As they stood thus, a third person entered, swiftly, silently, without being seen either by von Hardenberg or the Guardian of the Cave. Those in the gallery saw who it was: the man was the sheikh, the Black Dog of the Cameroons.

Of the scene that followed the watchers in the gallery were amazed and horrified spectators. It seems that Captain von Hardenberg had not been idle during the time the Sunstone had been in his possession; with Teuton thoroughness and industry he had even learnt to speak in the Maziri tongue.

"Who are you?" said he to the old man, so strange and terrible to behold.

"I do not ask who you may be," answered the other, "because I know."

Word by word, the following conversation was afterwards repeated to Harry by Fernando.

"You know!" cried von Hardenberg. "What do you know?"

"I know that you are he who bears the Sunstone on your person. I order you to deliver it up!"

Von Hardenberg drew back a pace. The Black Dog was crouching like a tiger behind one of the pillars, unseen by either of the speakers.

"Who told you?" cried von Hardenberg. "Who told you I have the Sunstone?"

"These things," said the old man in a great, solemn voice, "these things I know because I am one who holds converse with the gods. Me you cannot deceive. A short time ago I was asleep, and in my sleep I dreamed a dream-that the Sunstone had returned."

"You are mad!" cried the Prussian in brutal derision.

"Aye," said the man, "I am mad; but I am wiser than those who are sane. Deliver up the Sunstone!"

"By what right?"

"By every right. I am the Guardian of the Cave. I have lived five score years, and never once have I ventured beyond the entrance of the Caves of Zoroaster. Come, deliver up the Sunstone."

"And if I refuse?" asked von Hardenberg.

"If you refuse," said the man, "you die!"

Von Hardenberg looked about him with a quick, furtive glance. Softly his hand crept to his belt, where he carried the holster of his revolver.

What happened next was the work of a few seconds. Those in the gallery had no time to interfere. As for the sheikh, he evidently intended that the tragedy should be played out to its end, to the falling of the curtain.

The old man, seeing von Hardenberg's action, lifted his great two-handed sword and flourished it on high. Then, with a spring like that of a tiger, he hurled himself upon the Prussian.

Three shots rang out in quick succession. There were three flashes of fire, like jets of flame, and then three puffs of smoke. The cave was filled with an echo that went on and on as if it would never cease.

And when the smoke cleared, there was the old man lying upon his face upon the floor, silent and still. A century had rolled above his head, for a hundred years he had stood guardian of the Caves of Zoroaster-and now his task was ended.

Harry sprang to his feet, and would have fired then and there at von Hardenberg had not Cortes held him down by force.

"It was murder!" he whispered.

"If you fire, we are lost," cried Cortes. "It is too dark to shoot straight, and the Black Dog will escape us."

Harry resumed his kneeling position and waited.

A horrid silence reigned in the great, domed chamber. The scene was more tragic, more fantastic than ever. The shafts of light from above struck the body of the murdered man; the lamp still flickered before the altar. Even yet, the echoes of the shots were murmuring in the deeper recesses of the place.

Captain von Hardenberg stood stock-still, his revolver in his hand, thin wreaths of smoke issuing from the muzzle. From out of the heart of the stillness there came a chuckle: the Black Dog was pleased to laugh.

Murder was nothing to him. He had dealt for years in human lives. He was implacable, relentless. And even at that same moment he himself contemplated a greater crime, for the commission of which he was hiding in the darkness like a snake, biding his time to strike.

Captain von Hardenberg took two steps towards the body and turned it over with his foot.

"He is dead," said he in German.

The old man, who had been so terrible in life by reason of his madness, now looked sane and beautiful in death. The worn, agonized expression had gone altogether from his features, which were now calm and wholly at peace. With his white hair and ragged clothes, he was like one of the patriarchs of old.

Captain von Hardenberg was not himself. It was plain to see that it was all that he could do to control within him a feeling that was akin to terror. He looked about him with widely opened eyes-at the vast pillars, at the darkened corners of the aisles, at the shafts of sunlight that pierced the darkness like the blades of swords.

With trembling hands he attempted to unbutton his coat. His nerves were so shaken, and he in such feverish haste, that he could not at first succeed. In the end, as if grown desperate, he took a knife from his pocket, opened the largest blade, and cut off the buttons one by one. Then he ripped open his waistcoat, and, a moment after, drew forth the Sunstone and placed it on the altar by the side of the burning lamp.

