smart for me, and I got the worst of it this time," he added.

"What is that rope doing there?" asked the Doctor, as the light revealed
a long rope extending from a tent-peg to a considerable distance into
the darkness.

"Oh, it is there for a purpose, and it answers too well to suit me, for
it has given me one of the heaviest falls I have had for a long time. A
man was there listening to us, and it would have made no difference
which way I had come round the tent, for the eavesdropper would have
gone in the opposite direction. When I heard him making off I dashed
after him, and his comrade, who was at the far end of the rope, jerked
it taut when it was between me and the man I was after, with the result
that I came a most terrific cropper; then they promptly fled, and are
safely away by this time," explained Burton.

"But how did you know there was any one outside?" asked Tom. "I never
heard a sound."

"I saw the side of the tent shake, and there is not a breath of air
stirring. The man who was listening must know English, I feel sure; and
I am afraid we have made a terrible mistake in not taking precautionary
measures against being overheard. If they understood what I said about
suspecting who they are, I may make up my mind to having a rather lively
time." Burton said in a whisper, for he did not know but some one might
still be listening screened in the darkness.

"They may have only come to watch us, and probably did not grasp the
meaning of our conversation," said the Doctor, in a low voice.

"Let us hope so, for it may mean life or death," was Burton's serious
reply, and that night guards were set over the camp.

Early next morning Burton left, but before going he slipped a letter
into the Doctor's hand, saying as he did so, "Don't open it unless I am
not back by eight o'clock to-morrow morning. Inside you will find full
instructions what to do if I have not returned."

CHAPTER IV.--A CAPTURE

Soon after Burton had left the camp the Doctor received a letter from
Professor Muirson in which he said, "The only word on the rubbing you
sent me from the last fragment of slab you found means 'Cave,' and I
think it should be placed before the words 'of Hydas'; thus you have a
reference to the 'Cave of Hydas,' in which there is, or was, something
to be carefully guarded."

"Then, putting two and two together, the men who hold Mark a prisoner
are either anxious to learn where this Cave of Hydas is, or t