This notation can, with sufficient exactitude, be compared to that which is employed in the musical scale. The horizontal lines 1 and 2 represent the _compass_. We there also see _notes_; these are the bars indicating the pressure, of which the value--that is to say, the duration--is represented by the length of these bars. It is the same with regard to the intervals of _silence_: these are expressed by the intervals which separate the pressures, and correspond to the moments in which, during certain paces, such as running, the body is raised from the ground. Besides, we see intervals of this kind on the notation reproduced (Fig. 118) relative to the running of man. [Illustration: FIG. 118.] [Illustration: FIG. 119.] In order to make the signification of these tracings still better understood, we reproduce four varieties of them (Fig. 119). The first notation is that of ordinary walking. The pressures succeed each other regularly. The second shows what takes place during the ascent of a staircase. At a certain moment, the weight of the body is upon both feet at the same time, one of them not quitting the lower step, until the other is already in contact with the step above. Accordingly, there is thus produced an overriding of the pressures. The third is relative to running, and has already been represented in Fig. 118. The pressures of the feet are separated by the times of suspension. The fourth also represents running, but in this case more rapid and characterized by the shorter pressures, the slightly longer periods of suspension intervals, and the quicker succession of movements. Before putting aside the indications relating to the walking movements of man--indications which it was necessary to give in order to render intelligible those which are connected with the paces of the horse--we have yet to fix the value of that which we call 'a step.' It is generally admitted that a step is constituted by the series of movements which are produced between the corresponding phases of the action of one foot and that of the other--for example, between the moment at which the right foot commences its pressure on the ground and that at which the left foot commences its own. It is necessary to adopt here another method of looking at it, and to regard the preceding as being but a _half-step_. The step should then be defined as being constituted by the series of movements which are executed between two similar positions of the same foot--as, for example, between the commencement of a pressure of the right foot and the similar phase of the following pressure of the same foot. We shall soon understand the importance of this definition. Before entering on the details of the paces of the horse, it is necessary to see how the limbs of the latter oscillate during the period of a complete step; or, which is the same thing, to determine what the displacements are which a limb executes between two similar positions of its foot.