"He has that; but he has also a body of the best detectives, for the purpose, to be had. Maloney and Mike are puzzling him considerably, Stone; they are very close to one another always, and seem quite intimate."

"Yes," I replied. "I have noticed it. They both show a great deal of interest in these alterations. Have you noticed how Maloney is watching O'Brien? He keeps him continually in sight."

We had approached the front door of the Mansion as we spoke. Oakes was standing just outside, his eyes likewise upon the two gardeners. Our last remarks were made in his presence, and he entered the conversation with a quiet observation to the effect that Maloney seemed to fear that Mike might not attend to his business, but that Mike _would_, nevertheless.

I was obliged to acknowledge that I did not quite understand.

"Oh, Mike is a good laborer," he explained; "he needs no such watching,"

and there seemed to be a peculiar significance in his words. They were stated in a slow, indifferent manner that caused me to look at the speaker, but his face wore the inscrutable expression which I had frequently seen before, and I learned nothing. I knew him well enough by this time, however, to realize that something was taking shape in his thoughts.

"Now, let us go inside," said he. "After lunch we will attack the final solution of the manner in which these mysterious a.s.saults were performed. Like all such things, it will be simple enough, I know, and the point remaining to determine will be not _how_ it was done, but _by whom_.

"I feel confident that that door in the cellar room leads upward to an inters.p.a.ce which communicates with the dining-room through panels in the walls. The peculiar noise--the swish--that I heard, resembled the sudden sliding of a board, and it was the conviction that the person who a.s.saulted Moore disappeared into the wall which made me run downstairs.

I felt sure there would be some explanation of it below."

That afternoon a systematic search of the entire house was made. The cellar room in which the a.s.sault upon Oakes had occurred was thoroughly lighted and examined. The heap of rubbish which Mike had been investigating at our previous visit proved to be composed of plaster and bricks.

The wall in which the door was cut was found to be about three feet thick, and one of the foundations of the house. It was solid, save for a chimney-like opening which had been trapped with the door. Above, at the level of the dining-room floor, the great wall ceased. From one edge was continued upwards the original part.i.tion between that room and the next--the parlor; but it was thin, and had evidently been recently strengthened by another wall, slightly thicker, and built from the opposite edge of the foundation, leaving a s.p.a.ce between the two. Into this s.p.a.ce entered, at a certain point, the opening from the cellar room below.

It was a peculiar arrangement. As Oakes remarked, the new wall had been made with no regard to the economizing of s.p.a.ce; for, had it been built immediately back of the old, considerable room would have been saved for the parlor. One of the "carpenters" thought that the original idea had been to utilize the s.p.a.ce for closets. The only other possible use for it, so far as we could discover, was the one which Oakes had surmised--ventilation for the cellar. Still, to our ordinary minds, a chimney would have answered that purpose quite as well.

A little further investigation, however, showed the top of the foundation wall to be covered with cement well smoothed, and the walls themselves were plastered. It was generally conceded, therefore, that the first idea had been to use it as closet room, which could easily have been done by cutting doors through the walls. As Oakes said, the notion had evidently met with opposition and been abandoned, so communication had been made with the cellar instead, and the roof opened to afford ventilation.

The opening into the cellar was large. A man could easily enter it, and, standing, reach the top of the foundation wall; then, by a little exertion, he could raise himself into the intermural s.p.a.ce. Oakes, Moore and I proved this by actual experiment and found that the pa.s.sage was quite wide enough to accommodate a man of average proportions.

I have said that the dining-room was finished in oak panels. These had been reached from our side of the wall by removing the bricks and mortar--the same stuff evidently which helped to form the rubbish heap in the room below. One of the larger panels had been made to slide vertically. It had been neatly done and had escaped detection from the dining-room because of the overlapping of the other panels. Some debris still remained between the walls.

"The fellow we are after knew of the s.p.a.ce between the walls and worked at the panel after the repairs were completed," was Oakes's remark.

"How do you know that?" asked Moore.

