"Great Cæsar," exclaimed Walter, in disgust. "Can't we ever get a good night's sleep?" "Don't look that way," said his chum grimly, as he pulled on his clothes. Outside the tent the lads found the Captain and engineers just emerging from their shelters. Along the road for a mile in front of the machine, huge bonfires were burning. "They have fired the woodpiles!" Charley exclaimed. "Well, let 'em burn. There's more wood where that came from. Let's make for the machine; that's where the shooting came from." A few minutes' walk brought them to the digger, where they found the Spanish guards excited but unhurt. They had fired the guns to let the camp know of the fire. They were so apparently nervous, however, that McCarty volunteered to stay with them the balance of the night. "Well, it might be worse," said Charley, as the little party made their way back to camp. "They have just made more work for the teamster and woodchopper, that's all." But, as they approached close to the camp, they were met by one of the guards. "_Señors_," cried the man, his voice trembling, "there is frightful groaning coming from the darkness behind our picket line." "Where?" demanded the teamster, who had joined the little party. "Toward the North Star, not far from our picket line," answered the shaky sentinel. "Go back to your post, _hombre_," Charley ordered. "We'll get the lanterns and come right out and see what it is." The frightened sentinel obeyed, but he moved so slowly that the boys overtook him before he reached his post. CHAPTER XXIII. EXCITING EVENTS. EVEN before they reached the guard line, the little party could plainly hear the groans that had so frightened the Spanish sentinel. The sounds came from a point some two hundred feet beyond the line. Between the spells of groaning would come noises like a struggle going on, a heavy fall, then more groans. Suddenly the teamster with an oath broke into a run and the boys followed close at his heels. It was a pitiful scene that the lanterns revealed when they reached the spot. The teamster, with tears in his eyes, was swearing vigorously as he untangled the hitching ropes from the legs of the two mules whose sufferings were frightful to behold. Their bellies were swollen up to twice their natural size and their eyes were glassy with pain. Occasionally one would stagger to its feet, stand swaying for a few minutes, then fall heavily to the ground, where it would lay groaning in spasms of pain. "What's the matter with them?" Charley demanded anxiously. "The Lord only knows," said the teamster, "that swelling of the stomach looks as though they had been foundered, but that can't be. I only gave them their usual feed for the night--just what they always have." "Can we do anything for them?" inquired the lad. Canady shook his head. "I am afraid they are too far gone," he said. "But I'll try. I've got all kinds of medicines in my tent. I'll run and get them."