Frank's wound fortunately turned out to be nothing very serious—though painful enough—and after it had been treated with antiseptics from the medicine chest he declared that, aside from the stiffness and soreness, he felt no ill effect.
"Those fellows certainly gave us a sample of what we may expect," remarked Harry, examining the hole in his shirt where the arrow had ripped through.
"It was quite as narrow an escape as I care to experience," agreed Frank. "How about you, Ben?"
"Wall," said the old adventurer, "I don't know as how I think that kind of excitement is as beneficial fer the health as the rest cure."
Meanwhile the Golden Eagle, plowing through the clear African air at fifty miles an hour, rapidly drew nearer and nearer to the mysterious Moon Mountains.
As they neared the range the extraordinary character of it was revealed more and more clearly. Seamed with deep gloomy abysses and almost bare of vegetation, except a few scanty groves of palms and the hardier tropical trees, they seemed indeed fitted to be the theater of dark mysteries and the haunt of savage tribes.
"Well," exclaimed Harry, as he scrutinized the strange mountain mass through the glasses, "I should say that if those Winged Men are to be found anywhere, here is where they'd reside."
"I should think they'd use their wings to get out—a nastier looking lot of mountains I never saw," was Ben's reply.
Frank made no comment, but the sinister character of the mountains they were so rapidly approaching impressed itself on his mind nevertheless. Eagerly he scanned the range for the first sign of "The Upturned Face." Harry and Ben, too, gave quite as eager scrutiny toward the discovery of this striking mark of the ivory's hiding-place.
All at once it shot into view with a suddenness that made the boys' beads swim.
It was as clear as daylight. The line of the mountain for which Frank had the Golden Eagle II now directly headed was unmistakably the outline also of a hawk-nosed facet.
If the mountains themselves had an evil, menacing look, the stone face possessed this same quality in an infinitely greater degree.
"Well, if we've got to go looking for ivory right under that face the sooner we find it the better," exclaimed Ben. "I'd hate to be shipmates with the fellow who sat for that portrait."
"No human being ever sat for it, Ben," laughed Frank; "it's a mere freak of nature which has so disposed the mountain mass at this point as to give the semblance of what the map-maker terms The Upturned Face."
"Well, if I had a mug like that I'd turn it down instead of up before some one did it for me," was Ben's comment.
The Golden Eagle landed on a plateau about halfway up the mountain, beneath the upturned face. It made an almost ideal camping-place, considering the rugged nature of their surroundings. In one part of it a small grove of bananas and palms had taken root, and their smiling greenery offered a refreshing contrast to the dark oppressive gloom of the giant rock masses piled all about. From the center of this oasis in the rocky range bubbled a tiny spring of water as clear and cold as if it had been filtered and iced. Frank's first act was to send out a wireless to the River Camp, telling of their arrival.
"Well, thank goodness, we've got something green and pleasant to look at," remarked Ben, as they set about transforming the chassis of the Golden Eagle into a comfortable tent by means of running up the canvas curtains on the aluminum frames provided for that purpose. Thus equipped, the chassis served the uses of an improved tent, as the floor was well above the ground and out of all danger of the unwholesome, vapors rising from the ground and also the scorpions and other reptiles.
But if the oasis itself was a pretty spot, it was made doubly so by the contrast it afforded to the scenery surrounding it. On all sides shot up frowning walls of rugged black rock which seemed to have been torn and ripped in some remote period by a terrific convulsion of nature. In places, too, the rock masses seemed to have been seared by subterranean fires. Frank gazed upward at the terrific character of the scenery about them.
"We shall need the rope-ladder," he announced suddenly after a long silence.
"The rope-ladder?" inquired Harry, "what for?"
Frank laughed.
"I mean the rope-ladder we use in the Golden Eagle. As you know, the only way to locate the cache is to strike a direct line down from the nose of the upturned face. That will bring us to the small cairn or pile of rocks that marks the Arab's hiding-place."
"He could hardly have chosen a better," remarked Harry. "Who would ever guess, unless they had the key to the mystery, that these mountains held such a fortune in tusks."
The rest of that day was spent in overhauling the outfit which they would need to use on their expedition of the morrow. Luckily the boots they wore had been fitted with "hob-nails" so that they were ideal for the tough climb that they had ahead of them. Each member of the three was to carry a pick and of course they all were to be armed, carrying several rounds of ammunition each in their cartridge-belts.
