The mysterious cries were not repeated that night although the boys laid awake till daylight listening for any repetition. No theory they could advance, although these ranged all the way from cannibals and gorillas to ghosts, had any effect on the solution of the mystery. They finally agreed to trust to solving it in some chance way, and like sensible boys did not continue to worry themselves over the unsolvable.
Frank's first action was to send out a wireless to the river camp and to his great relief he found that events there were still proceeding with the same regularity as before. Nothing had occurred to mar the even life of the young adventurers left behind. This was the tenor of the message, but there was something about it that worried Frank. Lathrop, he knew, was an expert wireless operator, but the sending that he performed that morning was so jerky and irregular that the rankest amateur might have done better.
"What is the matter?" asked Frank sharply after the sending had become even more unskilled and shaky.
There was no answer; which caused Frank a vague feeling of apprehension. He speedily drove this impression from his mind, however, with:
"Pshaw! the sleepless night I passed has made me nervous."
After breakfast there was so much to be done that there was no more time to waste on gloomy forebodings and the boys started, as soon as the camp had been put in order, on their expedition up the mountain-side to the Upturned Face—which was to be the starting point for the uncovering of the secret ivory hoard.
The climb was quite as stiff as Frank had anticipated and, laden as they were with the rope-ladder and the other equipment, it was rendered even tougher. All three carried water-canteens covered with wet felt, containing half-a-gallon each. Frank had insisted on this as it was doubtful if they could find water at the summit of the mountain.
As the sun rose higher in the sky and beat down on the bare rock ridges over which the adventurers were making their way, it became as uncomfortable as any expedition on which the boys had ever beer engaged.
"Talk about New Mexico or Death Valley," exclaimed Harry, "I feel like a piece of butter rolled up in a paper and I've melted."
"I feel like a Welsh rarebit myself," laughed Frank, "how about you, Ben?"
"I feel like a pot of boiling tar with a fire lighted under me," growled the veteran angrily; "consarn these rocks, I'd give a whole lot for a bit of that shade we left behind us."
Despite the discomfort and the heat, however, they struggled on up the mountain-side, frequently using the rope-ladder to get over rough places, and at about noon they stood beneath the steep rock cliff that formed the nose of the upturned face.
It was easy enough then to reach a spot below the tip and Frank, with a long cord he had brought for the purpose, laid out a straight line from the point down the southern slope of the mountain-side. While they were busy about this they were startled by a repetition of the same strange cry, half-warning, half-savage, that they had been so alarmed by the night before.
"A-ho-o-o-o-AH-H-O-O-O-a-h-o-o-hoo-o-o-o-o!"
"Great Scott," yelled Harry, "what on earth do you think of that?"
Frank—considerably startled himself—had, however, made a determined effort to ascertain the source of the sound as it rose and fell in its strange cadence.
"I've got it!" he shouted; now with a cry of triumph.
"Got what?" cried Harry, as if he feared his brother had suddenly become infected with some strange complaint—"rabies or the pip?"
"The noise—I mean I know where it comes from," cried the excited boy.
"Where?" chorused Ben and Harry.
"From somewhere about the Upturned Face," cried Frank triumphantly, "Hark!"
The strange wailing cry rang out once more. They all listened intently.
Sure enough it seemed to proceed from the sinister countenance carved in the living rock above them.
"Well, here's where we end this mystery for all time," shouted Frank, drawing his revolver, "who is game to follow me?"
Of course Harry and Ben rushed to his side, and while the echo of the mysterious cry was still sobbing and sighing among the crags they dashed back up the mountain-side utterly oblivious now to the heat or anything but their determination to discover who or what had uttered the extraordinary cry. The side of the nose—or the nostril so to speak—was formed of a wall of rock fully twelve feet in height.
"You fellows give me a boost up there and I'll travel right along the face till I find out where the racket comes from."
On Ben's strong shoulders Frank was soon hoisted up to a height where he could lay hold of a projecting bit of rock and shin himself up on to the top of the nose.
"Look out he doesn't think you are a fly and try to brush you off," laughed Harry from below.
"No danger of that," shouted back Frank, "unless I lit on him in the Golden Eagle."
The surface of the face was as remarkable as its profile.
Apparently some forgotten tribe had at some time or other been struck by the facial outline of the rocks and had cut into the flat surface, which was upturned to the sky, eyes and a mouth, the latter well provided with teeth, in each of which was drilled a tiny triangular hole.
While Frank was puzzling over the meaning of these apertures there came a repetition of the weird cry, but this time the lad was so startled that he almost lost his balance and fell backward.
The call seemed to proceed from his very feet. Then, all at once, he realized what it was.
The strange sounds proceeded from the mouth of the stone face.
Frank ran to the edge of the steep declivity that formed the nose.
"Say, Harry, and you too, Ben, examine the surface below there very carefully for any holes. They will probably be small ones and in a row."
"None this side," announced the searchers after a lengthy quest.
"Try the other," ordered Frank.
They did so and after a few minutes of careful scrutiny Harry shouted that they had found a row of small holes pierced in the rock just below where Frank stood.
"Then we have solved the mystery of the voice," exclaimed Frank.
"What do you mean?" demanded Harry.
"That it is nothing more or less than an arrangement of holes through which, when the wind blows in a stiff puff, air is forced with violence enough to cause the cry that disturbed us so much last night," was the reply.
This indeed was the solution, and had the boys known it there are many such rocks in Africa, carved out by some forgotten race, and the weird cries that the vent-holes give out in the wind doubtless acted as a powerful "fetish" to keep away troublesome enemies.
"No wonder the niggers down below don't come near the Moon Mountains," said Harry, as they all buckled over the simple explanation of the phenomenon that had caused them so much alarm. "I wouldn't care to, myself, unless I knew just what made that cry."
"It certainly was as depressing as anything I ever heard," said Frank, "and now having solved the great mystery—let's get back to work."
The three adventurers went at the job with a will. The line was about a hundred feet long and the method of procedure was this: Frank tested the straightness of the line, as accurately as possible with his eye, while Ben and Harry carried it stretched between them. The end of each hundred feet was signalized by a stone, and Harry, who was at the end of the line, carried his end to this mark before they laid out a fresh hundred feet. In this way they must have measured off very nearly half-a-mile of the mountain-side when Frank gave a sudden sharp cry and pointed to a depression in the dark range immediately below them. As the others looked they echoed his cry and gave a dash forward.
Directly beneath them, about in the center of the little dip, was a cairn of rough stones perhaps four feet in height. In a few bounds they had reached the pile, which they knew meant the discovery of the ivory cache and the end of the most difficult part of their expedition. Little did they imagine the amazing things that were yet to happen to them and of which they were but on the threshold.
"Good Lord, look at that, boys!" exclaimed Frank, as they stood at the foot of the cairn.
There was a good reason for the boy's exclamation.
Distributed around the base of the pile were a dozen or, more human skulls.
"Are they those of white men?" asked Harry in an awed tone. Frank shook his head.
"No, they are those of negroes I believe," he replied after a careful examination, "and I imagine that Muley-Hassan killed them after they erected the cache so that they would not be able to spread the knowledge of its whereabouts to any of the marauding tribes who might even brave the ghostly voice when such a great treasure of ivory tempted."
A shout from Ben, who had been walking round the pile examining it from every view-point interrupted them. They looked up and saw the old adventurer pointing to the mountain summit where it cut the sky. Outlined against the deep azure was the object that had caused his exclamation. It was the figure of a man that had apparently been watching them intently.
But as they gazed the strange, crouched form suddenly vanished.
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