The three scouts lay quiet for ten minutes after the volleying had ceased.  Silence settled over the plain; but, presently, a low moaning sound came indistinctly from the east.  It lasted only a moment, then died away; and almost at once, the slight wind that had been blowing from the sea hushed itself to a strange calm.

Rrisa gave anxious ear.  His face grew tense, but he held his peace.  Neither of the white men paid any heed to the slight phenomenon.  To them it meant nothing.  For all their experience with the desert, they had never happened to hear just that thing.  The Arab, however, felt a stab of profound anxiety.  His lips moved in a silent prayer to Allah.

Once more the Master raised his hand in signal of advance.  The three man-stalkers wormed forward again.  They now had their direction, also their distance, with extreme precision; a simple process of triangulation, in which the glow of the beach-fire had its share, gave them the necessary data.

Undaunted, they approached the camp of the Beni Harb; though every moment they expected to be challenged, to hear the crack of an alarm-rifle or a cry to Allah, followed by a deadly blast of slugs.

But fortune’s scale-pan dipped in their direction, and all held still.  The sun-baked desert kept their secret.  Onward they crawled, now over sand, now over cracked mud-flakes of saline deposit where water had dried at the bottom of a ghadir.  All was calm as if the spirit of rest were hovering over the hot, fevered earth, still quivering from the kiss of its great enemy, the sun.

“Peace, it is peace until the rising of the morn!” a thought came to the Master’s mind, a line from the chapter Al Kadr, in the Koran.  He smiled to himself.  “False peace,” he reflected.  “The calm before the storm!” Prophetic thought, though not as he intended it!

On and on the trio labored, soundlessly.  At last the chief stopped, held up his hand a second, lay still.  The others glimpsed him by the starlight, nested down in a shallow depression of the sand.  They crept close to him.

“Lieutenant,” he whispered, “you bombard the left-hand sector, toward the fire and the sea.  Rrisa, take the right-hand one.  The middle is for me.  Fire at will!”

Out from belts and pockets came the lethal pistols.  With well-estimated elevation, the attackers sighted, each covering his own sector.  Hissing with hardly audible sighs, the weapons fired their stange pellets, and once again as over the woods on the Englewood Palisades—­really less than twenty-four hours ago, though it seemed a month—­the little greenish vapor-wisps floated down, down, sinking gently on the Sahara air.

This attack, they knew, must be decisive or all would be hopeless.  The last supply of capsules was now being exhausted.  Everything had been staked on one supreme effort.  Quickly the attackers discharged their weapons; then, having done all that could be done, lay prone and waited.