“Uncle Silas was so afraid last night!” Katherine whispered.

“That,” Bobby cried, “is the fact we must have.”

He paused.

“What’s that?” he asked sharply.

They sat for some time, listening to the sound of wheels on the gravel, to the banging of the front door, and, later, to the pacing of men in the room of death overhead.  They tried again to thread the mazes of this problem whose only conceivable exit led to Bobby’s guilt.  The movements upstairs persisted.  At last they became measured and dragging, like the footsteps of men who carried some heavy burden.

They looked at each other then.  Katherine hid her eyes.

“It’s like a tomb here,” Bobby said.

He arranged kindling in the fireplace and touched a match to it.  It hadn’t occurred to him to ring for Jenkins.  None of them wished to be disturbed.  Eventually it was the detective who intruded.  He strolled in, glanced at them curiously for a moment, then walked to the door of the enclosed staircase.  He grasped the knob.

“To-night,” he announced, “I am trying a small experiment on the chance of clearing up the last details of the mystery.  Since it depends on the courage of whoever murdered Mr. Blackburn I’ve small hope of its success.”

He indicated the ceiling.  “You’ve heard, I daresay, what’s been going on up there.  Mr. Blackburn’s body has been removed to his own room.  The room where he was killed is empty.  I mean to go up and enter and lock the doors as he did last night.  I shall leave the window up as it was last night.  I shall blow out the candle as he did.”

He lowered his voice.  He looked directly at Bobby.  His words carried a definite challenge.

“I shall lie on the bed and await the murderer under the precise conditions Mr. Blackburn did.”

“What do you expect to gain by that?” Graham asked.

“Probably nothing,” Howells answered, “because, as I have said, success depends upon the courage of a man who kills in the dark while his victim sleeps.  I simply give him the chance to attack me as he did Mr. Blackburn.  Of course he realizes it would be a good deal to his advantage to have me out of the way.  I ask him to come, therefore, as stealthily as he did last night.  I beg him to match his skill with mine.  I want him to play his miracle with the window or one of the locks.  But I’ll wager he hasn’t the nerve, although I don’t see why he should hesitate.  He’s a doomed man.  I shall make my arrest in the morning.  I shall publish all my evidence.”

Bobby wouldn’t meet the narrow, menacing eyes, for he knew that Howells challenged him to a duel of slyness with the whole truth at stake.  The detective’s manner increased the hatred which had blazed in Bobby’s mind when he had stood in the bedroom over his grandfather’s body.  For a moment he wished with all his heart that he might accept the challenge.  He did the best he could.