100 LECTURE XII. 



thousand pounds. In all these cases the muscles principally employed are 

 the extensors of ^he legs and thighs, hut the passive strength of the hones is 

 more concerned than the active force of the muscles. In the instance, 

 mentioned by Lahire,* of a young man who raised an ass from the ground 

 by cords tied to the hair of his head, the sensibility of the nerves of the 

 skin must have been diminished by habit, so as to allow the hair to be thus 

 forcibly extended without immoderate pain. 



The application of animal force is usually performed by means of a 

 progressive motion. The muscles employed in this process are in general, 

 if not always, the strongest of the body, both by nature and by habit ; so 

 that when force alone is required, it is most advantageously obtained from 

 their exertions. In walking, the centre of gravity is moved forwards with 

 a velocity nearly uniform. If the legs were perfectly inflexible, the centre 

 of 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 be- 

 tween walking and running is this, that in running, one foot is removed 

 from the ground before the other touches it ; while in walking, the hind- 

 most 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 legs makes the progressive motion much more uni- 

 form, 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 impos- 

 sible, 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 which 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 direction of the line of draught : but we much 



* Hist, et Mem. de 1'Acad. 1699, p. 153, H. 96. 



