vision could
not see that the bush hidden pair on shore, guessing his intention
from his stop by the conning tower, had silently taken to the water.
With them they towed the metal cylinder, which floated. To the cylinder
was attached one end of the light wire.

Some distance out from the shore the pair halted, treading water, only
their eyes above the surface. But Williamson could not make out such
small objects at the distance. Then he went below.

"Now, for it," breathed one of the swimming pair, tensely.

Both swimmers struck out strongly, yet silently, making fast progress
through the water by means of some of the best strokes known to swimmers.

When they reached the port side of the submarine Williamson was still
below. Nor had the attention of the marine guard on the "Waverly"
been attracted.

In just another swift instant the swimmers made a dive that carried them
and their cylinder below the surface.

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