CHAP, vii.] SOME SPECIAL MECHANISMS. 1103 



body is resting vertically on one leg, say the right, while the 

 other is inclined obliquely behind. The two legs and the plane 

 of the ground form a right-angled triangle, of which the left leg 

 is the hypothenuse, the right angle being between the right 

 leg and the ground. At a certain moment the foot of the right 

 leg will be flat on the ground and the line of gravity will pass 

 through its heel. But the centre of gravity is moving for- 

 wards: even if there had been no previous steps, and so no 

 momentum, the body and with it the centre of gravity, unless 

 prevented by muscular effort, would have fallen forward ; we 

 may therefore speak of the line of gravity as travelling for- 

 wards; it passes from the heel to the toe (of the right foot). 

 If the body were simply falling forwards the centre of the hip- 

 joint would move downwards as well as forwards, describing a 

 circle with the leg as a radius. But at the moment of which 

 we are speaking the (right) leg is somewhat flexed, both at the 

 ankle and still more at the knee. And, as the line of gravity 

 is travelling forward from the heel to the toe, the active part 

 of the performance intervenes. The foot is raised from the 

 ground from the heel forwards, until it is only the ball of the 

 great toe which is resting on the ground, and the whole leg is, 

 by muscular effort, straightened. In this act the right leg acts 

 as a lever, the ball of the great toe serving as a fulcrum ; and 

 the effect of the act is to prevent the centre of gravity, or the 

 hip-joint, from moving downwards, and to carry it forwards 

 only in more nearly a straight line. In thus carrying the hips 

 (and body) forward the leg has changed its position; from 

 being vertical and flexed with the whole sole resting on the 

 ground, it has become inclined forwards obliquely, extended 

 straight, with the toes only resting on the ground. It has 

 assumed the same posture as that of the left leg at the moment 

 at which we started. 



Even at that moment the left leg was behind the line of 

 gravity, and unless it moved would became more and more so as 

 the changes in the right leg went on ; hence if left to itself it 

 would swing forward much as a pendulum which had been 

 raised up would swing forward when let go. And during the 

 changes in the right leg which we have just described the left 

 does swing forward, its movement being chiefly determined like 

 that of a pendulum by gravity, though it may be assisted by 

 direct muscular effort, and is certainly so guided, being for 

 instance slightly flexed during the transit. It swings forward 

 in front of the line of gravity and is thus brought to the 

 ground, the toes in proper walking making contact first and 

 the heel later, though many people who wear shoes bring the 

 heel down at least as soon as the toes. It swings we say in 

 front of the line of gravity; but that line of gravity is travel- 

 ling forwards, so that in a very short time the body is resting 