That night after a supper of fried ham, canned corn and pancakes—all cooked by the skilful Ben over a fire of wood collected from the little grove—Frank sent out a wireless to the members of the camp on the river bank and felt much reassured when Lathrop's "All well—good luck," came back through the air. It was delightfully cool on the mountain-side after the oppressive fetid air of the river and its neighborhood, and as Ben had remarked before they turned in:
"Fine weather for sleeping."
But sleep would not come to Frank. He tossed and turned on his transom bed and several times gazed out into the night through the canvas curtains. An unaccountable feeling of unrest possessed him. Could they get the ivory out of the cache before Muley-Hassan and his band arrived by land?
Fast as they had traveled through the air Frank realized that the Arab, who doubtless by this time had been informed by the treacherous Diego of the boys' bold dash, would push on at furious speed in order to head them off. That he would come accompanied by a well-armed band Frank could not doubt. He and Harry and Ben could only put up a feeble resistance against such an attack. There was only one chance to secure the ivory and that was to get at it before the Arab arrived. It all depended then on how quickly they could find the cache. Frank lit the lantern and shielding it so that it would not strike in the eyes of his sleeping brother, drew out the map and scanned it attentively.
Yes, here were the directions written in the queer hand of Muley-Hassan's follower.
"A line from the nose straight down to the cairn of stones."
It seemed simple enough and certainly the nose of the Upturned Face was as clearly to be made out as a ship at sea. But Frank had been too long trained in the hard school of adventure to underestimate the difficulties of any piece of work. They faced a hard job and none realized the fact better than the young leader.
At last he blew the lantern out and once more composed himself to sleep. He was just dozing off when a sufficiently startling interruption occurred. One which drove all further thoughts of rest from his head.
It was an extraordinary sound that brought the boy out of his bed with a bound and caused him to clutch his revolver with a heart that beat loud and thick in spite of himself.
Clutching his weapon the boy rushed to the door of the chassis tent and gazed out.
There was a bright moon which threw into inky blackness the depressions of the rugged mountains and threw up their projections into a blue glare. It was almost as light as day under that wonderful African moon. Had there been any one near the boy must have been able to see them.
But look as he would there was not a soul in sight. All about him stretched the barren frowning mountains sleeping under the moon.
But the sound that he had heard?
There was no mistaking it. It had been too like the low humming of a human voice for him to have been misled. Perhaps he had been dreaming?
But as if to give the lie to any such supposition the strange sound that had so alarmed him at that moment made itself manifest once more:
"A-hooo-A-AH-HOOO-00-a-ho-ho-ho-o-!"
It started softly and gradually ran up the scale till it reached a crescendo shout and then died out in a soft sound like a woman's wail. Heard anywhere the sound would have been alarming enough, but coming as it did in the midst of these unknown, mysterious Mountains of the Moon it struck a chill to the boy's heart and caused his scalp to tighten in a manner that even the bravest man or boy in the world would have had no reason to feel shame over.
A human enemy, a foe he could see, Frank would have faced with iron nerve; but this strange wailing noise coming from what quarter of the compass he could not judge—was so uncanny that he was really disturbed. He bounded into the chassis and roused Ben and Harry. He had hardly whispered to them the extraordinary intelligence when again the voice arose.
"A-ho-ho-h-o-o-o-A-h-hoo-ho-AH-HO-HO-O-O-O-AH-ho-h-o-o-o-o-o-o!"
"Well, who?" roared Ben angrily, "come out and show yourself, you human hyena, and I'll put so much lead in your system you'll be worth a nickel a pound. Come, you old Ah-Hoo, and I'll show you who I am quick enough—shiver my topsails!"
But the only reply to Ben's tirade was the dismal echo of his voice among the rocky chasms.
"Shiver my topsails!" roared the echo and then the hills bandied the cry about from ridge to ridge till it died out in a whisper:
"My topsails!"
"Hum," remarked Ben, "I don't think I'll talk so loud around here. There seem to be a lot of folks listening. Such a dreary hole as this I never—"
"Never," sighed the echoes, "—never."
"Here, I can't stand this," cried Harry. "I'm going to send a bullet up there the next time that fellow starts 'Ah-hooing."'
But as the strange mournful cry rang out once more the boys paused in bewilderment.
There was no locating the sound.
It seemed to fill the air. To come from every quarter of the compass at once.
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