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, 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."