And next he did a strange thing indeed. He burst suddenly into loud laughter-laughter that was hysterical, delirious.

He had gone through so much; he had faced so many dangers; he had been guilty of a score of crimes; he had lost everything-good name and honour and position-in order to possess himself of the treasure that lay beyond the red granite rock.

And now that all this wealth was as good as his, he could do little else but laugh, in a kind of wild delirium, whilst tear-drops in quick succession coursed down his cheeks.

After a while he mastered himself a little, but not completely. He went to the nine wheels and turned them all ways in a fever of excitement.

Then he remembered what he had to do. He studied the wheels and took notice of the cuneiform writing on the "tyres". At that he returned for the Sunstone and brought it to the Bramah lock.

But, since it was too dark there to see the writing on the stone, he took it back to the altar, and laid it down once more before the lamp. Then he studied the character in the first segment, and, having committed it to memory, he went back to the wheels.

Slowly he turned the first wheel, noting each character as it appeared above the golden bar. At last he appeared satisfied. The cuneiform figure, or character, which lay immediately above the golden bar corresponded to that upon the Sunstone.

Then, in a like manner, he turned the second wheel. Always when he got the wheel in the correct position he compared the two characters-that upon the Sunstone and that upon the wheel-to make sure they were the same.

Finally, he came to the ninth wheel. His excitement was now so great that those in the gallery could see that he was trembling violently in every limb.

He troubled no longer with the Sunstone. He turned the wheel very slowly, with his eyes fixed upon the red granite rock. Presently there was a "click" like the sound of the turning of an enormous lock. Captain von Hardenberg held the wheel quite still.

There came another "click" even louder than that which had gone before. And then slowly, like some great living monster, the rock began to turn, as if it revolved upon a pivot.

It turned evenly, slowly, noiselessly, and, as it turned, the light from the lamp caused the quartz and mica and felspar in the granite to glisten like a thousand fire-flies on a summer's evening.

And then, in the moving rock itself, appeared a narrow archway about four feet across; and when this was immediately opposite the altar there was another "click" and the whole rock was still.

Those in the gallery sprang to their feet and looked on with bated breath. The thing was like a miracle. As for von Hardenberg, he gave vent to a cry that was half a cheer and half a sob. Then, snatching the lamp from the altar, he rushed through the archway into the darkness beyond.

From the gallery they could see the light grow smaller and fainter as the Prussian descended a narrow flight of steps. Then the light went out, and there came up from the vault beyond a faint cry of exultation. Captain von Hardenberg had attained the treasure of Zoroaster.

And it was at that cry that the Black Dog glided from his hiding-place. Now that the lamp had gone, the cave was darker than before. But by the light that came from above, and through the entrance, those in the gallery could see his white robes as the man glided noiselessly across the hall.

He went straight to the altar, picked up the Sunstone, raised it to his lips, and kissed it. For a moment he gazed at it, long and lovingly, before he thrust it into a pocket.

He moved on tiptoe towards the wheels. As he did so he passed through one of the shafts of light, and his features were illumined. On his face there was an expression that was diabolical. It was the face of a beast of prey, a tiger that stalks its victims. His white robes contrasted strangely with the swarthiness of his countenance. His eyes were very bright and now looked yellow like those of a cat.

When he reached the wheels, he let out a great shout that filled the vastness of the cave.

"Die!" he cried. "Die the death you merit!"

At that he set the wheels in motion, and immediately the great granite rock revolved again. And Captain Carl von Hardenberg was buried alive in the midst of the treasure that was his.

The sheikh passed rapidly down the centre aisle. Half-way to the entrance he stopped, looked back, and shook his fist at the rock.

"Lie there," he cried, "and rot! In my own good time I will return."

Before the last word was from the man's lips, Fernando had lifted his rifle and fired. The bullet flattened itself against a pillar not three inches from the Arab's head. The Black Dog glanced up at the gallery and then dashed out of the entrance, so swift and agile in his movements that it would have been sheer folly to fire again.

"You hit him?" cried Harry.

"No," cried the man, with a sullen oath. "I missed. It was too dark to see."

"Too dark to see!" repeated Harry. "But he is gone! Make haste, or he'll escape!"

