The stream soon widened, and recurring areas of phosphorescent rock lighted my way.

I made good time, but was convinced that I was nearly a day behind those I was tracking. Neither Woola nor I had eaten since the previous day, but in so far as he was concerned it mattered but little, since practically all the animals of the dead sea bottoms of Mars are able to go for incredible periods without nourishment.

Nor did I suffer. The water of the river was sweet and cold, for it was unpolluted by decaying bodies-like the Iss-and as for food, why the mere thought that I was nearing my beloved princess raised me above every material want.

As I proceeded, the river became narrower and the current swift and turbulent-so swift in fact that it was with difficulty that I forced my craft upward at all. I could not have been making to exceed a hundred yards an hour when, at a bend, I was confronted by a series of rapids through which the river foamed and boiled at a terrific rate.

My heart sank within me. The sorapus nutshell had proved a false prophet, and, after all, my intuition had been correct-it was the left-hand channel that I should have followed.

Had I been a woman I should have wept. At my right was a great, slow-moving eddy that circled far beneath the cliff's overhanging side, and to rest my tired muscles before turning back I let my boat drift into its embrace.

I was almost prostrated by disappointment. It would mean another half-day's loss of time to retrace my way and take the only passage that yet remained unexplored. What hellish fate had led me to select from three possible avenues the two that were wrong?

As the lazy current of the eddy carried me slowly about the periphery of the watery circle my boat twice touched the rocky side of the river in the dark recess beneath the cliff. 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.