136 ANIMAL MECHANISM. 



curves given before, for it does not indicate the phases of 

 variable pressure exerted by the foot upon the ground ; but it 

 is much more simple, and allows the two modes of progression 

 to be compared much more easily than the other. It will be 

 seen farther on, when speaking of quadrupedal locomotion, 

 that the complication of the subject renders it indispensable 

 to employ this very simple notation of the rhythm of move- 

 ment. 



Definition of a pace in any kind of progression. It is usually 

 considered that a pace is produced by the series of movements 

 which are executed between the action of one foot and that 

 of the other, whether we choose for the commencement of 

 the pace the instant that the feet reach the ground, or that 

 when they rise from it. Thus, in measuring a pace on the 

 ground, we usually take as its length the distance which 

 separates one portion of the print of the right foot from 

 a similar point of the impression made by the left. 



We shall be obliged to depart from this usage. Although 

 we regret any innovation, yet we shall consider the standard 

 pace only as half a pace, and we shall thus define it : A pace 

 is the series of movements executed between two similar positions of 

 the same foot between the two successive treads of the right 

 foot, for example, or two successive elevations of the left 

 foot, &c. 



In the same manner the extent of a pace on the ground 

 will be the distance which separates two homologous points 

 taken in the two successive impressions of the same foot. 

 The pace is estimated in this manner in Mexico. This is the 

 only method of counting which will prevent errors in the very 

 complicated moments of quadrupedal progression. 



SYNTHETIC REPRODUCTION OF THE MODES OF PROGRESSION 



EMPLOYED BY MAN. 



Since wo have completed the analysis of a phenomenon of 

 which we now seem to understand all the details, it is by 

 synthesis that we will endeavour to construct a counter-proof. 

 This method has proved very useful in verifying our theories 

 concerning certain physiological actions, as, for instance, the 

 circulation of the blood. It consisted in representing, by arti- 



