"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.