Page:Equitation.djvu/250

 concentric circles, as the center of gravity travels from O to M; otherwise the equilibrium will be lost. Evidently, the two left legs must travel farther than the two right legs.

But the length of the stride does not affect the velocity or the momentum; consequently, it does not change the center of gravity. The left front leg, if it is to gain more ground than the right, must be unloaded, since the rule is that any leg cannot leave the ground before the weight which it carries has been transferred to another support. This we accomplish by flexing the head slightly to the left, and at the same time we establish the fixed point of the rhomboidus and mastoido-humeralis muscles at the left side of the atlas region. Thereupon the unloaded left shoulder will cover the longer distance CL while the loaded right shoulder is covering the shorter distance DP and serving as pivot and support for the center of gravity, which remains on OM.

But for the impulsion of the hind quarters, both the rider's legs are necessary. The left prevents the haunches from yielding to the effect of the right, and thus departing from the proper path ACL. The right leg of the rider pushes forward the right hind leg of the horse, and since this is prevented from moving toward the left off the line BDP, the center of gravity must remain on the line OM; and momentum, velocity, and equilibrium remain altered.