A third time it struck, gently as it had before, but the contact resulted in a different sound—the sound of wood scraping upon wood.

In an instant I was on the alert, for there could be no wood within that buried river that had not been man brought. Almost coincidentally with my first apprehension of the noise, my hand shot out across the boat's side, and a second later I felt my fingers gripping the gunwale of another craft.

As though turned to stone I sat in tense and rigid silence, straining my eyes into the utter darkness before me in an effort to discover if the boat were occupied.

It was entirely possible that there might be men on board it who were still ignorant of my presence, for the boat was scraping gently against the rocks upon one side, so that the gentle touch of my boat upon the other easily could have gone unnoticed.

Peer as I would I could not penetrate the darkness, and then I listened intently for the sound of breathing near me; but except for the noise of the rapids, the soft scraping of the boats, and the lapping of the water at their sides I could distinguish no sound. As usual, I thought rapidly.

A rope lay coiled in the bottom of my own craft. Very softly I gathered it up, and making one end fast to the bronze ring in the prow I stepped gingerly into the boat beside me. In one hand I grasped the rope, in the other my keen long-sword.

For a full minute, perhaps, I stood motionless after entering the strange craft. It had rocked a trifle beneath my weight, but it had been the scraping of its side against the side of my own boat that had seemed most likely to alarm its occupants, if there were any.

But there was no answering sound, and a moment later I had felt from stem to stern and found the boat deserted.

Groping with my hands along the face of the rocks to which the craft was moored, I discovered a narrow ledge which I knew must be the avenue taken by those who had come before me. That they could be none other than Thurid and his party I was convinced by the size and build of the boat I had found.

Calling to Woola to follow me I stepped out upon the ledge. The great, savage brute, agile as a cat, crept after me.

As he passed through the boat that had been occupied by Thurid and the therns he emitted a single low growl, and when he came beside me upon the ledge and my hand rested upon his neck I felt his short mane bristling with anger. I think he sensed telepathically the recent presence of an enemy, for I had made no effort to impart to him the nature of our quest or the status of those we tracked.

This omission I now made haste to correct, and, after the manner of green Martians with their beasts, I let him know partially by the weird and uncanny telepathy of Barsoom and partly by word of mouth that we were upon the trail of those who had recently occupied the boat through which we had just passed.

A soft purr, like that of a great cat, indicated that Woola understood, and then, with a word to him to follow, I turned to the right along the ledge, but scarcely had I done so than I felt his mighty fangs tugging at my leathern harness.

As I turned to discover the cause of his act he continued to pull me steadily in the opposite direction, nor would he desist until I had turned about and indicated that I would follow him voluntarily.

Never had I known him to be in error in a matter of tracking, so it was with a feeling of entire security that I moved cautiously in the huge beast's wake. Through Cimmerian darkness he moved along the narrow ledge beside the boiling rapids.

As we advanced, the way led from beneath the overhanging cliffs out into a dim light, and then it was that I saw that the trail had been cut from the living rock, and that it ran up along the river's side beyond the rapids.

For hours we followed the dark and gloomy river farther and farther into the bowels of Mars. From the direction and distance I knew that we must be well beneath the Valley Dor, and possibly beneath the Sea of Omean as well—it could not be much farther now to the Temple of the Sun.

Even as my mind framed the thought, Woola halted suddenly before a narrow, arched doorway in the cliff by the trail's side. Quickly he crouched back away from the entrance, at the same time turning his eyes toward me.

Words could not have more plainly told me that danger of some sort lay near by, and so I pressed quietly forward to his side, and passing him looked into the aperture at our right.

Before me was a fair-sized chamber that, from its appointments, I knew must have at one time been a guardroom. There were racks for weapons, and slightly raised platforms for the sleeping silks and furs of the warriors, but now its only occupants were two of the therns who had been of the party with Thurid and Matai Shang.

The men were in earnest conversation, and from their tones it was apparent that they were entirely unaware that they had listeners.

"I tell you," one of them was saying, "I do not trust the black one. There was no necessity for leaving us here to guard the way. Against what, pray, should we guard this long-forgotten, abysmal path? It was but a ruse to divide our numbers.

"He will have Matai Shang leave others elsewhere on some pretext or other, and then at last he will fall upon us with his confederates and slay us all."

"I believe you, Lakor," replied the other, "there can never be aught else than deadly hatred between thern and First Born. And what think you of the ridiculous matter of the light? 'Let the light shine with the intensity of three radium units for fifty tals, and for one xat let it shine with the intensity of one radium unit, and then for twenty-five tals with nine units.' Those were his very words, and to think that wise old Matai Shang should listen to such foolishness."

"Indeed, it is silly," replied Lakor. "It will open nothing other than the way to a quick death for us all. He had to make some answer when Matai Shang asked him flatly what he should do when he came to the Temple of the Sun, and so he made his answer quickly from his imagination—I would wager a hekkador's diadem that he could not now repeat it himself."

