e said; "this stone is loose, and goes down when I stand
upon that corner. It's hollow, too, underneath."

He stamped as he spoke, and there was a strange echoing sound came up.

"Hush!" said Yussuf quickly, and he glanced round to see if they were
observed; but they were hidden from the other occupants of the place;
and, stooping down, Yussuf brushed away some rubbish, placed his hands
under one side of the stone where it was loose, and lifted the slab
partly up.

The air came up cool and sweet, so that it did not seem to be a vault;
but it was evidently something of the kind, and not a well, for there
was a flight of stone steps leading down into the darkness.

It was but a moment's glance before Yussuf lowered the stone again, and
hastily kicked some rubbish over it, and lowered a piece of an old
figure across it so as to hide it more.

"What is it?" said Lawrence quickly.

"I do not know," replied Yussuf. "It is our discovery. It may be
treasure; it may be anything. Say no word to a soul, and you and I will
get a lamp, escape from the prison to-night, and come and examine it,
and see what it is. It may be a way out."

Lawrence would gladly have gone on at once, but Yussuf signed to him to
be silent; and it was as well, for he had hardly time to throw himself
down on a block of stone, and sham sleep, when the guards came
sauntering in and looked suspiciously round. Then, not seeing two of
their prisoners, they came on cautiously, and peered over the stones
that hid them from where the professor was drawing, to find Yussuf
apparently asleep, and Lawrence sharpening his pocket-knife upon a
stone.

One of the men came forward and snatched the knife away, saying in his
own tongue that boys had no business with knives, after which he stalked
off and returned to his old place outside.

"You see," said Yussuf quietly, "it was no time now for examining the
place; wait till night."

For the first time since he had been a prisoner the hours passed slowly
to Lawrence. It seemed as if it would never be night, and every time he
met the professor's or Mr Burne's eye, they seemed to be taking him to
task for keeping a secret from them.

Then, too, Mrs Chumley appeared to be suspecting him, and Chumley drew
him aside as if to cross-examine him; but it was only to confide a long
story about how severely he had been snubbed that day for wanting to
follow the professor to the ruins where he was making his drawings.