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, peering eagerly at the negro.

By this time the negro was taking to the water, towing his miniature scow and its explosive cargo as he swam.

“He must be a good swimmer, and also a good diver,” concluded Tom.  “With my men patrolling the sea wall he must have to dive, some distance away, swim under water, and remain there until he has secured one of the tubes in place.  Then he has to get back, out of range of the lanterns’ rays, and get his breath before he goes back to the next job.  But maybe I can interfere with his work to-night.”

Though he rose and moved away, Reade, despite the darkness of the night, was careful to keep himself concealed behind the bushes, so that he could not be observed from beach or water.  Shortly the young engineer was over at the point in the jungle from which he had seen the negro emerge with scow and explosives.

“The fellow must use a magneto, attached to wires running under the water,” concluded Tom.  “At that rate, the first real job is to find the magneto.  My, but Mr. Sambo Ebony may be wondering, to-night, why his blow-out doesn’t work as easily as usual!”

Simple as the search ought to have been, Tom Reade was soon on the point of despair.

“If it isn’t a magneto, or if I can’t find it in time,” Tom muttered uneasily, “the mystery may remain nearly as great as ever, and the explosion may be pulled off to-night, after all.”

Twenty minutes passed before Reade, with all his senses alert, stumbled on the concealed magneto.  It had been so well hidden, under a mass of rocks, that it would not have been astonishing had Tom missed it altogether.

Attached to the magneto was the wire that must connect, in some way, with the series of tubes that would soon be fastened in the retaining wall out yonder.  Yet this wire ran into the ground, and then vanished.