WALKING}. 115 



sixlli prrt cf tlic dura! ion of a single impact or pressure of 

 the loot. 



Intensity of the pressure of the foot upon tlie ground. The 

 curves traced by walking 1 may also furnish the measure of the 

 effort exerted by the foot upon the ground. The experimental 

 shoes constitute a kind of dynameter of pressure; they com- 

 press the drum, less or more, according 1 to the effort they 

 exert ; and consequently they transmit to the registering level- 

 more or less extensive movements. In order to estimate, 

 according to the elevation of the curve, the pressure exerted 

 by the foot, we must substitute for the weight of the body a 

 certain number of kilogrammes. We see thus that, if the 

 weight of the body (75 It ilograi nines, for example) is sufficient 

 to raise the lever to the height which it attains at the com- 

 mencement of each curve, an additional weight will be required 

 to raise it to the maximum elevatiou which it attains towards 

 the end of its period of pressure. 



That proves that, in walking, the pressure of the foot on 

 the ground is not only equal to the weight of the body which 

 the foot has to sustain, but that a greater effort is produced at 

 a given moment in order to give the body the movements of 

 elevation and progression which we have just been studying. 



According to Mons. Cavlet, this additional effort is not more 

 than 20 kilogrammes, even in rapid walking, but it is much 

 greater in running and leaping. 



Reactions. We shall designate by this name the movements 

 \\hich the action of the leg produces on the mass of the body. 

 These movements are very complex ; they are effected at the 

 same time in every direction, and give to the trajectory 

 which a point of the body describes in space, some very com- 

 plicated sinuosities. The graphic method alone can enable 

 us, at least as yet, to appreciate the real nature of these 

 movements. 



In the first place, what point of the body shall we choose 

 in order to observe the displacement caused by the act of 

 walking? Almost all authors have taken for this purpose the 

 centre of gravity, the point which Borelli places inter nates ct 

 pulim. But if we reflect that the centre of gravity changes 

 as soon as the body moves, that in the flexion of the legs this 



