PROGRESSIVE MOTION IN QUADRUPEDS. 341 



already mentioned, more frequently happens, through a point 

 a little in front of the exact centre. At the time when the 

 hind foot which hegan the action is raised from the ground, 

 the centre of gravity, having heen by that action impelled 

 forwards, still remains above the base formed by the other 

 three feet, and wdiich is now reduced to a trian^i-lc. That 

 hind foot being set down, while the corre?pondino- fore foot 

 is raised, a new triangular base is formed by the same hind 

 foot, together witli the tw^o of the other side, which have 

 not yet been raised. The centre of gravity is still situated 

 above this triangle, and the body is consequently still sup- 

 ported on these three feet. The fore foot may now be ad- 

 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 o-ra- 

 vity has arrived above the centre of this new base. But at 

 this moment the centre of gravity is ao;ain urn-ed for- 

 wards by the other hind foot, which now comes into action 

 and repeats on the other side the same succession of actions 

 which are attended 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 quadrupeds 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 on the same spot, or even rather 

 in advance of the impression made by the former. But 

 the time during which the body is thus supported only by 

 two feet is so short as not sensibly 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 occa- 

 sioned; but these undulations are only lateral. A trot may 

 be considered as a succession of short leaps made by each 

 set of feet taken diagonally; that is, by the right fore foot, 

 and the left hind foot; or, vice versa, the one set being raised 



