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