Toot! toot! sounded the boat's pneumatic whistle.

"Foreman Corbett is signaling to us to wait and he'll put 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 a.s.sented.

"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 hara.s.s 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?"

"_I_ don't want you to quit," replied Mr. Prenter positively. "I admire fighting grit, and I want to see you keep hammering away at the work until you win and the job is finished. The board of directors will stand with me on that, if I can sway them. As for Mr. Bas...o...b.. you mustn't take him too seriously. He's a first rate fellow in a lot of ways, but there's no fight in him, and he's a bit close-fisted, too. As for me, Reade, and as far as I can speak for my fellow directors, go ahead, just the way you've started.

If you can find any way to hammer camp vice harder than you've been hammering it, then go ahead and do some harder work with your little hammer."

"I'll do it," promised Tom. "Now, Mr. Prenter, I don't believe anything more will happen here to-night---perhaps not for two or three nights. So I think the wisest thing for you to do will be to get back to the house and get some sleep. The same for you, Harry!"

"What are you going to do?" Hazelton wanted to know.

"I?" repeated Reade. "For to-night I'm going to remain up, and be out here around this threatened wall."

"Then that ought to be good enough for me, also," Harry suggested.

"Not much, chum. I'm going to take the night trick for the present, and put on you the burden of all the day work. So you'll need your sleep."

"I can swing the day work easily enough," laughed Hazelton. "It will be all the more easy as the next few days will be taken up simply with repairing the breaks that have been made."

"Swing the boat in toward land, Mr. Corbett," Tom directed the foreman.

At the little landing Hazelton and Mr. Prenter joined the waiting president and superintendent.

"Did you really find out anything?" called Mr. Bas...o...b..eagerly.

"It's as big a mystery as ever."

"There's just one thing we'll have to do," sighed Mr. Bas...o...b.. "and that will be to stop running the camp on a basis of old Puritan laws."

"You talk Reade into it, if you can," chuckled Treasurer Prenter. "You won't find him easy to convince, either."

Tom didn't wait to discuss the matter. Instead, he signaled to Foreman Corbett to run the craft out again.

"If you want to, Corbett," suggested Tom, with a laugh, as the boat moved over the salt waters again, "you might go ash.o.r.e and go to bed. You can easily claim that you engaged with us as a foreman, and that being captain of a motor boat amounts to breach of contract."

"I'm not fussing," smiled the foreman. "As long as I can sleep daytimes running this motor boat is easier than working."

"It probably will be," nodded Reade, "unless the enemy go in for a new line of tactics."

"Such as what, sir?" asked Corbett.

"If this boat hampers them too much they may decide to send it to the bottom with a torpedo."