54 



THE POPULAR SCIENCE MONTHLY. 



body leaves the ground for an instant at each step. Fig. V shows the 

 principal characters of this mode of progression. The pressures of the 

 feet are more energetic than in walking ; in fact, they not only sustain 

 the weight of the body, but impel it with a certain speed both upward 

 and forward. It is clear that, to give a mass a rising motion, a greater 

 effort must be exerted than would be sufficient simply to sustain it. 

 The duration of the pressures on the ground is less than in walking ; 

 this brevity is proportional to the energy with which the feet tread 

 on the ground. These two elements force and brevity of pressure 

 increase generally with the speed at which a person runs. The essen- 



Fig. 7. Tracing produced by Running (in Man). D (curve formed by a full line), impact and 

 rise of right foot ; G (dotted line), action of the left foot ; 0, oscillations and vertical reac- 

 tions of the body. 



tial character of running is the time of suspension, during which the 

 body remains in the air between two foot-falls. Fig. 7 clearly shows 

 the suspension by the interval which separates the descent of the curves 

 of the right foot from the ascent of the curves of the left foot, and vice 

 versa. The dui-ation of the time of suspension appears to vary but 

 little in an absolute manner ; but, if we compare it with the speed of 

 the runner, we see that the relative time occupied by this suspension 

 increases with the speed of the course, for the duration of each tread 

 diminishes in proportion to this speed. How is this suspension of the 

 body, at each impulse of the foot, produced ? We might at first think 

 that it is the effect of a kind of leap in which the body is projected 

 upward in so violent a manner, by the impulse of the feet, that it 

 would describe in the air a curve, in the midst of which it would attain 

 its maximum elevation from the ground. We may convince ourselves 

 that such is not the case by reference to Fig. V. The upper line ( 0) 

 is a tracing of the vertical oscillations in running. It shows that the 

 body executes each of its vertical elevations during the downward press- 

 ure of the foot, so that it begins to rise the moment the foot touches 

 the ground ; it attains its maximum elevation at the middle of the 

 pressure of this foot, and begins to descend again in order to reach its 

 minimum at the moment when one foot has just risen, and before the 

 other has reached the ground. This relation of the vertical oscilla- 

 tions to the pressure of the feet shows plainly that the time of suspen- 

 sion does not depend on the fact that the body projected into the air 

 has left the ground, but that the legs have withdrawn from the ground 





