494 THE MECHANICAL FUNCTIONS. 



vanced without endangering the stability of the 

 body : and by the time this foot is set down, and 

 has thereby formed a new quadrilateral basis 

 with the other feet, the centre of gravity has 

 arrived above the centre of this new base. But 

 at this moment the centre of gravity is again 

 urged forwards by the other hind foot, which now 

 comes into action, and repeats on the other side 

 the same succession of actions, which are at- 

 tended with the same consequences as before. 

 Thus, during its whole progress, the animal is 

 never for an instant in danger of falling ; for 

 whichever of the feet may be raised from the 

 ground, the other three feet are always so placed 

 as to form a stable base of support. 



In quick walking it often happens that qua- 

 drupeds raise their fore foot on either side a little 

 before the hind foot comes to the ground. This 

 is shown by the impression made by the latter 

 being in the same spot, or even rather in ad- 

 vance of the impression made by the former. 

 But the time during which the body is thus sup- 

 ported only by two feet is so short as not sen- 

 sibly to influence the results. 



In consequence of the obliquity of the alternate 

 impulses given to the centre of gravity by the 

 successive actions of both the hind legs, a slight 

 degree of undulation is occasioned ; but these 

 undulations are only lateral. A trot may be con- 

 sidered as a succession of short leaps made by 



