It was like the flight of two big birds in the air.  Now the smugglers’ craft would be mounting upward, with the Falcon after her.  Again she would shoot toward the earth, and Tom would follow, with a great downward swoop.

Ned kept the great lantern going, and, though occasionally the craft they were after slipped out of the focus of the beams, the young bank clerk would pick her up again.

To the right and left dodged the forward airship, vainly endeavoring to shake off Tom Swift, but he would not give up.  He followed move for move, swoop for swoop.

“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.

“I don’t know!  Something wrong.”

“Are you going to give up the chase?”

“I am not.  Stick to the light.  Koku, tell Mr. Damon to hold her on the course I set.  I’ll try to get this pressure down!” And Tom Swift began to work feverishly, while his ship rushed on through the night in danger, every moment, of being blown to atoms.  Yet the young inventor would not give up, and descend to earth.

CHAPTER XX

SUSPICIOUS ACTIONS

The chase was kept up, and Tom, when he had a chance to look up at the speed register, as he labored frantically at the clogged gas machine, saw that they were rushing along as they never had before.

“Are we catching them, Ned?” he cried to his chum, who was not far away, playing the powerful light on the smugglers’ craft.

“I think we’re coming closer, but it’s going to be a long chase.  I don’t see why we can’t close up on ’em.”

“Because they’ve got a very fast ship, Ned, and they are flying much lighter than we are.  But we’ll get ’em!”