When he grasped the first horn of the last head, the hand that did so was shaking with excitement and suspense. It seemed, like the rest, to possess no attribute other than mere decoration. And yet, and yet--surely he had missed some vital point. He would go over them again. There remained, however, the last horn, and as he took hold of it with a premonitory dread of disappointment, he felt that it was loose in its socket, and that he could by an effort turn it completely over. With a triumphant cry he twisted it round, and at the same moment Lady Ruth started back with an exclamation of alarm. She was standing where he had left her, and was nearly knocked down by the great slab of stone which, as Gimblet turned the horn of the bull, swung sharply out from the end of the pediment, till it hung like a door invitingly open and disclosing a hollow chamber within the stone. Within the opening, on the floor at the far end, stood a large tin despatch-box. The door was a good eighteen inches wide; plenty of room for Gimblet to climb in, swollen with exultation though he might be. In less than three seconds he had scrambled through the aperture and was stooping over the box. It seemed to be locked, but a key lay on the top of the lid. He lost no time in inserting it, and in a moment threw open the case and saw that it was full of papers. Suddenly there was another cry from Lady Ruth as, for no apparent cause and without the slightest warning, the stone door slammed itself back into position, and he was left a prisoner in the total darkness of the vault. He groped his way to the doorway and pushed against it with all his strength. He might as well have tried to move the side of a mountain. But, after an interval long enough for him to have time to become seriously uneasy, the door flew open again, and the agitated countenance of Lady Ruth welcomed him to the outside world. "Do get out quick," she cried. "If it does it again while you're half in and half out, you'll be cracked in two as neatly as a walnut." Gimblet hurried out, clutching the precious box. No sooner was he safely standing on the turf than the door shut again with a violence that gave Pandora the appearance of shaking with convulsions of silent merriment. "I wasn't sure how it opened," said Lady Ruth, "but I tried all the horns and got it right at last. How lucky I was with you!" "Yes, indeed," said Gimblet. "I am very thankful you were." They twisted the horn again, and stood together to watch the recurring phenomenon of the closing door. "It must be worked by clockwork," the detective said, and taking out his watch he timed the interval that elapsed between the opening and shutting. "It stays open for thirty seconds," he remarked after two or three experiments. "No doubt the mechanism is concealed in the thickness of the stone. At all events it seems to be in good working order."