TIIK IIORSK IN MOTION. 97 



It seems, at first thought, from the manner in which the labor is 

 thrown from the leading fore foot to the diagonal hind one, during 

 which the body has no support from cither, that the theory of constant 

 support and continuous propulsion does not hold true, that a perfect 

 machine should require no such hiatus. If the machine had been 

 constructed of inorganic and inanimate material, incapable of fatigue, 

 it could have been so arranged that the hiatus would not have been 

 necessary; but the Creator did not, if he could, build an animate 

 machine that would not tire. The animal is shown in the above 

 figures as moving his feet in the same order, and, but for some ar- 

 rangement that would permit of a change, fatigue would be inevitable; 

 but that change would not be possible until all the feet are clear of 

 the ground. If the attempt should be made while one foot is on the 

 ground, the result would be called a misstep and a fall. The opportu- 

 nity is afforded when the extraordinary propulsive force, given by the 

 fore leg that leaves the ground last, projects the body upward, giving 

 a time equal to one fifth of a stride for the hind foot of the same side 

 to take the place of the one that would have followed had the same 

 order continued. The coincidence of the act of changing the order of 

 the feet with the exposure of the negative plate in the camera, which 

 is too short a time for computation, must be very rare, and has not 

 been observed in any of the many pictures taken at Palo Alto ; but 

 the change is felt by the driver when the feet again take the ground, 

 and it is said to have been discovered in the changed order of the 

 footprints. 



While the run requires that each limb in turn should act as 

 propeller and supporter in regular order, it cannot be executed at 

 a low rate of speed, for the base of support is confined to one foot, 

 and it must be rapidly adjusted to the changes of the position of the 

 centre of gravity, for the same reason that a boy on stilts requires 

 to be continually in motion. If the horse's speed is diminished to 

 a great degree, he will change to a canter, which is a pace modified 

 from the run, as may be seen by reference to Plate XXVII., or, 

 what is more usual, to a trot, in which he uses two diagonal feet as 

 bases of support. 



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