the transverse dilatation of muscular fibre in the process of contraction. If we substitute in this figure an experimental shoe for each of the myographical clips, 1 and 2, we shall have the arrangement of the apparatus necessary for the study of footsteps or impacts of the foot on the ground.
The entire apparatus, as adjusted to the person of the operator, is seen in Fig. 3. The piece of machinery on the head will be described when we come to speak of the movements communicated to the trunk by the actions of the legs.
To understand the indications which the working of the apparatus gives, the reader is referred to Fig. 4, which has been furnished by an experiment in walking. Two tracings are given by the intermittent pressure of the feet on the ground. The full line D corresponds with the right foot; the dotted line G with the left. The lines are read from left to right.
Knowing the arrangement of the apparatus, we can understand that each impact of the foot on the ground will be represented by the elevated part of the corresponding curve. The pressure of the foot on the ground compresses the India-rubber sole and diminishes the capacity of the included air-chamber; a part of the contained air escapes by the connecting tube, and passes into the registering drum. The elastic side of the latter is thus elevated, carrying up the point of the lever, which in turn leaves the mark of its movement on the paper carried by the revolving cylinder. Were the lever to remain undisturbed, it would simply make an horizontal line running round the cylinder; but, with the apparatus in use, each impact of the foot lifts the pen-point of the lever, thus giving the curves traced in the figure. It will be seen that the pressure of the right foot commences at the moment when that of the left begins to decrease; and that in all the
tracings there is an alternation between the impacts of the two feet. The period of support of each foot is shown by an horizontal line which joins the minima of two successive curves. The impacts of the right and left feet are seen to have the same duration, showing that the weight of the body passes alternately from one foot to the other. It would not be the same in respect to a lame person; lameness corresponds essentially with the inequality of the impacts of the two feet. The curves traced by walking may also furnish the measure of the effort exerted by the foot upon the ground. The experimental shoes