right hind leg, therefore, stops, and the left hind leg moves forward into position for the impulse, while the right hind leg becomes the support. The right front leg also becomes a support; but the left fore leg extends forward to receive the impulsion. It follows, then, that before the left hind leg has made contact with the ground and taken over the duty of impulsion, the horse is upon a diagonal biped. (Figure 24.)
In order, therefore, to execute the change of lead from right to left, the cavalier should, at the instant of change, lean to the right, in order to load the right lateral biped. This, thereupon, becomes the support, and leaves the left lateral biped unloaded and off the ground for the very quick movement called "change of foot in the air." This whole motion, but especially the action of the left hind leg, is so rapid that the eye cannot perceive the relations of the different limbs. Even photography is inadequate to show the action clearly. The camera can, however, be made to exhibit the left hind leg in the different parts of its stride. Thus in Figure 25, the left hind leg first disappears behind the right; and next after that the right fore leg is flexed. Finally, in Figure 26, the left hind leg is on the ground, in front of the right and ready for propulsion. The left fore leg is already raised. It will extend forward as soon as the right hind leg has arrived near the left, to assist as support and thus allow the left hind leg to continue the impulse.
