The Scotchman had signed his name to the document certifying to the stop of the flyers at Kuka,--the paper on which they were to secure certifications at every scheduled airport,--and they were just in the act of starting over to the field tank to get some water for the airplane's radiators, when, without a moment's warning a hair-raising chorus of yells broke out on the brooding night air, and scores of savage-looking figures sprang from the shadows of the buildings into the open field. They emerged in a long straggling line, hooting and brandishing guns, spears and bows. They advanced toward the airplane in peculiar hops and side jumps, as if fearing an attack upon themselves. Not once did they cease their blood-curdling shouts. Rapidly they neared the objects of their anger and hatred. For a full five seconds the boys stood as if rooted in their tracks, too horrified and astounded to think or act. The sharp voice of the Scotchman, however, brought them to their senses. "You've fooled here too long; it's too late to get away now! They're mad as wet hornets. Jump inside your cabin quick, and defend yourselves as well as you can!" "But you, sir?" cried Tom. "They won't harm me, because I'm not a flyer." The boys dashed into the cabin and shut the door, while the Scotchman hurried away from the airplane. It was certain that there was no time to get out and crank the propeller and rise before the mad Fulbees would be upon them. Cornered in the little cabin of the machine they would sell their lives as dearly as possible. As they stood, guns in hand, watching through the windows, while the frenzied blacks drew cautiously nearer, spreading a cordon of hundreds all around the Sky-Bird, they could see in the moonlight that the Fulbees were grotesquely painted on arms and faces, while their bodies were entirely naked except for a dirty-looking cloth wrapped around their loins in the form of a short skirt. Every one of them was armed, and as they contracted their circle, guns, spears, and bows were frequently raised in threatening position; but for some reason no shots were fired. The inmates knew, however, that when nearer approach brought more assurance of hitting their target, the blacks could be counted upon to open up actual hostilities. And now this thought brought a sudden and grave fear to their minds, one unnoticed before. The helium-gas tanks in the hollow wings and rear fuselage! Bullets, spears and arrows striking them would penetrate, and the tanks thus punctured would lose their last ounce of the precious gas! It was a terrible predicament in which the flyers now found themselves, to be sure. By fighting they might preserve their lives, but that very act would make their world-trip impossible. What could they do? As the drowning man catches with hope at the floating straw. Bob now conceived an almost impossible but startling idea for delivering them from their dilemma.