126 PHYSIOLOGY CHAP. 



suspended by its centre of buoyancy, and to be in equilibrium it is 

 necessary that the centre of gravity and the centre of buoyancy 

 shall be on the same vertical plane. And for the equilibrium to 

 be stable the centre of gravity of the floating body must be below 

 the centre of buoyancy. Ships are all constructed on this 



FIG. 85. Diagram of four different gaits, from man. (After Marey.) 1, walking on flat ground ; 

 2, walking uphill and upstairs ; 3, running : 4, fast running. 



principle, i.e. so that their centre of gravity shall be as low as 

 possible in comparison with the centre of buoyancy. The same 

 principle has recently been applied to dirigible airships and 

 aeroplanes. 



On an average the human body as a whole is heavier than fresh 

 water (1*010), but its gravity differs little from, and is even some- 

 what less than, that of salt water. While lying on his back, so 



FIG. 86. Diagram of galloping and jumping. (Marey.) 1, galloping to the left ; 2, to the right ; 

 3, series of rhythmical jumps on both feet ; hops on right foot alone. 



that only his mouth and nose are above the water, an adult man 

 (especially if very fat) can easily float on the sea, if he keeps all 

 his muscles relaxed. Thin people, however, whose average specific 

 gravity is rather higher than that of salt water, are unable to 

 float in the supine position without the help of slight impulsive 

 movements of the feet, produced by rhythmical extension of the 

 legs. In order to move in this position it is necessary to supple- 



