"Say, Paul," cried Bob just then, "we're losing on the _Clarion_. She's clear out of sight now." "Why don't you tell me something I don't know?" growled Paul in a tone very queer for him. "What's the matter with you, Buddy?" demanded John, stepping up. "You seem to have an awful grouch on, some way!" "Got a good reason for it," snapped Paul. "This is enough to make a preacher almost swear." "Don't talk, but speed her up a bit if you don't want them to get away," advised John. "She doesn't act right, somehow," said Paul. "The Sky-Bird ought to be hitting it up to a hundred and fifty right now, but she's only making a hundred and fifteen. She acts groggy; don't you notice it?" "I thought myself she was riding a little rocky--sort of out of balance," admitted John. "Take the stick and try her yourself," said his brother. John did so. For fifteen minutes he said nothing, but worked the throttle and watched the speedometer. Then he called Paul again to the seat. "You might as well take her, Buddy," declared John with a puzzled shake of his head; "I can't do any better with her than you. She wallows along like a man with a load of buckshot in his pockets--heavy--and seems out of equilibrium, too!" "What do you suppose is the matter, John?" asked Tom Meeks. "I'll bet Pete Deveaux and that Chuck Crossman have been tampering with her, back there in Georgetown," declared Bob. "I don't know; it certainly looks kind of suspicious," admitted John Ross. He thought a moment. "Cattails and jewsharps!" he exclaimed very suddenly. "What now?" asked Bob. "I believe I've hit the trouble," stated John, with his brown face a shade paler. "You know we saw those fellows monkeying around our wings. It would be an easy matter to trip one or more of those valves and let some of the helium out! That would make us heavier, and if more gas were let out from one wing than from the other, we would be out of balance in the bargain." This declaration of John's brought a startled and troubled look to the faces of his companions. All knew that if Pete Deveaux had engineered such a dastardly trick as John hinted at, a handicap would be in store for the Sky-Bird's crew all through the remainder of the race, for it would be impossible to get a renewal of their helium-gas supply before reaching their own country again, and then it would be too late. "What shall we do?" came from Bob. "Do? There's nothing to do now, but to keep on flying at the best gait we can until we reach Para," decided John. "When we get there we'll have a chance to find out what is really wrong." This seemed the wisest course to pursue. So Paul, vexed though he was at the contrary actions of the airplane, buckled down to the job of guiding the machine and complained no more. But he made up his mind that if investigations proved the rival crew had been tampering with the Sky-Bird II he, for one, would do his part in giving them a warm time should they meet on the ground again.