“I saw you turn about and use your search light a lot,” Reade answered.

“Did you notice, sir, that I turned the light right up at the sky, first-off?”

“I believe I did notice that,” Tom assented.

“It seemed to me, sir, that nothing but an airship could plant a charge of high explosive on the wall in that fashion.”

“I don’t believe the airship theory will explain it either,” said Tom, shaking his head.

“Then what theory can explain it?” asked Mr. Prenter, anxiously.

“I’d pay a reward out of my own pocket for the right answer,” Reade replied.

“Then you haven’t a theory?” asked the treasurer.

“Not even an imitation of a theory,” Tom laughed, shortly.

All this time the motor boat was gliding out toward the scene of the wreck.

“Now, you can see the damage that has been done,” suggested Mr. Corbett, turning the light fully on the scene of the latest blow-out.  “You see, a long strip of the wall has been cleaned out.  Not a trace of the damaged part shows above water.”

“It wasn’t as big an explosion as the other two, though,” Reade declared.  “Really, it looks as though the folks behind this found themselves running low on explosives.”

“There must be a trace or a clue left,” urged Mr. Prenter.

“High explosives don’t leave many traces of anything with which they come in contact,” muttered Harry.  “If we do find any traces, I guess it will have to be in broad daylight.”

“And I guess that’s right,” agreed Tom.  “Mr. Corbett, did none of your men patrolling on the wall report any signs of strangers?”

“No such report was made, sir.”

“At all events, we can be thankful that the explosion didn’t blow one or two of our men into the other world,” Tom went on.

“Even that is bound to happen if there are many more of these explosions,” muttered Corbett, grimly.

“Which is another reason,” remarked Tom Reade, “why we’re going to solve the mystery of said explosions at the earliest minute that we can.”

“One thing is certain,” observed Mr. Prenter, with the nearest approach to gloom that he had yet shown.  “If you don’t soon penetrate this grim mystery, and find a way to stop these outrages, then the wall will be destroyed more rapidly than you can build it.”

“The outrages may cease after a while,” suggested Harry.

“No,” answered Reade.  “As long as the unknown enemy feels that he can harass us without much risk of being caught red-handed, just so long will he go on with his outrages—–­unless we give in.”

“Give in?” asked Mr. Prenter, with a rising inflection in his voice.

“Unless we give in,” supplied Tom promptly, “by allowing gambling and rum-selling to go on openly in our camp of workmen.”

“Have you any notion of giving in to that extent?” asked Mr. Prenter.

“Not an idea!” retorted Tom Reade promptly.  “It wouldn’t be my way to surrender to the Devil.  I’ll fight to the last ditch—–­unless your company really prefers to have Hazelton and myself cancel our contract and get out of this work.  Do you?”