EFFECT OF INSTABILITY OF EQUILIBRIUM ON SPEED. 69 



of it from side to side. As, however, the impetus derived from the hind 

 limb is greater than that from the fore leg, and as the centre of gravity 

 is nearer the latter than the former, there is a certain tendency to lateral 

 displacement, which has to be corrected by muscular effort. This 

 tendency to a rocking movement from side to side is naturally greater 

 in the amble than it is in the trot. In the gallop there is a strong tendency 

 to lateral displacement ; for at this pace, each leg gives its own separate 

 stroke (Figs. 156 to 191). When either of the near ones act, the centre 

 of gravity is canted to the right ; when the off ones make their re- 

 spective effort, it is thrown to the left. The retention of the centre 

 of gravity in a more or less straight line, entails a certain expenditure 

 of muscular work, and consequent fatigue. 



It is evident that the nearer (Fig. 43) a is to b, and d to e, the less 

 will be the lateral displacement of the centre of gravity, and the smaller 

 the loss of speed from this cause. Hence we find that in animals of 

 great speed, like the cheetah, antelope, greyhound, and race-horse, 

 both the fore and hind legs are respectively placed much closer together 

 than in animals of comparatively greater strength, but of slower move- 

 ment. The faster the speed (p. 140), the more do the footprints of the 

 horse tend to come into the line of the direction in which movement 

 is taking place. This principle is well exemplified in man ; for all fast 

 walkers, runners and skaters try to place the feet, at each step, as nearly 

 as possible, in the same straight line. 



As I have already said, the tendency to lateral displacement of weight 

 is corrected to a certain extent, in his various paces, by the horse, who 

 has, however, to do it by a muscular effort which is a lost force, as 

 regards propulsion. 



Effect of Instability of Equilibrium on Speed. — The 



more unstable, in a forward direction, is the equilibrium of a horse's 

 body during each step at any particular pace, the greater will be the 

 speed which can be developed at that pace ; because the more un- 

 stable the equilibrium, the more easily can the centre of gravity be 

 brought forward. This fact needs no mathematical investigation, for 

 we all know that if a person is bending forward, it is much easier to 

 push him to the front, than if he was leaning back. As the area of 

 the base of support depends to a great extent on the number of feet 

 on the ground, it follows that when there is a reduction in the number 

 of these feet, at each step, the instability of equilibrium, and conse- 

 quently the speed, will be increased, as a rule. Thus, in the walk 

 of the horse (p. 108) there are never less than two feet on the ground ; 

 three as often as two ; and sometimes even four. In the canter 

 (p. 116), we have respectively in three steps, a support of one foot, of 

 two (or of three) feet, and of one foot. In the trot and amble, two feet 



