Already the "Morton" had turned, and was chugging back to the scene of this latest outrage, the searchlight flashing back and forth, in the vain effort to detect any small craft stealing away from the vicinity. "I---I can't race on a narrow runway like that," faltered Mr. Bascomb, halting at the beginning of the narrow wall. "I---I'll wait here, Mr. Renshaw, will you keep me company?" "If you so direct, sir," replied the superintendent. "For that matter, what Reade and Hazelton can't find out, out yonder, will probably never be discovered." "Do you share Mr. Prenter's infatuation for those two young men?" asked the president of the Melliston Company. "I can't say about that, sir," Renshaw replied, with a puzzled air. "But this much I know---I never worked with two more capable men of any age. They always know what to do, and they never lose their heads." Mr. Bascomb compressed his lips tightly. In the meantime Tom, Harry and Treasurer Prenter covered nearly a quarter of a mile along the retaining wall when the motor boat, putting about, picked them up with the searchlight. 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 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."