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.