Straight up against the bottom of the hull the pair went.

When they returned to the surface the metal cylinder was in place below.

Glancing backward only once, to make sure that Williamson was not yet on deck, and that the gunboat’s marine guard had not detected their stealthy work, the swimming pair struck out lustily for shore.

Back into the same clump of bushes they made their way.  In the first few moments neither of the recent swimmers appeared to dare a glance into the face of his comrade.  In silence they fitted the shore end of the wire to the battery.

Then one of the pair seized the handle to pomp the fatal electric spark along the wire to the hidden mine under the “Benson’s” hull.

“Remember what happened to the ’Maine’!” this wretch chuckled hideously.

CHAPTER XIX

A joke on the secret service!

“What’s that noise?” wondered Williamson.

He stopped, listening intently, for he was still below.

Against the bottom of the “Benson’s” hull he heard a steady, slow, monotonous bumping.  As he listened, his face took on an anxious look.

“We’re in a friendly port,” muttered the machinist.  “It can’t be anything very wrong, and yet—­”

That slow steady bumping continued.

“Anything bumping against the bull of a boat at anchor, in that fashion may be wrong,” concluded the man, swiftly.

His mind made up to this much, the rest was not difficult to decide.  The cause of that bumping required instant investigation.  Williamson caught up the tool that came quickest to hand, a pair of nippers, thrust them into his jumper and raced up to the deck.

“If it’s any real mischief,” he muttered, “I hope I won’t be too slow—­too late!”

With that he dived overboard, at the starboard rail, the side nearest the gunboat.  There was a splash—­then the waters closed over the machinist.

He came up at about the point he had planned, where he had heard the bumping.

Held below water as he was by the under-hull of the submarine, he could move with certainty, though but slowly.

Groping, the machinist encountered the metal cylinder.  Quickly he felt for its connections which, like a flash, he knew must exist.  He found the wire, but reached for another.  It all had to be done swiftly, for his reserve “wind” was fast giving out.  Not finding a second wire, he fastened his nippers against the first wire—­then cut.  Now, steering the metal cylinder, he pushed it out from under the hull.  Cylinder and man rose together.

Whew!  What a powerful breath the man took!  Then he steered the cylinder carefully against the hull, and managed to hold it there until he could reach a piece of cordage and make the cylinder fast.

This done, he dashed below, thumping hard on the door of the stateroom occupied by Captain Jack Benson and Hal Hastings.