Nayland Smith bent and examined the mechanism intently.

“Freshly oiled!” he commented.  “You know into whose room it opens?”

Well enough I knew, and, detecting that faint, haunting perfume which spoke of the dainty personality of Karamaneh, my anger blazed up anew.  Came a faint sound of metal grating upon metal, and Smith pulled open the door, which turned outward upon the steps, and bent further forward, sweeping the ray of light about the room beyond.

“Empty, of course!” he muttered.  “Now for the base of these damned nocturnal operations.”

He descended the steps and began to flash the light all about the arched passageway wherein we stood.

“The present dining-room of Graywater Park lies almost due south of this spot,” he mused.  “Suppose we try back.”

We retraced our steps to the foot of the stair.  In the wall on their left was an opening, low down against the floor and little more than three feet high; it reminded me of some of the entrances to those seemingly interminable passages whereby one approaches the sepulchral chambers of the Egyptian Pyramids.

“Now for it!” snapped Smith.  “Follow me closely.”

Down he dropped, and, having the lamp thrust out before him, began to crawl into the tunnel.  As his heels disappeared, and only a faint light outlined the opening, I dropped upon all fours in turn, and began laboriously to drag myself along behind him.  The atmosphere was damp, chilly, and evil-smelling; therefore, at the end of some ten or twelve yards of this serpentine crawling, when I saw Smith, ahead of me, to be standing erect, I uttered a stifled exclamation of relief.  The thought of Karamaneh having been dragged through this noisome hole was one I dared not dwell upon.

A long, narrow passage now opened up, its end invisible from where we stood.  Smith hurried forward.  For the first thirty of forty paces the roof was formed of massive stone slabs; then its character changed; the passage became lower, and one was compelled frequently to lower the head in order to avoid the oaken beams which crossed it.

“We are passing under the dining-room,” said Smith.  “It was from here the sound of beating first came!”

“What do you mean?”

“I have built up a theory, which remains to be proved, Petrie.  In my opinion a captive of the Yellow group escaped to-night and sought to summon assistance, but was discovered and overpowered.”

“Sir Lionel?”

“Sir Lionel, or Kennedy—­yes, I believe so.”

Enlightenment came to me, and I understood the pitiable condition into which the Greek butler had been thrown by the phenomenon of the ghostly knocking.  But Smith hurried on, and suddenly I saw that the passage had entered upon a sharp declivity; and now both roof and walls were composed of crumbling brickwork.  Smith pulled up, and thrust back a hand to detain me.

“Ssh!” he hissed, and grasped my arm.