constitute a sort of dynameter of pressure; they compress the drum less or more according to the effort they exert, and consequently transmit to the registering lever more or less extensive movements. In order to estimate, according to the elevation of the curve, the pressure exerted by the foot, we must substitute for the weight of the body a certain number of pounds. We see thus that, if the weight of the body (150 pounds for example) is sufficient to raise the lever to the height it attains at the commencement of each curve, an additional weight will be required to raise it to the maximum elevation which it attains toward the end of its period of pressure. This proves that, in walking, the pressure of the foot on the ground is not only equal to the weight of the body which the foot sustains, but that a greater effort is produced at a given moment in order to elevate and move the body forward. This additional effort, in a man of average weight, is estimated at about forty pounds, and it is much greater in running and leaping.
There are certain oscillations of the body, both vertical and horizontal, produced by the actions of the legs, which M. Marey has carefully traced, but which, owing to their extreme complexity, are difficult to explain. We shall therefore pass them with only a glance, referring the reader to the work itself for details. With each step there is an up-and-down movement of the body, which varies with the length of the step and the rapidity of the pace. In ordinary walking it has an amplitude of from half to three-quarters of an inch. The maximum of these vertical oscillations is constant, and occurs during the pressure of the foot upon the ground, at the moment when the leg is brought into a straight position. The minima, and consequently the extent of the oscillations, will be determined by the length of the step; the longer the step the greater the obliquity of the legs, and, of
Fig. 5.—Instrument to register the Vertical Reactions during the Various Paces.
course, the greater the lowering of the trunk. Put in another form, it amounts to this: in ordinary walking, the body does not rise above the line of its greatest height when standing still, and the distance which it sinks below this line will increase as the length of the step increases.
The instrument by which M. Marey obtains the tracings of these vertical reactions is represented in Fig. 5.
It is an experimental lever-drum, fixed on a piece of wood, which is fastened with moulding-wax on the head of the experimenter, as
