FOR a few minutes Charley was almost a prey to vague, suspicious fears, which lie hidden deep in most of mankind. The suddenness of his chum's disappearance, the ages-old stockade, the ancient fort, with its grewsome occupants, all gave force to weird imaginings; but, with an effort, the practical lad shook off his gloomy thoughts with the simple logic that age is no more mysterious than youth, and that dead men are less to be feared than live ones. But, in spite of his sound reasoning, the worried lad could not imagine what had become of his chum. He was not in the stockade; he was not in either clearing; he was not among the orange trees; he was not back with the ponies, yet he had passed out of the fort not five minutes ahead of himself, but at this point in his reasoning Charley gave a start. He had found the flaw in his own logic. He had no proof that Walter had passed out of the fort. Affected as the lad had been by the grewsome sights, he might have fainted before reaching the open air and he might well have passed him by in the dim light without noticing him. Hastily gathering some dry sticks, Charley held them in one hand and fired the ends. As soon as his torch was blazing good, he entered the fort, and, holding it aloft, inspected the lower chamber. Near the middle of the chamber he found Walter's rifle lying on the stone floor, but a close search showed no other trace of the missing lad. Puzzled, he ascended to the upper chamber, but here he found everything as he had left it, and he descended again to the lower chamber, convinced that in it must lie the key to the mystery, for he was certain Walter would not have left the fort without taking his gun with him. Walking around the stone chamber, Charley held his torch aloft and inspected the solid floor and walls, in the vain hope of discovering some clew to his chum's mysterious disappearance. Suddenly he gave a frightened cry, and flung out his arms to save himself, for something had given way beneath his feet, and he felt himself sinking downward. Fortunately, his instinctive action had been so quick that his extended arms caught on the stone floor and saved him from sinking into the gaping black hole beneath him. Summoning up all his strength, the lad drew himself up out of the trap into which he had partly fallen, and, seizing the torch he had dropped, surveyed the spot. A large stone slab was slowly lifting back into place. In a flash, the lad grasped the situation. The slab had been so cunningly contrived as to appear part of the solid floor, but, when a person stepped on one end the slab would tilt down, sending the victim down to the depths below, and, when his weight was removed, the slab would tilt back into place again.