The mystery of the man on the hill bade fair to be an unsolved one, for although the boys watched for some time with considerable anxiety he did not reappear. This feature of the incident set them to comparing notes and they found that their impression of the apparition differed considerably. Both Frank and Harry were ready to swear that he was a black man, while Ben Stubbs was equally convinced that his skin was of a reddish hue. All three, however, agreed that he was weaponless so far as could be seen, and his attitude appeared to be more one of interested curiosity than of actual hostility.
"Well, there's no use wasting time in speculation," said Frank at last, "more especially as it does not look as if we can get any nearer to solving the problem in that way. The thing to do now is to get at the ivory and that as quickly as possible. If that man is the forerunner of a band that means to attack us, it is all the more reason that we should get a move on."
"Right you are, Captain," assented Ben, "and here goes!"
With a mighty swing of his pick the former prospector dislodged a pile of the rough stones of which the cairn was composed and the boys, too, laid on with a will. In an hour or so all that was left of the once lofty cairn was a few big rocks.
Excitement ran fairly to fever heat as the last obstruction that lay between the adventurers and the ivory hoard was cast aside.
In a few minutes now, if all went well, they would be in possession of the treasure. More than once as they worked, Frank drew his field-glasses out of their case and scanned the surrounding wilderness of rocky chasms and swept the green jungle that lay stretched like an emerald ocean far below, but each time he replaced them with a sigh of relief. So far there was no sign of any rivals' approach, although Frank well knew that by this time Muley-Hassan must be upon his way to contest the boys' claim to the ivory.
As the last stone was chucked aside with a mighty heave by the combined forces the perspiring adventurers broke into a hearty cheer.
Beneath it was a wooden trap-door which had a ring placed in the middle evidently for the purpose of lifting it. Frank gave it a heft, but the weight was too much for even his wiry muscles; but when Ben and, Harry assisted him the door gave with a jump that threw them all to their feet.
Scrambling up in a second they rushed to the edge of the hole revealed by the uplifting of the wooden cover. What they saw showed them instantly that their wildest hopes had not been overdrawn. There, at their feet, lay a king's ransom in yellow ivory.
From the hole rose a fetid, sickening odor that at first was almost overpowering. It came from the rotting flesh that still adhered to the roots of many of the huge trunks.
With a cheer Harry was about to spring down into the aperture when Frank, with a quick exclamation, drew him back.
"Jump back for your life!" he shouted.
Harry was accustomed to obeying his brother in everything, and jump backward he did with an agility that would have done credit to a gymnast. Before he could ask a question Frank's revolver cracked and a little spit of dust shot up almost at his very feet.
There lay a tiny snake viciously wiggling about in its death agony, pierced through by Frank's bullet.
It was a rock adder—one of the deadliest of African snakes. Barely more than three inches in length, and a dull gray in color, it was small wonder that Harry in his excitement had not seen it as he was about to jump almost upon it.
"We shall have to be careful," said Frank, as he kicked aside the still writhing body of the disgusting looking reptile. "There is just a chance that Muley-Hassan, with the cunning of an Arab, may have put several more of those customers in here to guard his ivory."
It was therefore cautiously that the boys proceeded to work at getting the ivory out of the hole and although they killed three more of the venomous reptiles it seemed more probable that they had got in by accident than that the Arab slave-dealer had deliberately placed them there. By mid afternoon a big pile of ivory lay ready for transportation to the Golden Eagle Il and only a few more tusks remained in the hole.
"How are we ever going to get the tusks down the hill to the Golden Eagle II?" asked Harry as he gazed at the formidable pile.
"I have a better plan than that," replied Frank, "we will bring the Golden Eagle II here."
"What?" gasped both his listeners.
"Why not? It will be a ticklish job to land her on this spot, but I think I can do it. I mean to try anyhow."
"You are risking breaking up the ship," objected Harry.
"On the other hand, if we don't get this ivory out of here in jig time Muley-Hassan will be here with a big force and we shall assuredly all have our throats cut."
This argument proved insurmountable, and while Ben was left by the ivory Harry and Frank hurried down the steeps to the plateau on which they had left the Golden Eagle II. It was the work of a few minutes to tune her up. In a brief time from the moment they had left the ivory cache, considering the clamber they had had, the boys were in the air and headed for the spot where they had left the hoard.
