PROGRESSIVE MOTION IN MAN. 543 



upon one foot, with a view to fix it on the ground ; 

 and then the other foot, being at liberty, is 

 brought forwards. By this action the centre of 

 gravity is made to advance, till it passes beyond 

 the base of the foot : in this situation the body, 

 being unsupported, falls through a certain space, 

 and would continue its descent, were it not that 

 it is received on the other foot, which, by this 

 time, has been set upon the ground. This falling 

 of the body would, if not immediately checked, 

 become very sensible ; as happens when, on 

 walking inattentively, the foot we had advanced 

 comes down to a lower level than we were pre- 

 pared for ; in which case the body, having ac- 

 quired a certain velocity by its greater descent, 

 receives a sudden shock when that velocity is 

 checked, and thus a disagreeable jar is given to 

 the whole frame. 



While the weight of the body is thus trans- 

 ferred alternately from one foot to the other, the 

 centre of gravity not only rises and falls, so as 

 to describe at every step a small arch, but also 

 vibrates from side to side, so that the series of 

 curves it describes are somewhat complicated in 

 their form. This undulation of the body from 

 one foot to the other would scarcely ever be per- 

 formed with perfect equality on both sides, if we 

 trusted wholly to the sensations communicated 

 by the muscles, and if we were not guided by 

 the sense of sight, or some other substitute. Thus 



