"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 they know where it is and do not wish any one else to obtain the knowledge," said the Doctor.
"I am inclined to think that Mark is in that very cave at the present moment," said Tom.
"Quite possible. By the way, Tom, tell the natives who are crowding about the camp to continue the search for Mark. Burton wishes it to be kept up for some reason or other," said the Doctor as he went into his tent.
"Hi! Tom; come here a moment," almost immediately shouted the Doctor; and as soon as Tom had joined him he said, "I have just found this—listen: 'I have been asked to say that I am all right, and to advise you to do what my captors have requested you. Your reply is to be written on the blank part of this paper and placed where you put the last. Mark.' There can be no doubt about the writing—it is Mark's, and my mind is greatly relieved," said the Doctor.