as he crouched low behind the fringe of
bushes, peering toward the beach.

It was now somewhat past midnight. For three hours Tom had been scouting
stealthily along this shore section, well to the west of the breakwater.

For, in pondering over the explosions, Tom had come to the conclusion that
the blow-outs on the retaining wall, however accomplished, were controlled
from a point to the westward of the sea wall.

This conclusion had been rather a simple matter to a trained engineer.
Tom had witnessed the flash of one explosion, and that, as he remembered,
had sprung up at the west side of the wall. Moreover, the appearance and
condition of the wall, at the point of each explosion, had shown that the
attack in each case must have been made at the west side of the wall.

And now, after nearly three hours of work, Tom Reade had come upon a real
clue.

"Another blow-out is arranged for to-night, just as I had expected," Reade
muttered, with an angry thrill, as he glanced at a figure down on the
beach. "Moreover, my guess that the huge negro is the fellow who touches
off the blow-outs has proved to be the correct one."

Down on the beach a big, black man was moving about stealthily. Though the
spot was a lonely one, this scoundrel plainly intended to take no
unnecessary risks of detection.

Just at the present moment the negro was placing in the water a
curious-looking little raft that he had brought on one shoulder from its
place of concealment. It was something like a flat-bottomed scow, the
sides being just high enough to prevent whatever cargo it carried, from
rolling off into the water.

The raft placed and secured to the shore, the negro crouched in his hiding
place in a jungle of bushes. He soon reappeared, carrying four metal
tubes.

"The explosive is in the tubes," guessed Tom easily. "And at one end of
each tube is a sharp metal point that permits of being driven into the
crevices in the wall. Four, or more, of these tubes are thrust into the
wall, I suppose, and connected in series, so that they can be fired by the
same electric spark. These tubes and the wires are water-proofed. The
negro is only the dastardly workman in this case. It was never he who
invented the trick. But he must be an excellent workman, who ought to be
employed in much more honest effort. I wonder if the fellow is going to
use more than four tubes?"

All of these thoughts ran through the mind of Tom as he crouched, p