"He'll be killed if he stays on," shouted Nat. "Yes, and he'll be trampled to death if he leaps off," called back John. "He's a balky horse, I guess." "I think he's a mad one." The next instant the animal, that had been rushing straight ahead, came to such an abrupt halt that Jack was actually flung from the saddle. He went right up into the air and slid along the horse's side. Only the grip he had of the neck and the mane saved him from falling. Before the horse could make another start the boy had wiggled back to his seat. Then came what was probably the hardest part of it all. The horse gathered its four feet under it and rose straight up in the air, coming down with legs stiff as sticks. Jack was not prepared for this and the resulting jar nearly knocked the breath from him. "He's a bucking bronco!" cried John. "Rise in your stirrups when he lands next time." This Jack did, with the result that the jar came on his legs, and was not so bad. Finding it could not thus rid itself of it's persistent rider, the horse began to run straight ahead again. It went so fast that the wind whistled in Jack's ears, and he was in fear lest he be thrown off at this terrific speed, and injured. He held on for dear life. But the horse had still another trick. Stopping again with a suddenness that nearly unseated Jack, it dropped to the ground and started to roll over, hoping to crush the boy on its back. "Get out of the way, quick!" called John, who was watching every move. Jack did so, just in time to escape having his leg broken. "The horse must be crazy," said Nat, who had never seen such antics in a steed before. "There's some reason for it," commented John. "There he goes!" The horse was up an instant later, and dashed off, but had not gone a hundred yards before the saddle fell to the ground, the holding straps having broken. At this the animal stopped, and seemed all over its excitement. "That's funny," said John. He dismounted from his horse and ran toward Jack's animal. The horse allowed himself to be taken by the briddle and lead, showing no sign of fear. John bent over and was examining the saddle. "I guess your yell must have scared him," spoke Jack. "It was the worst I ever heard." "It wasn't that," replied John. "Western horses are used to all sorts of yells. Ah, I thought so," he went on, "this explains it." He pulled something from the underside of the pad and held it up to view. It was a long cactus thorn. "That was what bothered the horse," John said. "It must have been torture to have any one on the saddle. See there," and he pointed to several drops of blood on the animal's back. "Why didn't it act so as soon as I got on?" asked Jack. "Some one has played a trick," said John "See, the thorn was trapped in cloth, so the point would not work through until the horse had been ridden some distance. I wonder who did it, and what for?"