ut in for us," said
Tom, coming to a halt. Soon the motor craft chugged in alongside, coming
close to the wall. Tom, Harry and Mr. Prenter jumped, landing safely
aboard.

"How did the enemy come to catch you napping, Corbett?" Tom inquired
good-humoredly.

"They didn't catch me napping, sir," protested Foreman Corbett. "It is the
strangest thing, sir---that explosion. Why, I had had my light turned on
that very part of the wall at least a dozen times in the last half-hour
before the blow-out came. Our light didn't pick up a soul around there at
any time. 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.

"Wh