"I don't know, myself, but Maloney said the outside opening by the well led into the cellar; Cook says so, too. 'Tis a pa.s.sage they used in wet weather, sorr."

"Mike, you and Cook go round and guard that outer door by the well. Open it. I'm going through."

"Mr. Clark, don't go in there alone!"

"I'll attend to that," said Oakes. "You go with Cook."

The two went to the well and lifted the hatch door. As they did so, Oakes held a lighted match inside one end of the tunnel. It blew strongly toward us; the air was rushing in, and we knew the pa.s.sage led to the opening. We heard their voices calling to us. Dr. Moore spoke.

"Oakes, you shall _not_ go in there; you have done enough to-day; you are a wounded man." I caught up the lantern and my revolver, and Moore followed.

"Hold on!" said Oakes. "You are in the most dangerous part; don't be rash. Here, Stone, you go first--and Moore, you follow about ten feet behind, without a light, in order that you may be undetected. Take matches. I'll stay here with the taper, and watch. When you get to the other end, don't go up the steps leading to the ground until both Mike and Cook show themselves. We know nothing about them, you know. Be cautious. The man we want went out this way, whoever he is."

I threw the light ahead and advanced some ten feet. I heard Moore following. "Careful!" said he in a whisper.

Again I threw the light ahead, and beheld only the walls of the square tunnel. I could hear the breathing of Moore behind me. I knocked on the wall here and there with my revolver; it rang true and solid. We gradually advanced until we beheld the daylight and saw the men waiting at the head of the stone steps.

I ascended. Moore took the lantern and called back to Oakes, addressing him as Clark. In a moment he came.

"Stay where you are, Stone," said he to me. "Come here, Mike."

Mike descended willingly enough. I watched Cook and looked all around.

"Open that door." Oakes pointed to a little wooden opening in the side of the stairs. Mike obeyed, but instantly closed it again with a bang.

"A man!" said he.

Oakes and Moore levelled their revolvers.

"Come out," said the detective, "or take the consequences. I shall shoot."

Mike opened the door again, hiding his figure behind it for protection as it swung out. I expected to see some one shot, but Moore threw the light in, and instantly Oakes dived forward into the alcove of stone. We could hear him chuckle. Cook, at my side, was standing on one leg in his excitement. Then Dr. Moore burst into laughter.

"What is it? What's the matter?" I cried. I could not see very well, and ran half-way down. Oakes was standing beside Moore, trying to look grave. In his hand was a red paper mask and a long black robe!

O'Brien looked on, his eyes twinkling, but his face serious. "I'm thinking it's lucky, Mr. Clark, sorr, that ye saved yer ammunition,"

said he.

"Yes," retorted Oakes, "and it's still more fortunate you're a good actor."

O'Brien's somewhat insolent manner changed instantly to one of civility, and Oakes turned to us.

"No wonder some said there was a woman in this affair."

Then he ordered the hatch door nailed down, and handed the things to me.

"Please take these upstairs, Stone; we must investigate this more fully," and we withdrew to discuss our findings.

"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.