My fear became a madness, and I twisted my body violently in a wild endeavour to free myself.  A dreadful pain shot through my left shoulder, and the whole nightmare scene—­the thing with the knife at the window—­the low-ceiled room-began to fade away from me.  I seemed to be falling into deep water.

A splintering crash and the sound of shouting formed my last recollections ere unconsciousness came.

I found myself lying in an armchair with Bristol forcing brandy between my lips.  My left arm hung limply at my side and the pain in my dislocated shoulder was excruciating.

“Thank God you are all right, Mr. Cavanagh!” said the inspector.  “I got the surprise of my life when we smashed the door in and found you tied up here!”

“You came none too soon,” I said feebly.  “God knows how Providence directed you here.”

“Providence it was,” replied Bristol.  “From the roof of Wyatt’s Buildings—­you know the spot?—­I saw the second yellow devil coming.  By God!  They meant to have it to-night!  They don’t value their lives a brass farthing against that damned slipper!”

“But how—­”

“Along the telegraph-wires, Mr. Cavanagh!  They cross Wyatt’s Buildings and cross this house.  It was a moonless night or we should have seen it at once!  I watched him, saw him drop to this roof—­and brought the men around to the front.”

“Did he, that awful thing, escape?”

“He dropped full forty feet into a tree—­from the tree to the ground, and went off like a cat!”

“Earl Dexter has escaped us,” I said, “and he has the slipper!”

“God help him!” replied Bristol.  “For by now he has that hell-pack at his heels!  What a case!  Heavens above, it will drive me mad!”

CHAPTER XIX

A RAPPING AT MIDNIGHT

Inspector Bristol finished his whisky at a gulp and stood up, a tall, massive figure, stretching himself and yawning.

“The detective of fiction would be hard at work on this case, now,” he said, smiling, “but I don’t even pretend to be.  I am at a standstill and I don’t care who knows it.”

“You have absolutely no clue to the whereabouts of Earl Dexter?”

“Not the slightest, Mr. Cavanagh.  You hear a lot about the machinery of the law, but as a matter of fact, looking for a clever man hidden in London is a good deal like looking for a needle in a haystack.  Then, he may have been bluffing when he told you he had the Prophet’s slipper.  He’s already had his hand cut off through interfering with the beastly thing, and I really can’t believe he would take further chances by keeping it in his possession.  Nevertheless, I should like to find him.”

He leaned back against the mantelpiece, scratching his head perplexedly.  In this perplexity he had my sympathy.  No such pursuit, I venture to say, had ever before been required of Scotland Yard as this of the slipper of the Prophet.  An organization founded in 1090, which has made a science of assassination, which through the centuries has perfected the malign arts, which, lingering on in a dark spot in Syria, has suddenly migrated and established itself in London, is a proposition almost unthinkable.