prung 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.

"We'll have to solve the question before we leave here," declared Tom.
"We can't have folks following us up in a ticklish place like this.
Besides, Harry, I'm willing to wager that your vision---whatever it
was---has some real connection with the mystery that we're going out
yonder to investigate. So we'll solve the puzzle that's right here before
we go forward to look at the bigger riddle that the dark now hides from us
out yonder. Use your eye