[Illustration: FIG. 115.--EXPERIMENTAL SHOES, INTENDED TO RECORD THE PRESSURE OF THE FOOT ON THE GROUND.]
When the membrane, as we have just seen, is elevated, the style is lifted, and then descends when the pressure of the foot ceases. It traces these displacements on a leaf of paper, the surface of which is covered with a thin layer of lamp-black, which it removes by its contact; different parts of this surface are successively presented to it, the paper being rolled round a cylinder which is turned on its axis by means of a clockwork movement. It is necessary to add that the inscription is made, in the study of the walk of man, by means of two styles, each corresponding to one of the feet.
The tracings thus obtained, which are read from left to right, are sufficiently simple; but to understand them properly, it is necessary to remember that the style undergoes a movement of ascensional displacement during each pressure of a foot, and that, on the other hand, it descends when the latter is separated from the ground. We also see, on the tracing which it leaves, a line which ascends and then descends; the meaning of this is that first the foot presses on the ground, and is afterwards raised from it.
[Illustration: FIG. 116.--RUNNER FURNISHED WITH THE EXPLORATORY AND REGISTERING APPARATUS OF THE VARIOUS PACES.]
On the tracing (Fig. 117), the line D relates to the right foot; the line G, which is dotted so that it may not be confused with the preceding, corresponds to the left foot. The line G first ascends; the meaning of which is that the left foot presses on the ground; afterwards it descends: this indicates that the pressure of the foot has ceased. It is the same for the right foot. As we see, the pressures succeed each other; when the left foot touches the ground, the right is separated from it; when the latter presses the ground, it is the left which no longer rests there.
The line O is related to the movements of the body, as indicated by the oscillations of the head. We will neglect these.
But this tracing, which serves us for an example, is not, it must indeed be said, of very easy reading; it would be still less so if the paces of a horse were registered, for there would then be four lines, the entanglement of which would cause greater complication.