seen in Fig. 3. 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. 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.
A complete horizontal oscillation occupies the time of two steps, and, consequently, of two vertical oscillations. The body is carried toward the right side, at the moment of the maximum of elevation, which corresponds with the middle of the pressure on the right foot, and toward the left at the middle of the pressure on the left foot. This lateral swaying of the trunk is the consequence of the alternate passage of the body into a position sensibly vertical over each foot.
The body is advancing at every moment during the step, but at some parts of it more rapidly than at others. The greatest rapidity of advance is at the end of the pressure of the foot.
With this brief sketch of the movements of the limbs and body in walking, and of the apparatus employed by M. Marey for studying these movements, we are prepared to consider the different paces common to man.
Fig. 6.—Tracing produced by Walking Up-stairs.—D, tracing of the pressure and rise of the right foot (full line); G, tracing of the left foot (dotted line).
In walking, the body does not leave the ground, the footsteps follow each other without any interval, and the weight of the body passes alternately from one foot to the other. The tracings in Fig. 4, obtained by walking on a level surface, illustrate these points. There are exceptions, however, to this definition. For example, in mounting a staircase it will be observed that the step-curves encroach on each other (Fig. 6), showing that each foot is still pressing on its support 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. Nothing like this is observed in the descent of a staircase; the step-curves cease to encroach on each other, following one another very much as in ordinary walking on level ground.
Running, though more rapid than walking, consists like it in alternate treads of the two feet, whose step-curves follow each other at equal intervals; but it presents this difference, that in running the
