On that evening, also, Bull made his next step forward toward the most difficult proposition of all—­he took a partly filled barley sack and put it on the back of Diablo.  The next moment the sack was shot into the air as Diablo leaped up and arched his back like a cat at the height of his leap.  He came down trembling and snorting, but Bull picked up the fallen sack and allowed him to smell it.  Diablo found that the smell was good and that the hateful sack even contained things very good to eat.  The next time the sack was put on his back he quivered and shrank, but he did not buck it off.

After that, Bull spent his evenings in gradually increasing the weight of that sack until a full hundred pounds caused Diablo no worry whatever, and when this point had been attained, Bull decided that he might venture his own bulk on the back of Diablo.  He confided his purpose to Tod, and the boy, greatly excited, hid himself at a distance to watch.

In the beginning it was deceptively easy.  Diablo stood perfectly unconcerned as Bull raised himself on the bars of the fence.  And when the long legs of Bull were passed over his back, Diablo merely turned his head and sniffed the shoe tentatively.  Slowly, very softly, steadying himself on the top bar of the fence, Bull lowered his weight more and more until the whole burden was on the back of the stallion—­and then he took his hands from the top rail.

But the moment he released that grip there was a change in Diablo, as though he realized that the man had suddenly trusted himself entirely to his mount.  Bull felt a sudden wincing of all that great body; the quarters sank and trembled.  He thought at first that it was because the horse was failing under the weight of this ponderous burden; but instinct told him a moment later that it was fear, and a mixture of suspicious anger.

Diablo took one of his long, catlike steps, and paused without bringing up his other foot.  In vain Bull spoke to him, softly, steadily.  Diablo took another step, quickened to a soft trot, and stopped suddenly.  That weight on his back failed to leave him.  He began to tremble violently.  Bull felt the sudden thundering of the great heart beneath the pressure of his knee.

To the stallion, this man had been a friend, a constant companion.  The touch of his hand was pleasant.  Pleasanter still was the continual deep murmur of the voice, reassuring, telling him of a superior and guardian mind looking out for his interests.  Now that hand was stroking his sleek neck and that voice was steadily in his ear.  But the position was the most hated one.  To be sure, there was no saddle, no cutting, binding cinch, no drag of cruel Spanish curb to control his head, no tearing spurs to threaten him.  But his flanks twitched where the spurs had dug in many a time, and he panted, remembering the cinches.  Those memories built up a panic.  He became unsure.  The voice reached him less distinctly.  Moreover it was a strange time of the evening.  The light of the day was nearly done; the moon was barely up, and all things were ghostly and unreal in that slant light.