“I’m some good little guesser,” he murmured to himself as he turned back to the ladder and descended to the floor below.

He moved quietly along the corridor to the fire escape and stepped out upon it.  Then, very quickly and expertly, he coiled a rope which he took from a paper parcel that had been under his arm.  At one end of the coil was a loop.  He swung this lightly round his head once or twice to feel the weight of it.  The rope snaked forward and up.  Its loop dropped upon the stone abutment he had noticed when he had been examining the exteriors of the buildings with Cole Sanborn.  It tightened when he gave a jerk.

Kirby climbed over the railing and swung himself lightly out into space.  A moment, and he was swaying beside the fire escape of the Paradox.  He caught the iron rail and pulled himself to the platform.

By chance the blind was down.  There was no light within, but after his eyes had become used to the darkness he tried to take a squint at the room from the sides of the blind.  The shade hung an inch or two from the window frame, so that by holding his eye close he could get more than a glimpse of the interior.

He tapped gently on the glass.  The lights inside flashed on.  From one viewpoint he could see almost half the room.  He could go to the other side of the blind and see most of the other half.

A man sat down in a chair close to the opposite wall, letting his hands fall on the arms.  A girl stood in front of him and pointed a paper-knife at his head, holding it as though it were a revolver.  The head of the man fell sideways.

Kirby tapped on the window pane again.  He edged up the sash and stepped into the room.

The young woman turned to him eagerly, a warm glow in her shell-pink cheeks.  “Well?” she inquired.

“Worked out fine, Rose,” Kirby said.  “I could see the whole thing.”

“Still, that don’t prove anything,” the other man put in.  He belonged to the staff of the private detective agency with which Kirby was dealing.

The Wyoming man smiled.  “It proves my theory is possible.  Knowing Olson, I’m willin’ to gamble he didn’t sit still on the fire escape an’ let that drawn blind shut him off from what was goin’ on inside.  He was one mighty interested observer.  Now he must ‘a’ known there was a clothes-line on the roof.  From the street you can see a washin’ hangin’ out there any old time.  In his place I’d ‘a’ bopped up to the roof an’ got that line.  Which is exactly what he did, I’ll bet.  The line had been tied to the posts with a lot of knots.  He hadn’t time to untie it.  So he cut the rope.  It’s been spliced out since by a piece of rope of a different kind.”

“How do you know that’s been done since?” the detective asked.

“A fair question,” Kirby nodded.  “I don’t.  I’ll find out about that when I talk with the landlady of the Wyndham.  If I’m right you can bet that cut rope has puzzled her some.  She can’t figure out why any one would cut her rope down an’ then leave it there.”