To the rear a bulky, hulking figure rose and stood erect. With the softest of steps this apparition of the night followed after them, until it stole along, ghost-like, just behind Hazelton. Then a huge arm was raised, threateningly, over Harry's head. At that particular moment, as though insensibly warned, Hazelton stopped, half-wheeling. In the next second Harry bounded back just out of reach of the descending arm, the hand of which held something. But in that backward spring Harry, in order to save himself from pitching into the water, was oblige to turn toward Reade. "Tom!" exploded the young engineer. "Flash the light here quickly!" In the instant, however, that Harry had sprung backward the figure had slipped noiselessly into the water to the left. As Reade wheeled about, throwing on the light, he let the ray fall in the water to the right of the wall. But no sign of the intruder appeared; the water had closed noiselessly over the now vanished figure. "What's the matter?" asked Reade, as he stood looking, then finally flashed his light over to the other side of the wall. "I saw---" began Hazelton. Then changed to: "I thought---er---I saw---oh, nonsense! You'll josh the life out of me!" "Not I," Tom affirmed gravely, as a thrill of pity, for what he deemed his friend's unfortunate "nervous condition," shook him. "Tell me what you saw, Harry." "Why, I thought I saw a big fellow---a black man, too---right behind me, arm upraised, just ready to strike me." "Well, where is he?" Tom demanded blankly, flashing the light on either side of the narrow wall-top. "See him anywhere now, chum?" Harry didn't. In fact, he hardly more than pretended to look. The thing that had been so real a moment before was now utterly invisible. Hazelton began to share his chum's suspicion as to the utter breakdown of his nerves and powers of vision. "It was nothing, of course," said Harry, shamefacedly, but Tom vigorously took the other side of the question. "See here, Harry, it must have been something," insisted Reade. "You're not dreaming, and you're not crazy. It would take either one of those conditions to make you see something that didn't really exist. No mere nervous tremor is going to make you see something as tall as a man, standing right over you, when no such thing exists." "Well, then, where is the fellow?" Harry Hazelton demanded, helplessly, as he stared about. "There isn't any human being but ourselves in sight, either on the wall or in the water. Your light shows that." The light did not quite show that, and could not, since the huge prowler was now swimming gently under water, some seven or eight feet from the surface.