Oakes looked at him and smiled, then said: "Moore, where is your reasoning ability? Do you think, if the panel had been tampered with at the time the repairs were made, that the debris would have been left behind? No! It would have been removed with the rest of the dirt."

We had gone to our rooms upstairs while the men were hunting through the tunnel to the well. They found nothing; everything was as we had left it after our adventures there.

It seemed to us that, all things considered, the work on the panel must have been done by someone within the household, or, at least, that some of its members must have been involved in the matter.

"It may have been accomplished at night, however, and by an outsider,"

said Oakes. "The servants' quarters are separate from the house. Anyone might easily have entered the cellar by the tunnel route. Still, there may have been collusion also."

"It seems a nonsensical idea to leave the debris in the cellar," I said.

"No, I think not," was the answer. "The care-takers are afraid even to enter that place. The miscreant knew that detection would be probable at the hands of strangers only."

That evening Elliott and Martin left for New York. They were to bring the negro boy, Joe, to Mona. Late at night, before we retired, Oakes asked us to go with him into the parlor.

"What for?" said I.

"To forge another link in the chain--the strongest yet," he said.

"What?"

"Do you remember the cartridge I found in the cellar?"

"Yes, yes; but you did not pay much attention to it, I thought."

He looked gravely at me. "Stone, that cartridge probably corresponds in calibre to the one which was used in the murder of Mark."

"Ah!" said Moore. "I had a notion of that myself. Why did you not tell us your opinion before?"

"Because, when I found it, we were working on the Mansion affair only. I divined the value of the find; but why should I have mentioned it? I was not hunting the Mark murderer then."

"Quintus, you consummate fox--you worked Hallen well!"

"Not at all; business is business. What is the use of gossiping? There are no ladies to be entertained in _my_ profession, Doctor."

He led the way to the parlor--we meekly following--to where a cl.u.s.ter of arms hung upon the wall: one of those ornaments of crossed swords, guns and a shield, so common in old houses.

He remarked that he had noticed these arms on his previous visit. He looked at a revolver hanging across the shield, with a pouch beneath it, and then suddenly, in surprise, said: "Last time I was here, a few weeks ago, there was a large old-fashioned revolver here of 44 or 45 calibre.

I remember it well, being interested in firearms.

"This one now here is of a similar pattern and appearance, but of smaller calibre, and newer. Look! The cartridges in this pouch are of about 45 size; they belong to the old weapon and cannot be used with this one."

"Again, some of them are missing; there were at least a dozen before, now there are only three or four. The old revolver and some cartridges have been taken away, and a newer weapon subst.i.tuted."

"Indeed! But why?" said Moore sceptically.

"Partly because"--and Oakes was decisive, curt, master of the situation--"because this one cannot be loaded. See!" He then tried to turn the chamber and showed us that the mechanism was faulty.

"The old revolver," said he in a low tone, "and some cartridges were taken away, and in order that its absence should be less noticeable, this one was left here--it being useless.

"Now, boys, the cartridge I found downstairs on the cellar floor is a 45-calibre and belongs to those of the pouch and the original revolver, as you see."

He took it from his pocket and showed us that it did _not_ fit the weapon in his hand but matched the cartridges in the pouch. It belonged to the _old_ weapon.

"We are closing in," said I.

"Yes--the man of the robe has the old revolver and cartridges; he took them within the last few days, finding his own weapon out of order. It is he who is responsible for the mystery in this house--and in all probability it is he who shot Winthrop Mark. You remember, the evidence at the inquest showed that a heavy revolver had been used--a 44 or 45 calibre--exactly such an one as the old weapon which I saw here."

"Excellent, Oakes," remarked Moore. "There's only one objection."

"Yes, I know," said Oakes. "You were going to ask why the fellow did not take all these cartridges and put his own in the pouch to match the weapon he left here."

"Exactly," said Moore.

"Well," said the detective, "he either had no cartridges of his own handy, or else, like all criminals, however smart, he tripped--the brain of no man is capable of adjusting his actions precisely in every detail."

"Guess you're right. No man can be perfect in his reasoning, and, no matter how clever the criminal, he is almost certain to make an error sooner or later," said Moore.