Tom headed his course for the shore end of the wall.  Here he would find men in abundance.  Moreover, now that the big black was a prisoner the men would hardly be needed on the wall.

“I think I know just how Sambo worked it, too,” the engineer reflected, as he ran.  “He swam out into the Gulf, towing that little scow behind him.  Neither his black head nor the little scow would be seen far on the water on a dark night.  Sambo, when he got near enough, could take one of the metal tubes, swim in under water to some point where no watchman was near, and stick the tube fast into the wall.  Then another tube, and another—–­all under water where they would not show to a passing watchman.

“Then, when he had all in place, and while no patrolling watchman was too near, Sambo could begin to attach the wires.  That would take but a few minutes.  Whenever any one came too near Sambo had but to swim out a little way and tread water until he could return to his job.  When, at last, all was complete, Sambo would attach a wire from the bombs to a wire moored at a stated point under water, and then swim in, work his magneto, and touch the whole thing off from a safe hiding place on shore.  The explosion itself would shatter the last length of wire.  Oh, but it was all slick and easy!”

Not increasing his speed, but keeping steadily at the jog-trot, Tom was at last near enough to the wall to raise his voice and shout.

“Hullo!” came back the answer.

“This is Reade, the chief engineer,” Tom answered, through the night.  “We’ve caught the fellow that has been blowing up the wall.  A half a dozen of you men hurry over here with your lanterns.  Come on the run.”

The man who had answered summoned several of his comrades as quickly as he could.  As the men had to come in from the wall, however, it took a little time.  Then six men reported, almost breathless, to Reade.  Still behind them came Corbett on the run, summoned from the boat.

“What’s this I hear, Mr. Reade?” puffed the foreman.  “You’ve solved the mystery and caught the fellow who has been dynamiting the wall?”

“Got him and he’s tied up, waiting for his ride to jail,” Tom chuckled.

“How did it happen, sir?” asked Corbett, staring with his eyes very wide open.

“I caught the fellow—–­a huge giant of a negro, the same fellow who got Hazelton the other night,” replied Tom.  “But before the fight was over the black ‘got’ me, instead, and had me tied up.  Then Nicolas came along and put the negro out of the fight, and—–­”

“Nicolas?” demanded Foreman Corbett incredulously.

“Yes.  Nicolas proved himself to be the most fiery little bunch of fighting material that I have ever seen,” laughed Reade, as they walked rapidly along.

“How could that Mexican wallop a giant?”

“I’ll ask Nicolas to show you, to-morrow,” Tom laughed mischievously.  “But, Corbett, I believe that four bombs are even now attached to some part of the retaining wall, ready to be set off.