Bob had not walked many yards along the rough floor when he heard a slight sound ahead. Thrusting his torch well forward, he advanced with great caution, feeling some anxiety now he had the evidence of his own ears that Mrs. Pottle had not been dreaming. If by any chance one of the enemy had found his way to the cleft, Bob felt that he himself would be at a great disadvantage, being seen while the other was unseen. "But I am in for it now," he reflected; "it would be as dangerous to retire as to advance." He was so intent upon watching the space in front of him that he stumbled more than once over the rocky, uneven floor of the cavern. At every few yards he stopped to peer more carefully, and to listen; always he saw nothing, always heard the sound of a light footfall ahead. Fearing that his quarry would escape him he hurried his pace; there was a corresponding acceleration in front; he hurried still more, so did the other; until Bob, his uneasiness now banished by set determination, was pushing forward at the utmost speed the rough ground permitted. Now he heard the sound of heavy breathing, and a scurrying noise as of more than one person in full flight. Stumbling, falling, scrambling on, Bob dashed in pursuit; he must know at all costs what this new peril might be. At length, at a point where the roof shelved downwards, he caught a glimpse of a dark form not many yards in advance; it was immediately lost in shadow, then again it was discovered by the torch. He did not venture yet to fire, but sprang forward to reduce the distance between himself and the hurrying form. He was gaining on it; its pace appeared to be slackening; he called on it to stop, or he would fire. Suddenly the form disappeared, sinking as if into the floor. Bob took a couple of steps, and then started back with a thrill of terror. He felt wind upon his cheek; the torch flared more brightly. He had emerged from the cleft; beneath him yawned a vast empty blackness. He was on the brink of an abyss. At that moment he heard from below a heavy thud, and started back from the edge with a sudden feeling of faintness. Then he became aware that his torch could be seen from far around, and flinging it upon the ground he stamped out the flame. Recovering from his momentary vertigo, he crawled cautiously to the brink of the precipice. There was nothing to be seen, save where a thin streak of still unmelted snow in a crevice a few yards below made a grayish patch on the black. He rose, picked up the torch, relit it when he had retraced his steps for several yards into the cavern, and made his way back to the other end. "Well?" exclaimed both the ladies in a breath, as they saw him in a circle of light some time before he reached them. "We have been so much alarmed about you." added Mrs. Pottle. Ethel gave a sigh of relief. "There was something, or somebody," said Bob quietly. "A Russian?"