MOTION. 



439 



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 

 level 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 

 spread out their wings as a sail, upon which 

 the wind acts with sufficient 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 pal mated 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- 

 ments in a minute they are incapable of re- 

 maining below the surface of the water for 

 lengthened periods. The lluminantia, 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 

 easy accomplishment. Quadrupeds swim by 

 the alternate extension and flexion of their 

 legs; the effective stroke is performed during 

 extension, and the back stroke during flexion, 



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 to the 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 always an 

 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 

 in the vertical direction upwards and in the hori- 

 zontal direction forwards as is sufficient to keep 

 the respiratory openings above the surface of the 



Sec Principles of the Resistance of Fluids. 



* Sec Resistance of Fluids. 



