What do you suppose I did, Mr.Reade, as soon as the explosion sounded ?" "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.