The gas chambers were fed with pure hydrogen, no common coal gas, and many thousand cubic meters were in the flow of this one envelope filling. "Guess they'd have to carry a hydrogen factory around with this outfit to keep it going," observed Billy, as he noted the elaborate process. "Not that bad," advised the man at his elbow, "this gas can be transported from the factory in cylinders under pressure." "Just think of it," put in Henri, "I heard them say just now that it took thirty gallons of petrol an hour to buzz these motors." "Biggest thing I know in the air business. I wish Captain Johnson could see an expense bill like this. He'd have a fit." Billy would, indeed, have counted it a red-letter occasion if his old friend, and the boss airman of Dover, were really at hand to take in this show. To go aloft in an airship about which they were not thoroughly posted was a brand-new experience for the boys, but they were not in the least degree like the proverbial cat in a strange garret. It was easy riding, and none of the guns pointed their way. Billy carried a memorandum of a British military biplane, with a record of 10,000 miles, which Henri and himself had once patched up, that had been hit by 250 rifle bullets and sixty fragments of shells. He wondered if the immense craft in which they were sailing could have floated with, proportionately, about ten times that amount of lead poured into her. But Billy, of course, did not then know much about Zeppelins. Roque, however, was eminently well satisfied, particularly with the improved method of distributing explosives where they would do the most harm. The airship had a special armored compartment for bombs near the propellers and a big gun mounted in front to destroy aëroplanes. "Get a fleet of these over the English channel," he proclaimed, "and somebody would think that hell had been moved upstairs!" "I'll say this much," announced Billy, "I'd take an ocean voyage for my health if I knew when they were coming." "But if the fighting crowd over there had the date and the hour, I'll promise you that the reception your fleet would receive would be warm enough to boil an egg." This was Henri's prediction. "We never advertise," grimly remarked Roque. When the Zeppelin had completed her trial trip and had again been housed by the small army of workmen, Roque informed the boys that he was going to give them the chance on the morrow to show their mettle in a biplane test, which was to decide the relative merits as to the speed of two special designs. "I am going to put you up to jockey the machine that I favor," he said, "and, mind you, the aviators that will drive against you are among the finest in our flying corps. I always pick my men by personally knowing what they can do in any line of action. They seldom fail me, and it is with you to make good." "We're going some, Herr Roque, when we come up to your standard," replied Henri.