"Let us not remain here longer, Lakor," spoke the other thern. "Perchance if we hasten after them we may come in time to rescue Matai Shang, and wreak our own vengeance upon the black dator. What say you?"

"Never in a long life," answered Lakor, "have I disobeyed a single command of the Father of Therns. I shall stay here until I rot if he does not return to bid me elsewhere."

Lakor's companion shook his head.

"You are my superior," he said; "I cannot do other than you sanction, though I still believe that we are foolish to remain."

I, too, thought that they were foolish to remain, for I saw from Woola's actions that the trail led through the room where the two therns held guard. I had no reason to harbor any considerable love for this race of self-deified demons, yet I would have passed them by were it possible without molesting them.

It was worth trying anyway, for a fight might delay us considerably, or even put an end entirely to my search—better men than I have gone down before fighters of meaner ability than that possessed by the fierce thern warriors.

Signaling Woola to heel I stepped suddenly into the room before the two men. At sight of me their long-swords flashed from the harness at their sides, but I raised my hand in a gesture of restraint.

"I seek Thurid, the black dator," I said. "My quarrel is with him, not with you. Let me pass then in peace, for if I mistake not he is as much your enemy as mine, and you can have no cause to protect him."

They lowered their swords and Lakor spoke.

"I know not whom you may be, with the white skin of a thern and the black hair of a red man; but were it only Thurid whose safety were at stake you might pass, and welcome, in so far as we be concerned.

"Tell us who you be, and what mission calls you to this unknown world beneath the Valley Dor, then maybe we can see our way to let you pass upon the errand which we should like to undertake would our orders permit."

I was surprised that neither of them had recognized me, for I thought that I was quite sufficiently well known either by personal experience or reputation to every thern upon Barsoom as to make my identity immediately apparent in any part of the planet. In fact, I was the only white man upon Mars whose hair was black and whose eyes were gray, with the exception of my son, Carthoris.

To reveal my identity might be to precipitate an attack, for every thern upon Barsoom knew that to me they owed the fall of their age-old spiritual supremacy. On the other hand my reputation as a fighting man might be sufficient to pass me by these two were their livers not of the right complexion to welcome a battle to the death.

To be quite candid I did not attempt to delude myself with any such sophistry, since I knew well that upon war-like Mars there are few cowards, and that every man, whether prince, priest, or peasant, glories in deadly strife. And so I gripped my long-sword the tighter as I replied to Lakor.

"I believe that you will see the wisdom of permitting me to pass unmolested," I said, "for it would avail you nothing to die uselessly in the rocky bowels of Barsoom merely to protect a hereditary enemy, such as Thurid, Dator of the First Born.

"That you shall die should you elect to oppose me is evidenced by the moldering corpses of all the many great Barsoomian warriors who have gone down beneath this blade—I am John Carter, Prince of Helium."

For a moment that name seemed to paralyze the two men; but only for a moment, and then the younger of them, with a vile name upon his lips, rushed toward me with ready sword.

He had been standing a little behind his companion, Lakor, during our parley, and now, ere he could engage me, the older man grasped his harness and drew him back.

"Hold!" commanded Lakor. "There will be plenty of time to fight if we find it wise to fight at all. There be good reasons why every thern upon Barsoom should yearn to spill the blood of the blasphemer, the sacrilegist; but let us mix wisdom with our righteous hate. The Prince of Helium is bound upon an errand which we ourselves, but a moment since, were wishing that we might undertake.

"Let him go then and slay the black. When he returns we shall still be here to bar his way to the outer world, and thus we shall have rid ourselves of two enemies, nor have incurred the displeasure of the Father of Therns."

As he spoke I could not but note the crafty glint in his evil eyes, and while I saw the apparent logic of his reasoning I felt, subconsciously perhaps, that his words did but veil some sinister intent. The other thern turned toward him in evident surprise, but when Lakor had whispered a few brief words into his ear he, too, drew back and nodded acquiescence to his superior's suggestion.

"Proceed, John Carter," said Lakor; "but know that if Thurid does not lay you low there will be those awaiting your return who will see that you never pass again into the sunlight of the upper world. Go!"

During our conversation Woola had been growling and bristling close to my side. Occasionally he would look up into my face with a low, pleading whine, as though begging for the word that would send him headlong at the bare throats before him. He, too, sensed the villainy behind the smooth words.

Beyond the therns several doorways opened off the guardroom, and toward the one upon the extreme right Lakor motioned.

"That way leads to Thurid," he said.

But when I would have called Woola to follow me there the beast whined and held back, and at last ran quickly to the first opening at the left, where he stood emitting his coughing bark, as though urging me to follow him upon the right way.

I turned a questioning look upon Lakor.

"The brute is seldom wrong," I said, "and while I do not doubt your superior knowledge, Thern, I think that I shall do well to listen to the voice of instinct that is backed by love and loyalty."

As I spoke I smiled grimly that he might know without words that I distrusted him.

"As you will," the fellow replied with a shrug.