MODES OF PROGKESSION USED BY MAN. 127 



From these alternate actions a current of air results, which, 

 transmitted by a tube to a registering lever, shows by a curve 

 the oscillatory movements of the body. 



Fig. 28.— Instrument to register the vertical re-actions during the various paces. 



We will not enter into the details of the experiments which 

 have served to verify the exactitude of the tracings thus 

 .obtained; they consisted in adjusting the weight of the disc 

 of lead and the elasticity of the membrane of the drum, 

 until the movements given to the apparatus are faithfully 

 represented in the tracing. 



We will call step- curves each of the curves formed by the 

 pressure of a foot upon the ground, and we will designate by 

 the name of ascending or descending oscillations, the curve of 

 the vertical re-actions on the body. 



1. Of walking. — We have already pointed out the distinc 

 tive character of walking considered as one of the modes of 

 progression in man. We have said that the body, in walking, 

 never leaves the ground, and that the footsteps follow each 

 other without any interval, so that the weight of the body 

 passes alternately from one foot to the other. 



But this definition cannot apply to walking on an inclined 

 surface, on yielding soil, or upstairs. Being obliged to pass 

 rapidly over these peculiar conditions of walking, we will only 

 give the tracing which corresponds with the act of mounting 

 a staircase (fig. 29). 



It is to be remarked that the step-curves encroach on each 

 other, showing that each foot is still pressing on the ground, 

 when the other has already planted itself on the next step. 

 Besides this, it is at the time of this double pressure that 

 the lower foot exerts its maximum force ; it is at this moment, 

 in fact, that the work is produced which raises the body to 

 the whole height of a step. 



