William Cavendish may refer to: (wikipedia)
You may observe in all my lessons, that I tell you how the legs go, and those who are unacquainted with that, are entirely ignorant and work in the dark.
Now being upon the haunches (as he necessarily must be in this case) is it impossible but he must be light in hand, because no horse can be rightly upon his haunches without being so.
And he that said that a horse was not dressed, whose curb was not loose, said right; and it is equally true that the curb can never play, when in its right place, except the horse be upon his haunches.
Without knowing this, no man can dress a horse perfectly.
But there is nothing to be done till a horse's head is settled.
The main secret for a horse that is heavy upon the hand, is for the rider to have a very light one; for when he finds nothing to bear upon with his mouth, he infallibly throws himself upon the haunches for his own security.
You should pull him back besides in all the lines before the quarter, just as you make the others advance.
But we ought to consider the natural form and shape of a horse, that we may work him according to nature.
I would have you fasten instead of holding it, the rein that comes back to your hand to the pommel of the saddle, because it has the greater force.
But what for the generality I call a light or gentle hand, is at the same time as light as a feather, and yet firm, except in extraordinary cases.