“Of course that threatening figure Harry saw behind him was an imaginary one,” Tom said to himself, but he felt uneasy nevertheless.

A few moments later Reade clutched at one of Evarts’s arms.

“Did you hear that, man?” the young engineer demanded.

“Hear what?” Evarts wanted to know.

“It sounded like a yell out there yonder,” Tom rejoined.

“Didn’t hear it, Mr. Reade.”

“There it goes again!” cried Tom, leaping up.  “Some one is calling my name.  It must be Harry Hazelton, and he must want help.  Conlon, slam it to that engine of yours!”

CHAPTER III

VANISHING INTO THIN AIR

Left by himself Harry had stood, at first, motionless, or nearly so.  He strained his hearing in trying to detect any unusual sound of the night, since it was so dark that vision would not aid him much.

There was nothing, however, but the mournful sighing of the wind and the lapping of the waves.  It seemed to Hazelton that the wind was growing gradually more brisk and the waves larger, but he was not sure of that until the water commenced splashing across his shoes.  The footway on the masonry became more slippery in consequence.

“With these rocks well wet down I wouldn’t care much about having to run back to the land,” muttered Harry, dryly.  “However, I won’t have to go back on my own feet.  Tom will have the boat out here, and undoubtedly he will plan to have us both taken back to shore after we get through cruising around here.  We should have brought the boat out in the first place.”

A night bird screamed, then flapped its wings close to Harry’s face in its flight past him.  The young engineer saw the moving wings for an instant; then they vanished into the black beyond.

Farther out some other kind of bird screamed.  The whole situation was a weird one, but Harry was no coward, though a less courageous youth would have found the situation hard on his nerves.

Still another night bird screamed, of some species with which Hazelton was wholly unacquainted.  The cry was answered by some sort of strange call from the shore.

“It’s a fine thing that I’m not superstitious,” laughed the young engineer to himself, “or I’d surely feel cold chills chasing each other up and down my spine.”

As it was, Harry shivered slightly, though not from fear.  With the increasing wind it was growing chilly out there for one who could not warm himself with exercise.

“It’s a long time, or it seems so,” muttered the young engineer presently.  “Yet I’ll wager that Tom is hustling himself and others on the very jump.”

Again the call of a night bird, and once more a sound from shore seemed to answer it.

“Real birds?” wondered Hazelton, with a start of sudden curiosity.  “Or have I been listening to human signals?  If so, the signals can’t cover any good or honest purpose.”