But the challenge did not come and he passed casually on along the walk. They were behind him now and, unless he turned his head to look back, out of range of his eyes. His ears, however, were busy and soon they caught the sound of voices. An intersection appeared ahead and unhesitatingly the cave lord cut diagonally across it and moved out of sight of the four sentries. If he expected to find this section of the wall unguarded, however, he was doomed to disappointment. Half way down the block a single lantern sent out feeble rays from a small niche directly above a single gate--a gate guarded by a patrolling sentry. Because of the comparative narrowness of this street and the high walls on either side, heavy shadows left it in almost total darkness. Tharn, across the street and still a good hundred and fifty yards away, had not yet been observed by that lone sentry. He might, Tharn realized, be able to pass the man once without arousing undue interest or suspicion. But should he attempt to retrace his steps later on the guard would be almost certain to take some sort of action. It was not that Tharn would be unable to handle the matter if that should happen, but there was always the possibility that others might be aroused by a warning cry. Stooping, Tharn removed his sandals and, hugging the wall where shadows lay deepest, began a slow, careful advance. Thirty paces the guard took in each direction before executing a brisk about face and retracing his steps. The leather soles of his sandals made crisp clear rhythmical sounds against the stone underfoot. Each time his measured pacing brought him toward Tharn, the cave lord remained frozen, hugging the wall; when he wheeled and started back Tharn raced lightly ahead, even while he counted off each step the sentry took. On the twenty-ninth pace Tharn would freeze again, then repeat the maneuver. Finally the man of the caves reached the point where he dared go no further. He was still fifty or sixty feet down the street and another fifteen feet to one side. Hardly daring to breathe, he stood as motionless as the wall at his back until the man finished the routine of thirty paces toward him; then, as he wheeled and started back, Tharn unslipped his bow with unthinkable swiftness, fitted an arrow to its string. Mighty muscles rippled smoothly across that bronzed back as a steady hand bent the stubborn wood, a single musical "twang" sounded against the still air and flint-tipped death flickered for an immeasurable instant between the two men. True to its target flew Tharn's arrow, the sharp point striking squarely at the juncture of neck and the skull's base. Wide flew the sentry's arms and he fell soundlessly in a crumpled heap, the spear still tightly clutched in one dead hand. Even while the body was still falling Tharn was bounding toward the now unguarded gate. Unbarring it, he drew the lifeless warrior out of sight beyond, then closed the gate with his back.