This last question, it seemed to me, was answered.  In some way the woman had learned of the substitution, and had tried to use her knowledge for blackmail.  Nina Carrington’s own story died with her, but, however it happened, it was clear that she had carried her knowledge to Halsey the afternoon Gertrude and I were looking for clues to the man I had shot on the east veranda.  Halsey had been half crazed by what he heard; it was evident that Louise was marrying Doctor Walker to keep the shameful secret, for her mother’s sake.  Halsey, always reckless, had gone at once to Doctor Walker and denounced him.  There had been a scene, and he left on his way to the station to meet and notify Mr. Jamieson of what he had learned.  The doctor was active mentally and physically.  Accompanied perhaps by Riggs, who had shown himself not overscrupulous until he quarreled with his employer, he had gone across to the railroad embankment, and, by jumping in front of the car, had caused Halsey to swerve.  The rest of the story we knew.

That was my reconstructed theory of that afternoon and evening:  it was almost correct—­not quite.

There was a telegram that morning from Gertrude.

“Halsey conscious and improving.  Probably home in day or so. 
          
                                  Gertrude.”

With Halsey found and improving in health, and with at last something to work on, I began that day, Thursday, with fresh courage.  As Mr. Jamieson had said, the lines were closing up.  That I was to be caught and almost finished in the closing was happily unknown to us all.

It was late when I got up.  I lay in my bed, looking around the four walls of the room, and trying to imagine behind what one of them a secret chamber might lie.  Certainly, in daylight, Sunnyside deserved its name:  never was a house more cheery and open, less sinister in general appearance.  There was not a corner apparently that was not open and above-board, and yet, somewhere behind its handsomely papered walls I believed firmly that there lay a hidden room, with all the possibilities it would involve.

I made a mental note to have the house measured during the day, to discover any discrepancy between the outer and inner walls, and I tried to recall again the exact wording of the paper Jamieson had found.

The slip had said “chimney.”  It was the only clue, and a house as large as Sunnyside was full of them.  There was an open fireplace in my dressing-room, but none in the bedroom, and as I lay there, looking around, I thought of something that made me sit up suddenly.  The trunk-room, just over my head, had an open fireplace and a brick chimney, and yet, there was nothing of the kind in my room.  I got out of bed and examined the opposite wall closely.  There was apparently no flue, and I knew there was none in the hall just beneath.  The house was heated by steam, as I have said before.  In the living-room was a huge open fireplace, but it was on the other side.