CHAP, vii.] SOME SPECIAL MECHANISMS. 



345 



of the next step merely repeat those of the one which we have 

 described. 



It is obvious from the above that in walking there are in each 

 step periods when both feet are touching the ground, and periods 

 when one or the other foot is raised from the ground, but there is 

 no period when both feet are off the ground. This is shewn in the 

 diagram, Fig. 192, which represents two steps. 



FIG. 192. DIAGRAM TO ILLUSTRATE THE CONTACT OF THE FEET WITH THE 

 GROUND IN WALKING. 



R, the right foot. L, the left foot. In each case the curved line represents the 

 time when the foot is not in contact with the ground, and the straight line 

 when it is in contact. 



During a b, the left leg (L) leaves the ground as indicated by 

 the curving of the line. During b c both feet are on the ground. 

 During c d the right leg (R) is above but the left (L) is still on the 

 ground. During d e both are on the ground and the double step 

 is completed, the next step beginning again at e with the left leg 

 leaving the ground. 



We have said that the centre of gravity is in walking pre- 

 vented from moving downwards as well as forwards, as it would 

 do in the act of falling forwards. It does not however describe 

 a straight line forwards, it, and with it the top of the head, rises 

 and falls at each step of each leg, and hence describes a series of 

 consecutive curves not unlike the line of flight of many birds. 



Since in standing on both feet the line of gravity falls between 

 the two feet, a lateral displacement of the centre of gravity is 

 necessary in order to balance the body on one foot. Hence in 

 walking the centre of gravity describes not only a series of vertical, 

 but also a series of horizontal curves, inasmuch as at each step the 

 line of gravity is made to fall alternately 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 



