"Luncheon ready, gentlemen," he said with the utmost suavity, as if there had been no such thing as an exciting rescue, and as if he had had nothing else to think about. "Commander Jackson's compliments, and he feels wonderfully better." "Then we will go to the saloon," said Andrew. "Joe seems as clever with a patient as with airships. Come, Colonel, we can leave the Commander to my nephew while we eat. Welcome aboard the airship!" "And now tell us how it happened that you turned up at such a fortunate and exciting moment," asked the Major, when lunch was finished and the friends were seated smoking about the table. "Remember, you were to return during the night. Adrianople is hardly a safe place for an airship at this moment. Think of the result of a shell bursting close to this vessel." "Precisely! Think also of the members of our party stranded in the city," smiled Andrew. "Joe and I discussed the matter." "And decided that we would risk everything," said that latter. "After all, it gave one the opportunity of conducting a valuable test. This ship is supposed to be transparent." "Extraordinary!" declared the Colonel. "Why, 'pon my word! but really, one can see right through her. There's a man patrolling the deck high overhead, a sailor by the look of him. Surely he's yards above us--almost, it seems, suspended on air. And yet one sees that there are beams and girders all about us. You mean to tell me, sir," and he addressed his question to Joe in particular, "you mean to say that those girders are of solid, strong material, and enclose a space filled with hydrogen? In fact, a space which supports this huge vessel?" "Yes and no. For the most part, certainly yes," declared the young inventor, blushing with pride. "But the gas happens to be merely coal gas. You see, I chose it with an object. On a long trip such as this, that is to say a voyage which is to circle the globe, one must expect to lose gas from the compartments which go to fill the bulk of this huge vessel. In the case of Zeppelins and allied vessels the loss is appreciable. Here, thanks to celludine, which happens not to be porous, the loss is, in fact, negligible. Still, accident may give rise to leakage. It may become necessary to refill the whole vessel." "Then you descend?" asked the Colonel, obviously interested in this explanation. "We should already have been forced to descend," Joe corrected him. "Precisely; and call at some gasworks?" the Colonel interjected. "No; we carry a gas producer. We have coal in abundance; the rest is easy." It might or might not be. To the Colonel it was wonderful; in fact, so interesting that, what with the excellent meal of which he had partaken and this discussion, he quite forgot that experience when being swung upward to safety. Indeed, he must needs go off at once with Joe on a tour of investigation, while Andrew chatted with the Major. "And so you determined to risk it," said the latter.