ii MECHANICS OF LOCOMOTOB APPAEATUS 125 



of the body, the movement is much more rapid than in coming 

 down again. While rising, the arms are pushed forward in order 

 to raise the centre of gravity and increase the impulse in the 

 direction of the leap ; during the descent, on the contrary, they 

 are thrown back to lessen the momentum of the body at the 

 moment at which it touches the ground. As soon as the feet 

 come in contact with the ground the knees are flexed to lessen 

 the counter-blow and the shock. 



In order to understand the essential features of the different 

 gaits which we have been discussing the diagram suggested by 

 Marey is useful. In this the duration of the contacts of the right 

 foot is shown by white lines, of the left foot by shaded lines, the 

 duration of the elevation of either limb in the air by the inter- 



FIG. 84. Instantaneous photographs of a long jump on fixed plate. (Marry.) 



vening black area. It is a kind of simplified notation, less 

 complete than that of the graphic method because it does not 

 indicate the pressure exercised by the foot upon the ground and 

 its variations ; but it is much clearer and shows at a glance the 

 fundamental difference between the different gaits (Figs. 85, 86). 

 This form of notation is almost indispensable in differentiating the 

 various gaits of quadrupeds. 



VII. Swimming differs from terrestrial locomotion inasmuch as 

 the body does not rest on the ground, but is immersed in water, 

 which is a fluid medium. 



The body floating in water may be compared to a body 

 resting upon a supporting plane, formed by the buoyancy of the 

 fluid. This is due to a great number of parallel forces which 

 act vertically from below upward on the lower surface of the 

 swimming body. The resultant of these forces is called the centre 

 of buoyancy, which corresponds to the centre of gravity of the 

 liquid mass displaced. The floating body may thus be regarded as 



