This was done, and then another careful search was made of the premises. No more bombs were found and Tom announced his intention of going home to get some much-needed rest. “But how do you suppose, with all your guards, Tom, and the electrical fence, those fellows planted the bombs?” asked Mr. Damon. “I don’t know,” replied the young inventor. “I’m afraid there is treachery somewhere in our working force. Without the aid of confederates those plotters couldn’t have put the bombs in here. I’ll have to make an investigation. But for the present the danger is past, I think.” They were all in need of rest and quiet after the exciting two days through which they had passed, especially Tom, and when he reached home Mrs. Baggert insisted on putting him straight to bed, in which place, to tell the truth, the young inventor was not at all averse to spending some time. The following day things had rather quieted down at the plant. The resulting débris was swept up, and the shattered lumber pile, devastated by the bomb, was examined for remnants of the infernal machine. Several pieces of cast iron were picked up, and Tom said he would investigate them to try to discover, if possible, where the bomb was made. It appeared to have contained no missiles, being merely a hollow shell filled with explosives, set to go off at a certain time, and Ned had hurled it away not an instant too soon. “The first thing we’ve got to do,” decided Tom, a few days later, when he was able to be about without his crutch and with much of his former energy restored, “is to investigate that secret stairway. Maybe some of the fellows are still on Barn Door Island.” But the delay, short as it was, had given the plotters time to vanish and to destroy some of their work. The stairs were in place, but after tearing up the planks, after the soil had been swept away, there was revealed only a blind passage. The tunnel had been caved in a short distance from the secret steps and it was impossible to traverse it. The same conditions prevailed on Barn Door Island. The place where Tom had emerged from the tunnel was found, but a short distance back in the passage dirt and rocks were piled up, preventing a further examination being made. “Maybe they’re walled up in the tunnel under the lake,” suggested Mr. Damon. “Not likely,” Tom said. “They probably cleared out after their bold plan didn’t succeed,” Ned remarked. “Yes, they’ve gone for a time,” Tom admitted. “But that doesn’t mean it’s forever. They’re still at large and they won’t give up so easily. I’m afraid for the success of my airline express plans. But I’m going to work on them.” That Tom’s fears were well grounded was borne out a few days later when, as the young inventor sat at his desk, his private telephone rang. Tom’s own instrument had a number not in the book and was known only to a few. Unless this number were given to the central operator Tom’s ’phone bell would not ring.