CHAPTER XXVIII-A Race for Life

Fernando stood motionless, his rifle in his hand. He had been within an ace of fulfilling his oath, and sending the Arab to the shades.

"I would have hit him," he complained, "had the lamp not been taken away."

Meanwhile Cortes dashed down the steps, and crossed the central aisle to the body of the murdered man.

The madman lay quite still. A life of fasting, of penance and privation, had closed in the heroic fulfilment of his duty. With his last breath he had demanded of von Hardenberg to deliver up the Sunstone; and there he was-a huddled, formless object, lying at the foot of the altar.

The first impulse of Harry Urquhart was to follow in pursuit of the sheikh. With this intention he hastened to the terrace, whence he could see nothing. The Black Dog had vanished into the white mists that wrapped the mountain-side. By now he was no doubt at the bottom of the great flight of steps on each side of which stood the strange, fantastic statues.

Harry, rifle in hand, was about to take up the chase, when he remembered that somewhere beyond that impenetrable granite rock was von Hardenberg-alone in the midst of the treasure.

He returned to the cave, and went to the rock and listened. He could hear nothing. Beyond, all was silent as the grave.

"What can we do?" reiterated the boy, looking about him in bewilderment.

Jim Braid went to the nine wheels and turned them at random, hoping that by chance the vault would open. In a little while he desisted and returned to Harry.

"We must follow the sheikh," said he. "We must endeavour to recover the Sunstone at every cost."

"And leave him here?" said Harry, with a motion of the hand towards the granite rock.

"We can do nothing," said Fernando.

"I bear the rascal no goodwill," said Harry. "He deserves but little pity. But this is terrible!" he added, and repeated the word again and again.

"Come," said Cortes, "we waste time in talking."

As he spoke, he led the way from the cave, followed by the others.

As they passed down the great flight of steps, Harry Urquhart turned and looked back. The entrance to the caves was no longer visible. A great cloud lay upon the mountain like a mantle. Near at hand, the strange beasts carved in stone were quite conspicuous and plain, but gradually, as they mounted one behind the other towards the terrace, they became lost in the mist. They resembled an army of quaint, primeval animals that were filing down from the clouds to inhabit the abodes of men.

The elder guide, shading his eyes with a hand, scanned the mountains to the north. Presently he let out a cry-a cry of exultation.

"There!" he cried, pointing across the valley.

Sure enough, far in the distance was a white speck that was moving rapidly upon the mountainside, disappearing for a moment to appear again, always bearing in the same direction-towards the north.

Cortes turned to the others.

"I can run," said he. "I was a tracker once by trade. I undertake to keep upon his trail. Do you follow as quickly as you can."

Fernando laid a hand upon his brother's shoulder.

"You will not kill him?" he said.

"No. The man's life is yours."

With these words Cortes sped upon his way, springing from boulder to boulder, supple in figure, agile despite his wound. He had spent much of his life hunting wild game in the midst of unexplored, inhospitable hills. He was quick of eye and sure of foot.

Outrunning his companions, he went rapidly upon his way, and was soon lost to sight. All that afternoon they followed in his tracks, and towards evening they heard a shot, high up in the mountains, many miles to the north.

A grim smile passed across the face of the elder guide, who calmly turned to Harry.

"Yonder," said he, "is the sheikh."

"It was he who fired?" asked Harry.

Fernando shook his head.

"That shot was fired by my brother," he answered. "I know the sound of my brother's rifle."

"Where are we going?" asked Jim.

The half-caste shrugged his shoulders.

"The Black Dog chooses the way," said he.

"He goes to his home?" asked Harry.

"His home!" repeated Fernando. "Has the wild dog a home? Does the hare burrow in the ground? The Black Dog sleeps where he finds himself. All the world is his home. He may go into Nigeria; he may cut back to the coast; he may pass through the mountains to the great Sahara Desert. But, wherever he goes, Cortes will follow him; he will be followed to the ends of the earth. And now and again Cortes will fire his rifle to guide us on our way, to let us know that he still holds the Black Dog in view."

Throughout the days that followed, the mountains witnessed the almost superhuman efforts of two men: Sheikh Bayram, the Black Dog of the Cameroons, and Cortes, the half-caste Spaniard of the Coast.

The one fled from justice, clutching the Sunstone in his hand, and the other followed, until miles grew into leagues, until they reached the rolling grasslands to the west of Lake Chad, where cattle grazed in herds.

