"Difference in gravitation," said the doctor, in response to Jackson's
questioning look. "Easy enough for the old-timers, perhaps." They
struggled up the flight as best they could, reaching the top after over
five minutes of climbing.

Perhaps it was the reaction from this exertion; at all events each felt
a distinct loss of confidence as, after regaining their wind, they again
began to explore. Neither said anything about it to the others; but each
noted a queer sense of foreboding, far more disquieting than either of
them had felt when investigating anything else. It may have been due to
the fact that, in their hurry, they had not stopped to eat.

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