind to employ it. 
The density and temperature of water produce 
at the moment of immersion an involuntary 
expulsion of air from the chest, added to which 
the consequent alarm and misdirected struggles 
facilitate the fatal catastrophe of drowning. In 
swimming, the hands and feet are employed so 
as to present the least surface to the water in 
the back and the greatest in the effective stroke ; 
in the former the hands are brought near the 
mesial plane, with the palmar surfaces parallel 
to each other ; they are then thrust forward by 
the extension of the arm, with the points of 
the fingers in advance to cut the water with the 
least resistance; when the hands have nearly 
reached their greatest distance from the centre 
of gravity, they are rotated by pronation, so 
that the palms are directed at an oblique angle 
outwards and downwards; they are then forced 
backwards by the abduction of the whole arm 
through a large arc of a circle, having the shoul- 
der-joint for its centre, and the length of the 
arm for its radius ; the fore-arm is then flexed, 
and carried into its former position preparatory 
to making another stroke. During the exten- 
sion of the arm, the feet are drawn towards 
the centre of gravity, with their convex surface 
directed obliquely backwards by the extension 
of the ankle and flexion of the hip and knee 
joints, and during the adduction of the arm 
the flat surfaces of the feet are driven forcibly 
backwards and downwards by the sudden ex- 
tension of the leg. From the ratio of the areas 
of the hands and feet, and the ratio of the dif- 
ference of their velocities in the two strokes, 
there results such a preponderance of the force 
iu the vertical direction upwards and in the hori- 
zontal direction forwards as is sufficient to keep 
the respiratory openings above the surface of the 
* See Resistance of Fiuids. 
