"Now," went on his captor, after a period of silence, "I shall have to bind and blindfold you again." "Why?" asked Mark, in some alarm. "Because I don't want you to see how I get in and out of this room, and that's the only way I can guard my secret. Though if you promise not to remove the bandage from your eyes within five minutes from the time I leave you, I will not have to tie your hands and feet. After I am gone you may take the handkerchief off, but when you hear me rap on the wall, ready to come back again, you must once more blindfold yourself. Otherwise I shall have to tie you up." Mark considered a moment. It was not pleasant to be tied with the cruel ropes, and he felt that in time he could penetrate the mystery of how the room opened, even if he did not see his jailer enter and leave. "I promise," he said finally. "That's good. It simplifies matters. Now you can blindfold yourself, and I trust to your honor. You may remove the bandage in five minutes, but when you hear me knock, you must replace it until I am in the apartment. Then you can take it off again." There was little choice but to obey, and Mark tied the handkerchief over his eyes. He listened intently, heard the man moving about the room, felt the wind on his cheeks, and then came silence. He waited until he thought five minutes had passed, and then took off the bandage. The candle was burning where the man had set it, but the fellow himself was gone. He had taken with him the broken dishes, and remains of the food Mark had not eaten. The glass and a pitcher of water stood on a broken table, and Mark took a big drink. "Now to see if I can't get out of this place," he murmured to himself. Mark had invented many pieces of apparatus, and he was considered a good mechanician. Consequently he went about his task in a systematic manner. He examined the walls carefully by the candle, which he carried in his hand, but no opening was apparent. "Of course, there must be some secret spring to press," said the lad. "That's how he gets in and out. A section of the wall moves, but where it is I can't see. It will take time. I must look at every inch." He was in the midst of his investigations when there sounded on the wall back of him three raps. "Ha! At least, that tells me where the opening is," thought the lad. "It's on that side, but now I have to put that blamed bandage on. Well, I may be able to escape yet." True to his promise, he blindfolded himself well, and presently he heard a noise, felt a draught of air, and he knew his captor was in the room. "You can now take off the handkerchief," said the man. "I have brought you some more bags for bed clothing. It isn't much, but it is all I have. They will keep you warm tonight." "Are you going to imprison me over night?" asked Mark. "Yes, and I'll stay here with you. No one can find us here. The secret room is well hidden. But first I have another matter that needs attention. I am going to ask you a question."