PROGRESSIVE MOTION IN QUADRUPEDS. 493 



foot is next raised and advanced : and then the 

 same succession of actions takes place with the 

 hind and the fore foot of the other side. 



An attentive examination of the conditions of 

 these successive positions will show that, amidst 

 all the changes which take place in the points of 

 support, the stability of the body is constantly pre- 

 served. It is an elementary proposition in me- 

 chanics that all that is necessary for ensuring the 

 support of a body on any given base, is that the 

 vertical line drawn from the centre of gravity 

 shall fall within that base. When the animal is 

 standing, the feet form a quadrilateral base, and 

 the centre of gravity is in a vertical line passing 

 either through the centre of the base, or, as, for 

 the reasons already mentioned, more frequently 

 happens, through a point a little in front of the 

 exact centre. At the time when the hind foot 

 which began the action is raised from the ground, 

 the centre of gravity, having been, by that action, 

 impelled forwards, still remains above the base 

 formed by the other three feet, and which is now 

 reduced to a triangle. That hind foot being set 

 down, while the corresponding fore foot is raised, 

 a new triangular base is formed by the same 

 hind foot, together with the two of the other side, 

 which have not yet been raised. The centre of 

 gravity is still situated above this new triangle, 

 and the body is consequently still supported on 

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



