MODES OF PROGRESSION USED BY MAN. 125 



apparatus which we have used in our previous researches. 

 The circular and horizontal track on which the experimenter 

 was obliged to walk must be exchanged for surfaces of every 

 kind and of every slope. 



If the new instruments to which we must have recourse 

 leave the experimenter more liberty in his movements, they 

 are, on the other hand, relatively less complete as to the indi- 

 cations which they furnish ; therefore, we can only require 

 from them two kinds of indications ; those of the pressures of 

 the feet on the ground, and those of the vertical re-actions 

 which are communicated to the body by these pressures. 



Fig. 27 shows a runner furnished with apparatus of the 

 new construction. He wears the experimental shoes which 

 we have already described, and holds in his hand a portable 

 registering instrument, on which are traced the curves produced 

 by the pressure of his feet. As the cylinder of this instru- 

 ment turns uniformly, the curves will be registered in propor- 

 tion to the time, and not to the space traversed during each 

 of the acts by which this curve is traced. 



In order to facilitate the experiment, and to allow the 

 apparatus to assume a uniform motion before it traces on the 

 paper, we have recourse to a special expedient. The points 

 of the tracing levers do not touch the cylinder ; but in order 

 to bring them in contact with the paper, an india-rubber ball 

 must be compressed. As soon as this compression ceases, the 

 points retreat from the cylinder, and the tracing is no longer 

 produced. In fig. 27 the runner holds this ball in his left 

 hand, and compresses it with his thumb. 



In addition to this, the runner, in fl'der to obtain the 

 tracings of the vertical re-actions, carries on his head an 

 instrument whose arrangement is represented in fig. 28. 



It is an experimental lever- drum fixed on a piece of wood, 

 which is fastened with moulding wax on the head of the ex- 

 perimenter, as seen in fig. 27. The drum is provided with 

 a piece of lead placed at the extremity of its lever ; this mass 

 acts by its inertia. 



While the body oscillates vertically, the mass of lead resists 

 these movements, and causes the membrane of the drum to 

 sink when the body rises, and to rise when the body descends. 



