Quietly, I reclosed the window, and stood by it for a moment listening.  Nothing occurred, and I returned to the writing-table, puzzled but in no sense alarmed.  I resumed the seemingly interminable record of the Si-Fan mysteries, and I had just taken up my pen, when ... two loud raps sounded upon the pane behind me.

In a trice I was at the window, had thrown it open, and was craning out.  Practical joking was not characteristic of Nayland Smith, and I knew of none other likely to take such a liberty.  As before, the court below proved to be empty....

Some one was softly rapping at the door of the chambers!

I turned swiftly from the open window; and now, came fear.  Momentarily, the icy finger of panic touched me, for I thought myself invested upon all sides.  Who could this late caller be, this midnight visitor who rapped, ghostly, in preference to ringing the bell?

From the table drawer I took out a Browning pistol, slipped it into my pocket and crossed to the narrow hallway.  It was in darkness, but I depressed the switch, lighting the lamp.  Toward the closed door I looked —­as the soft rapping was repeated.

I advanced; then hesitated, and, strung up to a keen pitch of fearful anticipation, stood there in doubt.  The silence remained unbroken for the space, perhaps of half a minute.  Then again came the ghostly rapping.

“Who’s there?” I cried loudly.

Nothing stirred outside the door, and still I hesitated.  To some who read, my hesitancy may brand me childishly timid; but I, who had met many of the dreadful creatures of Dr. Fu-Manchu, had good reason to fear whomsoever or whatsoever rapped at midnight upon my door.  Was I likely to forget the great half-human ape, with the strength of four lusty men, which once he had loosed upon us?—­had I not cause to remember his Burmese dacoits and Chinese stranglers?

No, I had just cause for dread, as I fully recognized when, snatching the pistol from my pocket, I strode forward, flung wide the door, and stood peering out into the black gulf of the stairhead.

Nothing, no one, appeared!

Conscious of a longing to cry out—­if only that the sound of my own voice might reassure me—­I stood listening.  The silence was complete.

“Who’s there?” I cried again, and loudly enough to arrest the attention of the occupant of the chambers opposite if he chanced to be at home.

None replied; and finding this phantom silence more nerve-racking than any clamor, I stepped outside the door—­and my heart gave a great leap, then seemed to remain inert, in my breast....

Right and left of me, upon either side of the doorway, stood a dim figure:  I had walked deliberately into a trap!

The shock of the discovery paralyzed my mind for one instant.  In the next, and with the sinister pair closing swiftly upon me, I stepped back—­I stepped into the arms of some third assailant, who must have entered the chambers by way of the open window and silently crept up behind me!