eight o'clock
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 , `` Do n'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 . '' 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 .