It was a struggle of Titans, a race for life or death between men who were well versed in the craft of the hunter, who knew each bridle-path and mountain-spring and solitary oasis between the bend of the Congo and the Atlas Mountains.

Day and night they raced onward, under the march of the southern stars. And Cortes clung to the heels of Black Dog like a leech. As often as the sheikh halted, he was obliged to push on again in greater haste.

At nightfall, every evening, Cortes fired his rifle, and this enabled his brother and the two boys to keep upon his track. The route taken by the sheikh was not a straight one: the course he followed was in the shape of the letter S. Harry and his party were often able to take short cuts, completing one side of a triangle when the Arab and his pursuer had accomplished the other two. Thus it was that upon the twentieth day they came to the place where the younger guide was encamped.

"He is close ahead?" asked Fernando.

Cortes pointed to the west.

"He is in the valley yonder," said he. "To-night he sleeps in the jungle that lies on the edge of the plateau."

They were now in a part of the globe of which little is known. They had left the cattle far behind them. This country is uninhabited except by wild animals, and is visited only by the caravans that come south-east from Timbuctoo.

The Black Dog, with the Sunstone in his possession, still held his course towards the north, setting forth across the illimitable, barren waste. He journeyed for two days without halting. Then he crossed a river, and, passing over a plateau, descended into the true desert, where the sun blazed like a furnace.

CHAPTER XXIX-The Temple

On the skirting of the desert lay a small Arab village-a place of a few dilapidated huts, accommodating not more than a score of inhabitants. For the most part these were people sunk to the lowest depths of poverty, living in a state of dirt unimaginable to those who are not acquainted with the Arab.

To this village came Harry and Braid and the elder guide. The headman of the village came forward on their approach, followed by a few children.

Fernando, who had an intimate knowledge of Arabic, was able to act as interpreter. The headman said the village had been rich in the possession of two camels; but, late on the previous evening, an Arab had come from the plateau who had purchased one of these camels. Early that morning had come another man, a white man-as he said-who, having purchased the other, had set forward without delay in the same direction as the Arab.

"Did the second man leave no message?" asked the guide.

The villager replied that the "white man" had left word that those who followed him were to wait in the village until he returned. He expected to be back that night with news of great importance.

Accordingly they halted for the afternoon, and, giving the villagers a wide berth, camped upon the sand, lighting a fire, upon which they cooked a meal.

"And all this time," said Harry, "von Hardenberg lies buried alive, starving to death in the Caves of Zoroaster."

Fernando shook his head.

"He cannot starve," said he. "I noticed he wore a haversack well filled with provisions. And I have heard it said that inside the vault is water; a small spring bubbles up in a great basin, forming a little fountain."

"You have seen it?" asked Harry.

"No," said the man; "but I know what I say is true. And, even were there no water in the cave, the Prussian carried a water-bottle."

Harry Urquhart sighed.

"All this is like a dream," he exclaimed.

"The scoundrel deserves no better fate," said the guide, hard of heart and pitiless where his enemies were concerned.

At nightfall they lay down to sleep, Jim Braid remaining on sentry for the earlier part of the night. They had small reason to trust the people of the village, who were not incapable of murdering them in the night for the sake of their possessions.

At about ten o'clock Jim was alarmed by a peculiar grating sound quite near to their camp. The moon had not yet risen, and, though he strained his eyes in the direction from which the sound had come, he was able to distinguish nothing. At last he rose to his feet and walked some little distance from the fire. There he discovered a camel lying down upon the ground, engaged in chewing a bundle of coarse hay. The camel had appeared as if by magic.

Jim returned to the fire, and there to his amazement found Cortes sitting before the embers.

"You have returned?" said he.

"Yes, I have returned."

"With news?"

"Of the Black Dog. Yonder in the desert is an ancient temple or mosque. It stands in an oasis where there are palm-trees, and around which melons grow. For many years it has been deserted. The sheikh himself is there."

"We must awaken the others," said Braid.

"There is plenty of time," said the man. "He will not move before daybreak. The night is yet young. We will surround the oasis at sunrise and take him alive. Fernando must fulfil the oath he has made to the saints."

Jim Braid had not such patience. With this news upon his mind he could not stay idle while Harry and Fernando were asleep. Despite the advice of Cortes, he awoke them both, and told them what had happened.

