"What do you think of O'Brien, Oakes?" I asked. "He seems to be a cool sort of a customer." "Yes, he is no ignoramus. He's a shrewd fellow, and a deep one; but I have learned a few things." _CHAPTER XI_ _The Night Walk_ Events were following each other rapidly at the Mansion. After leaving the cellar, Oakes led us back through the grounds, around the south side of the house. There was no entrance to the cellar there, apparently. When we reached our rooms and I had deposited the mask and gown on my table, Oakes turned to the care-taker, Cook, who accompanied us: "You have been several years here, have you not?" "Yes, Mr. Clark." "When did the first trouble begin?" "About three years ago, sir, following some repairs that were made after Mr. Odell Mark bought the place from his brother." "What do you know of those repairs?" "Well, sir, as perhaps you have noticed, the door from the dining-room to the parlor opens on a short hall about three feet deep. Now, sir, Mr. Odell Mark had the wall thickened between the rooms; he thought it was weak, and this hall represents the thickness of the wall." Oakes stood at the window, his hands in his pockets, looking out. "Did you see that wall being built yourself, Cook?" "I didn't notice particularly, sir." "Well, Stone, we'll try the simplest theory first. Will you kindly go with Cook up to the roof and look around carefully. I have an idea that the wall is double, and that you will find an opening up there somewhere." We went, and, as Oakes had surmised, soon found a small opening like a chimney, grated in solidly and protected by a covering, and so reported. "Good!" said Oakes. "The wall is double--in part at least--and the opening was carried into the cellar room and a door placed there." "What for?" said I. "Perhaps to ventilate it. We may find some other reason." "We seem to be solving the mystery," was Moore's comment. Oakes looked at him quizzically. "Are you satisfied, Doctor, that there is a physical agent at work here?" Moore grew red. "Certainly," he said. And Quintus smiled. "I thought probably you would be convinced in time. A thorough licking is an excellent argument. It is my belief that the escapes were made through that double wall, and that we shall find movable panels in the dining-room." "But the motive! We are strangers; we gave no provocation," I cried. "We have yet to learn the motive; also _why_ a man should wear a robe. The mask is sensible enough, but why he impeded himself with a robe is beyond us as yet. It would hide his body, to be sure, as the mask would hide his face, but it would certainly greatly affect his chances of escape, if pursued. Cook, why was no investigation ever made before?" "I don't know, sir. Mr. Odell was very timid." "Did you ever go through the tunnel to the well?" "Yes, sir. I used to go before the mystery began, but never afterward." "How about the place in the stairs where the robe was found?"