But as they rose into the air they were startled by the sound of a shout and then another and another, then carne a volley of shots.
What could be the matter?
The shooting evidently was taking place at the spot where they had left Ben to guard the ivory.
Muley-Hassan! was the first thought that shot through Frank's brain.
The next minute, however, he dismissed the idea as absurd. The Arab, even by the swiftest marching, could not have reached the Moon Mountains in such record time unless he also had an air-ship, which Frank knew was impossible.
As the ship soared higher and rushed straight as an arrow through the air to the ivory cache a strange sight was revealed to the two young voyagers. High up on the mountain-side they could see Ben struggling with what appeared to be dozens of naked savages. The boys could see his gallant resistance as he swung his clubbed rifle again and again at his savage opponents. Several of them lay dead on the ground about him, but those that remained were attacking him with what seemed demoniacal fury.
"Good Lord," gasped Frank, "what on earth can have happened?"
"They're cannibals!" gasped Harry.
"No—no," exclaimed Frank hastily, "they're—give me the glasses quick, Harry—that's right—I thought so. They're not savages, but worse almost."
"What do you mean?"
"That they are gorillas!"
At her utmost speed the big aeroplane bore down on the scene of the unequal combat between Ben Stubbs and the savage beasts.
The boys could see that one of the brutes had seized their stalwart companion's rifle from him and with incredible strength had broken it in half as if it had been a wooden toy. The next minute Harry's rifle spoke and the gorilla that had just performed the miraculous feat of strength fell dead. With a shriek of rage the others turned to see whence came this new enemy.
At the sight of the great aeroplane bearing down upon them they at first started to flee with howls of terror, but the next minute they rallied and with low growls of rage, that bared their cruel fangs, they deliberately waited to see what this strange object might be.
This gave Ben a brief respite and he occupied it by reloading his revolver. The boys were delighted to see by this that their brave comrade was not seriously injured.
But now the Golden Eagle II was ready to settle and Frank, guiding his aerial steed with one hand, grasped his revolver with the other, for it was evident that the rush would come as they struck the ground. And come it did. As the wheels of the aeroplane struck the earth and Frank threw in the brakes sharply crashing into a rocky wall, with a howl of defiance the whole horde of man-like brutes rushed down on the air-craft with wicked rage in their spiteful little red eyes.
The leader of them, a huge "old man" gorilla, brandished an immense stone which he hurled with vicious energy at the new arrivals. Luckily it fell short of the air-ship or it would have crashed through the plane covers and have seriously crippled, if not ruined, the air-ship.
The boys' rifles cracked simultaneously and two of the attackers rolled over, with horrible human-like cries, but the leader, the bad "old man," was still in the field. As he saw his fellows fall he gave a mighty yell of rage and hatred that seemed to come from the depths of his hairy chest, and beating rapidly on it, as if it were a war-drum he rushed straight at the aeroplane.
"Don't let 'em get near the engines," was all Frank had time to shout before the avalanche of hairy, ill-smelling brutes was upon them. Some of them had armed themselves with rocks which they hurled with ferocious force. Others used nothing but their bare hands. Some of them, wounded as they were, fought with added fierceness. Desperately the boys fought them off and when the magazines of the rifles and revolvers were emptied they fell back on their hunting knives.
Frank had made a furious lunge at the "old man" and missed him by a hair's-breadth when he felt two great hairy arms encircle him from behind and the hot breath of one of his horrible opponents whistling savagely in his ear. He tried to lunge backwards at the creature, but toppled over and fell sprawling. In a flash the "old-man" gorilla was on him when Ben's revolver cracked and the "old-man," badly wounded, sprang high into the air and rolled over and over, clutching his head with both his huge hands and screaming in an agonized manner.
The fall of their leader seemed to discourage the others. They fought on for a while but it was half-heartedly. The boys had had time in the brief pause that followed the killing of the "old-man" to reload, and with their rifles newly charged they were in position to make terrible reprisals on the gorilla band for the mischief they had wrought. The monsters evidently were about to quit the battle when suddenly a cry rang through the air that ended the fight more abruptly than even the boys' bullets could have done.
