CHAP, viz.] SOME SPECIAL MECHANISMS. 



1105 



much as at each step the line of gravity is made to fall alter- 

 nately on each standing foot. While the left leg is swinging, 

 the line of gravity falls within the area of the right foot, and the 

 centre of gravity is on the right side of the pelvis. As the left 

 foot becomes the standing foot, the centre of gravity is shifted 

 to the left side of the pelvis. The actual curve described by 

 the centre of gravity is therefore a somewhat complicated one, 

 being composed of vertical and horizontal factors. 



The natural step is the 'one which is determined by the 

 length of the swinging leg, since this acts as a pendulum ; and 

 hence the step of a long-legged person is naturally longer than 

 that of a person with short legs. The length of the step how- 

 ever may be diminished or increased by a direct muscular 

 effort, as when a line of soldiers keep step in spite of their hav- 

 ing legs of different lengths. Such a mode of marching must 

 obviously be fatiguing, inasmuch as it involves an unnecessary 

 expenditure of energy. 



In slow walking, which Fig. 193 may be taken to illustrate, 

 there is an appreciable time during which, while one foot is 

 already in position to serve as a fulcrum, the other, swinging, 

 foot has not yet left the ground. In fast walking this period is 

 so much reduced, that one foot leaves the ground the moment 

 the other touches it ; hence there is practically no period during 

 which both feet are on the ground together; this might be 

 shewn by omitting b c and d e in Fig. 193. 



bed 



f g 



FIG. 194. DIAGRAM TO ILLUSTRATE RUNNING. 



Z, the line of contact with the ground of the left, H of the right foot ; in 

 each case the curved portion of the line represents the time during which the 

 foot leaves the ground. 



When the body is swung forward on the one foot acting as 

 a fulcrum with such energy that this foot leaves the ground 

 before the other, swinging, foot has reached the ground, as 

 shewn in Fig. 194, there being an interval, b <?, d e in the 

 figure, during which neither foot is on the ground, the person 

 is said to be running, not walking. 



In jumping this propulsion of the body takes place on both 

 feet at the same time ; in hopping it is effected on one foot only. 



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