“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 eyes, lad, an I’ll do the same with mine!”

Neither Tom Reade nor Harry Hazelton are strangers to the readers of this series, nor of the series that have preceded the present one.

Tom Reade and Harry Hazelton, now engineers in charge of a big breakwater job on the Alabama gulf coast, were first introduced to our readers in the “Grammar School Boys Series.”  There we met them as members of that immortal band of American schoolboys known as Dick & Co.  Back in the old school days Dick Prescott had been the leader of Dick & Co., though, as all our readers know, Prescott was not the sole genius of Dick & Co.  Greg Holmes, Dave Darrin, Dan Dalzell and Tom and Harry had been the other members of that famous sextette of schoolboy athletes.