Page:Popular Science Monthly Volume 6.djvu/64

54 body leaves the ground for an instant at each step. Fig. 7 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 essential

 .—D (curve formed by a full line), impact and rise of right foot; G (dotted line), action of the left foot; O, oscillations and vertical reactions of the body.

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 duration 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. 7. The upper line (O) is a tracing of the vertical oscillations in running. It shows that the body executes each of its vertical elevations during the downward pressure 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 oscillations to the pressure of the feet shows plainly that the time of suspension 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