"She's turning around!" suddenly cried Mr. Damon. "She's given up the flight, Tom, and is going back!"
"That's so!" agreed Mr. Whitford. "They're headed for Canada, Tom. We've got to catch 'em before they get over the Dominion line!"
"I'll do it!" cried Tom, between his clenched teeth.
He swung his airship around in a big circle, and took after the fleeing craft. The wind was against the smugglers now, and they could not make such good speed, while to Tom the wind mattered not, so powerful were the propellers of the Falcon.
"I think we're gaining on them," murmured Mr. Damon.
Suddenly, from the engine room, came a cry from Ned.
"Tom! Tom!" he shouted, "Something is wrong with the gas machine! She registers over five hundred pounds pressure, and that's too much. It's going up, and I haven't touched it!"
"Mr. Damon, take the wheel!" exclaimed the young inventor. "I've got to see what's wrong. Hold her right on their trail."
Tom sprang to the motor room, and one glance at the gas generating machine showed him that they were in dire peril. In some manner the pressure was going up enormously, and if it went up much more the big tank would blow to pieces.
"What is it?" cried Ned, from his position near the light.