"Ah-o-o-o-o-AH-O-O-O-O-o-o-o-o-o-o-o-o-o-o-o!"
It was the voice of the mountain once more.
With yells of dismay and terror the remainder of the gorilla band instantly dashed up the rocky mountain-side dragging with them, in grotesquely human fashion some of their wounded. Several of these, however, still lay on the ground and the boys put them out of their misery with a few well-directed shots. A pathetically human look lingered in the eyes of some of the injured gorillas and Harry burst out with:
"This is awful work. I'd rather fight a dozen bands of cannibals than have to do this."
"And yet," replied Frank, "if we hadn't killed them they'd have killed us."
At last the unpleasant work was over and the ivory was rapidly loaded into the aeroplane. But here an unanticipated difficulty manifested itself. Obviously the aeroplane would be too heavily laden if she attempted to carry all or even a good part of the ivory.
"Now we are stuck," cried Harry.
"Hold on," exclaimed Frank with a smile, "I anticipated this. We are going to turn the Golden Eagle into a tow-boat."
"A tow-boat?"
"That's what I said."
"What do you mean?"
Frank, in reply, bent over the stem-locker of the aeroplane and drew out what Harry instantly recognized as the silk envelope of an experimental dirigible they had built the year before.
"Now then," said Frank, "give a hand here."
They all three pulled and hauled till the envelope was spread level on the ground, all folds and creases having been carefully shaken out.
"Well," said Harry, "this would carry an awful weight of ivory, but how are you going to inflate it?"
"With these cylinders," was the answer as Frank opened the store-room below the floor of the Golden Eagle and pointed to a dozen cylindrical steel receptacles. "They contain more than enough pure hydrogen gas at a high pressure," he explained, "to inflate the bag."
In his enthusiasm Harry waved his helmet and Ben did the same.
"An aerial express, hurray!"
The inflation hose was soon connected to the first of the cylinders and with a hiss the gas rushed into the bag when a turn of the wrench set free the precious stuff. Slowly the big yellow envelope swelled and assumed shape until by the time the last cylinder was empty it was tugging and straining to rise. But the boys had weighted it down with rocks and pegged its net ropes to the ground.
The ivory was loaded into a sort of rope basket, like those used to hoist cargo aboard a ship, and in a short time, so quickly did they work, they were ready for the air, so far as what Harry called "the airbarge" was concerned.
"We shall have to strip the Eagle," decided Frank, when the inflation job was finished.
"Of everything that we can spare," added Harry, setting to work at once to rip the transoms and detach the bolts that held the heavy wireless apparatus in place. As he did so, Frank was moved by a sudden thought.
"Hold on a second, Harry," he shouted, "I'll call up the river camp before we cut off all communication."
Rapidly he sent out the call. Again and again his nervous finger agitated the key—but there was no response.
"They—they don't answer," gasped Frank at last—heavy anxiety in his tones.
"Oh, Frank, do you think anything serious is the matter?" cried Harry.
"It may only be that the apparatus is out of order," replied the elder brother seriously; "but it looks bad. That field wireless was in prime condition and it would be next to impossible for them to fail to receive our call."
"Well, there is only one thing to be done," remarked the practical Ben Stubbs.
"And that is—?" queried Harry.
"To get back there as soon as possible, for if they need us they need us dern bad," was the energetic reply.
Half an hour later the Golden Eagle, stripped of all her heavy gear and only carrying just enough gasoline to get her to the river camp, where the adventurers expected to find a reserve supply, rose slowly into the air with her queer tow tugging behind on the wireless ground rope. The boys had cached the wireless apparatus and the other gear, to be called for at some more opportune time. To their great regret, also, they had had to leave some of the ivory behind them. But the majority of what they did not dare trust to the gas-bag they carried in the chassis. Luckily for them there was hardly a breath of wind and the novel carrier towed well.
As the occupants of the great aeroplane gazed back at the sinister Moon Mountains as they fast faded out—they saw silhouetted against the evening sky a dark figure.
It was recognized at once as one of the beaten gorillas scouting to see if the terrible white men had really gone.
"There's the man we saw this afternoon," laughed, Frank, as with rapidly beating propellers the Golden Eagle II winged her way with the convoy toward the River Camp.
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