Then the tug went back over by the fort.  Jack grew tired of waiting, for it was some two hours ere the tug finally left the ordinance wharf at Fort Craven.

It was warm out there, on the low, sandy cliffs, provided one got into a position sheltered from the ocean winds.  So Jack, in the weariness of his waiting, threw himself down in a sheltered hollow.

Finding that the sun shone disagreeably in his eyes, the submarine boy pulled his cap forward over his face.

Then, in the course of a very few minutes, the inevitable happened.  Jack Benson drifted off into sleep.

He awoke with a fearful start, for he had no idea how long he had slept.  Yanking out his watch and noting the time, the submarine boy concluded that he had not been asleep more than twenty or thirty minutes.

“But I might just as easily have slept for hours,” Benson reproached himself.  “Then what a hero I’d have felt.  Asleep on post!”

At that moment Jack Benson heard a faraway whistle, across the bay.  Showing just the top of his head above a ridge of sand, Captain Jack saw the Army tug just pulling out from the dock across the bay.

But Jack saw something else, too, in that brief instant.

A slim, soldierly-looking man of perhaps thirty, tall and of naturally good carriage, was skulking along in front of the submarine boy, yet hidden from the bay by a sand ridge.

Under one arm the stranger carried a draughtsman’s board and a book.  A strap over one shoulder held a field-glass case.

“Where in blazes have I seen that chap before?” wondered Captain Jack Benson, staring hard.  “For I have seen him—­somewhere.  I’d declare that under oath.”

Figure, carnage and face all strangely haunted the submarine boy, who crouched lower, watching.

“By the great turret gun!  He’s skulking for a reason!” muttered Benson.  “Is he spying on the mine-planting?  I wonder?  Yes!  That must be his work!  Long-legs, I’ll keep my eyes on you!”

The stranger hastened along for perhaps a quarter of a mile further.  Then he threw himself down on the sand, choosing a position in which he could lie flat, his head fairly well hidden behind a low ridge of sand.

Unslinging the field-glass, the stranger brought it to his eyes, closely watching the progress of the tug.

“Ha-ha!” muttered watchful Jack, who had followed, keeping behind another sand ridge.  “So, sir!”

The minutes passed, though Jack Benson was so absorbed in watching this long stranger that the boy had but the vaguest notions of the flight of time.

The tug had halted, now.  A great crane at the bow swung around, and a submarine mine hung poised in the air.  Then, with a rattle of chains not audible at the distance, the mine was slowly lowered until it touched on bottom.

While this was going on, the long-legged stranger, wholly absorbed in his own work, made some observations and some hurried calculations.  Then he pulled the drawing-board toward him, jotting down a point.