Steam already was up, as they could see from the thick black clouds of smoke that curled upward from her smokestack. Big cranes, operated by powerful winches on the vessel and on shore, were hoisting cases of various sizes and shapes upon the lower decks and into the hold. A small army of men helped complete the loading of the ship, and one group was experiencing considerable difficulty in trying to persuade unwilling mules to board the transport for Europe. The boys hurdled over piles of food and ammunition, wended their way through scores of stacks of ordnance, and finally over a gang-plank to the vessel. There they saluted and reported to the officer of the day, who directed them to go at once to the wireless room. As they entered there Lieutenant Mackinson was busily engaged in "tuning up" his instruments. He stopped when he saw them and reached into an inner pocket, from which he produced three large oblong envelopes. One was addressed to each lad, and as they accepted them they saw that each was closed to prying eyes by the official seal of Uncle Sam. Swept by various emotions, the boys stood there gazing first at the envelopes and then at Lieutenant Mackinson. "Well," said the lieutenant at last, with an amused smile, "do you want me to retire while you read your communications?" "Oh, no, not at all, sir," Joe hastened to say, and as if to prove the statement all three envelopes were ripped open and the single sheet of paper in each drawn forth. Especially addressed to each lad, the letters were identical and read: "I hereby convey to you my heartiest congratulations upon the efficient and heroic manner in which you and your two friends discovered and frustrated a plot to conceal enemy ammunition in the vicinity of this naval base. You all displayed true American courage; and I wish you every success for the future." The letters were signed by the commandant of the Philadelphia Navy Yard. "Look at that," said Slim, pushing his letter at Lieutenant Mackinson, utterly forgetful of the fact that the other man was his superior officer. "Ain't--isn't that fine, though? For the commandant to mention it that way, I mean." "Yes," admitted Lieutenant Mackinson, "but he wouldn't have mentioned it that way if you hadn't deserved it." "I'm not going to lose that letter," announced Jerry. "Nor I," added Joe, "although we only did what any other fellows would have done under the same circumstances." "Well," said Lieutenant Mackinson, "it showed that you were to be depended upon in an emergency, and emergencies are likely to crop up at any time in our work, so let's get down to business." He immediately began explaining the apparatus of the wireless room--how messages were sent and received; the power of the batteries and their auxiliaries; the switch-board regulating voltage; the automatic recording apparatus--in fact, every detail connected with the intricate mechanism of an up-to-date wireless.