mpse of a stone
floor. As the old man entered, Dr. Cairn grasped his son by the wrist.

"Down!" he whispered. "Now is our chance!"

They ran down the rickety stairs, crossed the narrow street, and
Robert Cairn cautiously looked in around the door which had been left
ajar.

Black against the dim light of another door at the further end of the
large and barn-like apartment, showed the stooping figure. Tap, tap,
tap! went the stick; and the old man had disappeared around a corner.

"Where can we hide?" whispered Dr. Cairn. "He is evidently making a
tour of inspection."

The sound of footsteps mounting to the upper apartments came to their
ears. They looked about them right and left, and presently the younger
man detected a large wooden cupboard set in one wall. Opening it, he
saw that it contained but one shelf only, near the top.

"When he returns," he said, "we can hide in here until he has gone
out."

Dr. Cairn nodded; he was peering about the room intently.

"This is the place I came to, Rob!" he said softly; "but there was a
stone stair leading down to some room underneath. We must find it."

The old man could be heard passing from room to room above; then his
uneven footsteps sounded on the stair again, and glancing at one
another the two stepped into the cupboard, and pulled the door gently
inward. A few moments later, the old caretaker--since such appeared to
be his office--passed out, slamming the door behind him. At that, they
emerged from their hiding-place and began to examine the apartment
carefully. It was growing very dark now; indeed with the door shut, it
was difficult to detect the outlines of the room. Suddenly a loud cry
broke the perfect stillness, seeming to come from somewhere above.
Robert Cairn started violently, grasping his father's arm, but the
older man smiled.

"You forget that there is a mosque almost opposite," he said. "That is
the _mueddin_!"

His son laughed shortly.

"My nerves are not yet all that they might be," he explained, and
bending low began to examine the pavement.

"There must be a trap-door in the floor?" he continued. "Don't you
think so?"

His father nodded silently, and upon hands and knees also began to
inspect the cracks and crannies between the various stones. In the
right-hand corner furthest from the entrance, their quest was
rewarded. A stone some three feet square moved slightly when pressure
was applied to it, and gave up a sound of hollowness ben