He grew thirsty and started to kneel down by the side of the pool to drink. Then he remembered that the horses had quenched their thirst on this side of the water-hole, and the water was likely to be roiled and muddy. So he started around toward the other side. The water-hole was twenty yards across and its edge was screened by bushes and brakes for most of the way. Dig looked for an opening where he could kneel and reach the water, intending to fill his canteen and bring it back with him to the camp. Poke stamped and whinnied; but Dig did not hear his mount. He kept on until he was fully half way around the water-hole. The plain seemed quite as silent and deserted as before. He could not see the spot where his chum lay nor even the gleam of the firelight now. Chet was quite given up to sleep. He lay on his back with the neck of his shirt open. He did not hear the restlessness of the horses, nor any other sound about the camp. Not at first, at least. But when a rifle exploded somewhere near, Chet Havens awoke with a start. “Hi! what’s that?” he ejaculated, and sat up suddenly, throwing off the final restraining folds of the blanket. “Dig! where are you?” he added and, getting no answer, he scrambled to his feet and picked up his own rifle that had been lying partly under him. His chum was nowhere to be seen. He shouted again: “Dig! Dig!” and then strained his ear to catch the reply. But there was no immediate answer and Chet found himself shaking with apprehension. What had become of his chum? CHAPTER XXI—A STARTLING DISCOVERY Chet’s second thought was, naturally, for the horses. If anything happened to their mounts out here on the plains, they would be in a bad way indeed. They were all of thirty miles from the Grub Stake trail, and if that trail were intersected with a line running directly south from this camp, such intersection would be about midway of the distance between Silver Run and Grub Stake. In other words, once back upon the trail the boys would have a choice of something like a hundred mile ride to either town. And if they had to walk it! With his rifle at “ready” Chet stumbled away from the edge of the water-hole until he could get a free sight of the plain on this side. He made out the horses almost immediately. They were feeding contentedly and nothing seemed to have happened to them. Chet raised his voice again and shouted for his chum. There was no reply, and the boy became more and more anxious as the moments passed. Where could Dig have gone? It was just then that Chet heard a strange sound. It must have been going on ever since he was aroused; only his senses had been too dulled with sleep to notice it. A throbbing sound, that was steadily growing fainter. The boy suddenly came to a sensible conclusion regarding it, and he dropped to his knees and put an ear to the ground.