In the bare portion of the wall thus revealed, about two feet square, was a wooden trap door, fastened by a button. Colwyn unfastened the button, and opened the door. A black hole gaped at him. The light of the candle showed that the wall was hollow, and the trap opened into the hollow space. There was nothing unusual in such a door in an old house; Colwyn had seen similar doors in other houses built with the old-fashioned thick walls. It was the primitive ventilation of a past generation; the doors, when opened, permitted a free current of air to percolate through the building, and get to the foundations. But a further examination of the hole revealed something which Colwyn had never seen before--a corresponding door on the other side of the wall. The other door opened into the bedroom which had been occupied by Mr. Glenthorpe. Colwyn pushed it with his hand, but it did not yield. It was doubtless fastened with a button on the outside, like the other. Colwyn, scrutinising the second door closely, noticed that the wood was worm-eaten and shrunken. For that reason it fitted but loosely into the aperture of the wall, and on the one side there was a wide crack which arrested Colwyn's attention. It ran the whole length of the door, along the top--that is, horizontally--and was, perhaps, a quarter of an inch wide. With the tightened nerves which presage an important discovery, Colwyn felt for his pocket knife, opened the largest blade, and thrust it into the crack. It penetrated up to the handle. He ran the knife along the whole length of the crack without difficulty. There was no doubt it opened into the next room. Colwyn closed the trap-door carefully, and started to push the wardrobe back into its previous position. As he did so, his eye fell upon several tiny scraps of paper lying in the vacant space. He stooped, and picked them up. They were the torn fragments of a pocket-book leaf, which had been written upon. Colwyn endeavoured to place the fragments together and read the writing. But some of the pieces were missing, and he could only decipher two disjointed words--"Constance" and "forgive." Slowly, almost mechanically, the detective felt for his pipe, lit it, and stood for a long time at the open window, gazing with set eyes into the brooding darkness, wrapped in profound thought, thinking of his discoveries and what they portended. CHAPTER XX Colwyn was astir with the first glimmering of a grey dawn. He wanted to test the police theory that the murder was committed by climbing from one bedroom to the other, but he did not desire to be discovered in the experiment by any of the inmates of the inn.