"Ugh! Disagreeable sort of game that," laughed the Major, shrugging his shoulders and staring upward. "Nasty thing to receive a bomb when slung even six thousand feet in the air. You'd come an awful crasher." "As to exploding," continued Joe serenely, "of course one no longer experiences at these high altitudes the normal fourteen pounds per square inch one is accustomed to on terra firma. The atmosphere is rarer, it weighs considerably less, and exerts decidedly less pressure. Hence, as you rightly assume, the envelope of a Zeppelin tends to tear. But, my dear sir, permit me to hand you a sample of sheet celludine. See, it is transparent, flexible, and extremely light. Please tear it, using as much force as you wish, and thereby prove that it is neither tough nor unstretchable." The inventor held out a single sheet of his wonderful yet simple material, while Dick craned his neck forward to get a closer view. As for Alec and Andrew, they were already versed in the characteristics of the stuff, but none the less interested. At once the Major complied with Joe's wishes. "Light, transparent, flexible," he said. "Yes, admitted. You agree, Colonel. Now tough and non-extensible; that is, won't stretch." "Like rubber," interjected Dick. "Quite so. Hang on--no, you're too light yet. Who ever heard of a midshipman having weight? The Colonel will suit my purpose. Now, Steven, pull with all your might." As was only to be expected the experiment proved the value of celludine conclusively. "I've been through the same sort of game," laughed Andrew. "I've tugged and pulled and stamped on the stuff till I was hot. Then, gentlemen, I put my money into this ship. I had had a practical demonstration." "But we were talking of exploding," said Joe. "Of course, each one of the gas compartments has a safety valve, so that if at any time the pressure from without should lessen to a dangerous degree, then the valves open and gas escapes. But you were looking at the aeroplane. I propose to make use of it presently; for our friend, the French airman whom Dick was sent to rescue, is now recovered and wishes to be landed."