"Just think what might be done if we had enough flying machines, Zeppelins and aëroplanes, to bring over an army corps every week or so." This idea expressed by the ever-enthusiastic Stanislaws. "You are not talking airship now, Stanny; it's an air castle you have in mind." This pleasantry on the part of Billy turned the laugh on the Austrian aviator, in which he joined himself. "There's one thing sure," finally declared Roque, "I know of at least two airships that are soon to sail over the heads of the Russians who are now blocking the way to the fortress." "I just knew he would be pushing something across before long," said Henri to his chum. "From the way he looked at us when he spoke, it's safe to believe that we will be somewhere behind the push." Billy had a hunch that his job was secure whenever Roque had work above ground. Schneider had heard enough to set him at the task of cleaning and polishing his personal stock of firearms. The four biplanes returned that very night to the besieged fortress, from which two of the machines were destined to leave in short order on a most important and perilous journey. Our boys had instructions to give the aircraft a thorough going over, fill the petrol tanks to utmost capacity, and carry all the condensed foodstuff possible. "Maybe he is figuring on a chance of a lay-up in the mountains," suggested Stanislaws, detailed to assist the younger aviators in the work of preparation. "'Maybe' is a good word to use in connection with the moves of the chief, for you can't prove anything by us." The present was all that counted with the busy lads, hustling to complete their immediate assignment. "Ready and waiting," they soon announced to the chief, who simply nodded approval, and went on with the work in which he was engaged--studying and making field maps. Henri put in the spare time afforded with continuous instruction of his chum in the German language, Billy having already acquired, by hard knocks, talking knowledge of French. They were thus occupied one morning, when Schneider appeared, in war-like array, with brief order. "Buckle up." Roque found everything in shipshape for the getaway, and smiled at the impatience of Schneider, who had been stamping around the hangars since the first glimpse of daylight. While the young pilots were drawing to the elbows their fleece-lined gauntlets, the secret agent was earnestly assuring the commander of the garrison of his belief that the way would very soon open for the long-expected relieving force. "I think I can advise them to good effect if we get through in safety," he said, mounting his perch in the biplane, and giving Billy the word to go. As the biplanes shot through space, only Roque, the directing power, had knowledge of their destination, though Schneider inferred that the finish would be somewhere in the thick of battle.