THE HUMAN FRAME. 479 



which he has to acquire, and which costs him in 

 the apprenticeship many painful efforts, and many 

 discouraging falls. But whenever nature is the 

 teacher, the scholar makes rapid progress in 

 learning; and no sooner have the muscles acquired 

 the necessary strength, than the child becomes an 

 adept in balancing its body in various attitudes, and 

 in a very short time is unconscious that these 

 actions require exertion. 



In walking, the first effort that is made consists 

 in tranferring the whole weight of the body 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 unsup- 

 ported, 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 prepared for ; in which case the 

 body, having acquired 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 transferred 

 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 souiewhat complicated in their form. This un- 



