The floor they were on was fairly well lighted with the usual oval windows. The space was open, except that it contained the same kind of dividing walls they had found in the library. Here, however, each compartment contained but one opening, and that not uniformly placed. In fact, as the three noted with a growing uneasiness, it was necessary to pass through every one of them in order to reach the corner farthest, from the ladderlike stairs. Why it should make them uneasy, neither could have said. When they were almost through the labyrinth, Van Emmon, after standing on tiptoes for the tenth time, in order to locate himself, noted something that had escaped their attention before. "These compartments used to be covered over," he said, for some reason lowering his voice. He pointed out niches in the walls, such as undoubtedly once held the ends of heavy timbers. "What was this place, anyhow? A trap?" Unconsciously they lightened their steps as they neared the last compartment. They found, as expected, that it was another stairwell. Van Emmon turned the light upon every corner of the place before going any further; but except for a formless heap of rubbish in one corner, which they did not investigate, the place was as bare as the rest of the floor. Again they climbed, this time for a much shorter distance; but Jackson, slightly built chap that he was, needed a little help on the steep stairs. They were not sorry that they had reached the uppermost floor of the mansion. It was somewhat better lighted than the floor below, and they were relieved to find that the triangular compartments did not have the significant niches in their walls. Their spirits rose perceptibly. At the corner farthest from the stairs one of the walls rose straight to the ceiling, completely cutting off a rather large triangle. The three paid no attention to the other compartments, but went straight to what they felt sure was the most vital spot in the place. And their feelings were justified with a vengeance when they saw that the usual doorway in this wall was protected by something that had, so far, been entirely missing everywhere else. It was barred by a heavy door. For several minutes the doctor, the geologist, and the architect stood before it. Neither would have liked to admit that he would just as soon leave that door unopened. All the former uneasiness came back. It was all the more inexplicable, with the brilliant sunlight only a few feet away, that each should have felt chilled by the place. "Wonder if it's locked?" remarked Van Emmon. He pressed against the dust-covered barrier, half expecting it to turn to dust; but evidently it had been made of the time-defying alloy. It stood firm. And to all appearances it was nearly air-tight.