"We must start at once!" cried Harry.

"There is no haste," said Fernando, with a shrug of his shoulders. "But, if you wish it, we will go."

They packed up their camp equipment and provisions, and their reserve ammunition, and these they loaded upon the camel. Then they set forward on their way, following a caravan route, whilst a full moon, red as blood, crept over the horizon and illumined the wasteland like a lantern.

In three hours they came to a place where a stone building, surrounded by a dozen trees, stood forth against the moonlight. Near by a hyena howled.

"The sheikh is within," said Cortes.

Drawing a little distance away, the four held consultation together. As far as they could make out, there was but one entrance to the temple, which was half in ruins. For all that, they thought it best to surround the place, and it was finally agreed that Harry Urquhart should enter the building, revolver in hand, whilst the other three guarded the walls to prevent the Arab's escape, should they have overlooked some other means of exit.

Leaving his rifle behind, with his revolver ready loaded in his hand, Harry passed on tiptoe through the entrance and found himself in a shallow, darkened chamber.

Though there was no roof to the building, the adjacent palm-trees shut out the light of the moon, and some seconds elapsed before the boy's eyes grew accustomed to the semi-darkness.

As far as he could make out, he was surrounded by high walls. Scattered here and there about the floor, upon which the sand of the desert lay like a thick, luxurious carpet, were great cylindrical boulders, which, in former times, had evidently composed the pillars that supported the roof. In the shadow of these boulders it was quite dark, and each shadow was large enough to conceal the form of a man.

The boy decided to act with caution. With such an opponent as the sheikh he knew he would be called upon to exercise not only promptitude but cunning. It had not been without difficulty that he had managed to persuade the guides to allow him to enter the temple. Fernando, who was filled with a strong desire for vengeance, had wished for the honour for himself. But Harry, as the leader of the expedition, would not give way, agreeing that the moment he fired the elder guide should hasten to his assistance.

Ready to fire at a moment's notice, Harry set about a systematic search of the ruined temple. Starting from the entrance, he worked his way around the walls, holding as much as possible to the shadows. He looked behind each boulder, he searched each crevice that appeared large enough to admit the body of a man. In the end he returned to the entrance. The place was evidently deserted.

His first thought was to leave the building, to tell Cortes that he had been mistaken, that the sheikh was not there; but then he remembered how seldom the judgment of either of the guides had been at fault, and, assuring himself that he had overlooked some hiding-place, he began his search anew.

He came to a place where a clump of cactus was growing against the wall, and here he discovered what he had not noticed before. Under the cactus plant was a little archway, a kind of tunnel, large enough to admit a man crawling on hands and knees.

The boy was in two minds what to do. It was one thing to search from boulder to boulder, ready to fire at a moment's notice; it was another to go head foremost on all-fours into what might prove to be a trap. If the Arab was hiding on the other side of the wall, beyond the cactus-bush-a stroke of the knife, and the matter would be ended. The boy had need of all the courage he possessed. To go back to Jim and the two guides would be to confess himself afraid.

Taking a deep breath, like a man about to dive, he lay flat upon his face, and as silently as possible worked his way forward through the sand, which was still warm from the rays of the sun of the preceding day.

If there were many holes in the wall such as this, it was well three of them had remained on guard without. Had all four entered the temple at once, the sheikh, if he lay anywhere in hiding, had a sure way of escape. Harry had no means of knowing whether the hole led to the desert or to an inner room.

On the other side of the wall it was quite dark. The boy looked overhead, and was able to see that he was sheltered by a roof-a roof in which there were great holes, through which he could see the stars. He could do nothing as yet, until his eyes had grown accustomed to the darkness.

For some moments he lay still, his heart thumping against his ribs, straining his ears to catch the slightest sound. From somewhere quite near at hand, at first almost imperceptible but gradually growing louder, came a low, soft, vibrating noise that seemed to proceed from somewhere under the ground.

Harry thought he had heard something like it before; he could not remember where. It was like the droning of a monster bee, or the noise of a kettle on the point of boiling over, or else the purring of a cat.

How long the sound continued he was never able to say. It seemed to him that he lay for an eternity, breathless, waiting for something to happen, with the sound continuously in his ears. And then he became aware of two great, yellow eyes, staring in the darkness, immobile, like flaming lamps.