Gradually the cradling swing, the quivering power of the airship, lulled his fevered spirit.  Sleep won upon him, dulled the excitements of the past twenty-four hours, sank him into oblivion.  His deep, regular breathing sounded in the gloom of the cabin that contained the Great Pearl Star, the Myzab, the sacred Black Stone of infinite veneration.

An hour he slept.  On, on roared Nissr, swaying, rising, falling a little as she hurled herself through the Arabian night toward the unknown Bara Jannati Shahr, hidden behind the Iron Mountains of mystery as yet unseen by any unbelieving eye.

Peace, all seemed peace, for one dark hour.

But as the hour ended, a shadow fell along the narrow gallery outside the cabin window.  A silent shadow it was, that crept, paused, came on again.  And now in the dark, had there been any eye to see, the shadow would have been identified as a barefoot man, lithe, alert, moving silently forward with the soundless stealth of an Arab versed in the art of asar, or man-stalking.

To the Master’s window this shadow crept, a half-invisible thing in the gloom.  It paused there, listening to the deep, regular breathing within.  Then a lean, brown hand was laid on the sill.  It still seemed to hesitate.

Something gleamed vaguely in that hand—­a crooked jambiyeh, needle-sharp at the point, keen-edged and balanced for the stroke that silently slays.

Motionless, unbreathing even, the shadow waited a long minute.  Then all at once over the sill it writhed, quick, lithe as a starved panther.

Dagger in hand, the shadow slid to the berth where lay the Master of the Legionaries.  There Rrisa paused, listening to the slow respiration of the White Sheik with whom he had shared the inviolable salt, to whom he owed life itself.

Up, in the gloom, came the dagger-blade.

Over the unconscious Master it poised, keen, cold, avenging in the dark of the cabin where lay the three supreme treasures of all Islam.

CHAPTER XXXV

INTO THE VALLEY OF MYSTERY

The upraised blade, poised for swift murder, did not descend.  With a groan from the heart’s core, Rrisa let fall his trembling hand, as he recoiled toward the vague patch of starlight that marked the cabin window.

“Bismillah!” he whispered hoarsely.  “I cannot!  This is my sheik—­’and thrice cursed is the hand that slays the sheik.’  I cannot kill him!”

For a moment he remained there, pondering.  Swift, passionate thoughts surged through his brain, which burned with fever.  In Rrisa’s fighting-blood the supreme battle of his whole existence was aflame—­duty of annihilating the violator of his Faith combating duty of loyalty absolute to one whose salt he had eaten, to one who had preserved his life.

So, in the dark he stood there, a shadow among shadows.  He peered about with white-rimmed eyes, striving to discover where now the Myzab and the sacred Black Stone might be.  The dim bulk of the blanket under the berth came to his senses.  He knelt, touched the blanket, felt the hard solidity within.