5 z THE POPULAR SCIENCE MONTHLY. 



constitute a sort of dynameter of pressure ; they compress the drum 

 less or more according to the effort they exert, and consequently trans 

 mit 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 esti- 

 mated at about forty pounds, and it is much greater in running and 

 leaping. 



There are certain oscillations of the body, both vertical and hori- 

 zontal, produced by the actions of the legs, which M. Marey has care- 

 fully traced, but which, owing to their extreme complexity, are diffi- 

 cult to explain. We shall therefore pass them with only a glance, 

 referring the reader to the work itself for details. With eacli 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 





