130 J^ECTUBE XII. 



gravity would describe, in succession, portions of circles, of which each leg 

 would alternately be the radius: but if the velocity were great enough to 

 create a centrifugal force more than equivalent to the force of gravity, the , 

 pressure would be removed from each leg after the first instant of its touching 

 the ground; the path would become parabolic instead of circular, and the 

 walking would be converted into running: for the difference between walk- 

 ing and running is this, that in running, one foot is removed from the 

 ground before the other touches it ; while in walking, the hindmost foot is 

 only raised after the foremost has touched the ground. Now supposing the 

 length of the inflexible leg three feet, the centrifugal force would become 

 equal to the weight, with a velocity which would be acquired by a heavy 

 body in falling through a foot and a half, that is, near 10 feet in a second, 

 or 7 miles an hour; and this is the utmost velocity with which it would be 

 mechanically possible to walk with inflexible legs. But the flexibility of the 

 iegs makes the progressive motion much more uniform, by softening the 

 angles of the path, which the centre of gravity describes, and rendering it 

 either more or less curved at pleasure; so that it becomes mechanically if not 

 physically possible, to walk with a velocity somewhat greater than 7 miles an 

 hour, and to run or dance with as small a velocity as we please, since we 

 may make the path of the centre of gravity somewhat less, or much more 

 curved, than a circle described on the point of the foot as a centre. (Plate 

 IX. Fig. 115, 116.) 



The flexions and extensions of the legs are also almost the only means 

 by which an impulse is given to the body; if the legs were perfectly inflexible, 

 it would be extremely difficult, although not absolutely impossible, to obtain 

 a progressive motion. The centre of gravity is principally impelled forwards 

 in the beginning of the ascending part of the curve which it describes, in 

 walking, by the action of the leg which is left behind, but in running or 

 hopping, by that of the only foot ^yhich touches the ground at any one time. 

 When we thrust against any obstacle, or draw a rope in a horizontal, or in a 

 descending direction, the* body is inclined forwards, and in some cases its 

 action is limited by the effect of the weight of the body reduced to the direc- 

 tion of the line of draught: but we much more usually draw or \n\\l in an as- 

 cending direction, so that our whole muscular force may be exerted Avithout 

 any limit of this kind. 



