is 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!"

"How are you making out with that gas machine?"

"Well, I'm doing all I can, but I can't seem to get the pressure
down. I can't understand it. Some of the pipes must be clogged with
a carbon deposit. I ought to have cleaned them out some time ago."

Ned gave a hasty glance at the gauge which showed the gas pressure.
It registered six hundred pounds now, having risen a hundred in a
short time.

"And she'll go up, sure, at eight hundred," murmured Ned, as he held
the light steadily on the smugglers' aircraft. "Well, if Tom sticks
to the chase, I will too, but I think it would be better to go down,
open up everything, and let the gas escape. We could get the rascals
later."

Tom, however, did not seem to think so, for he kept on with his
task, working away at the pipes, trying to force the obstruction
out, so that the gas from the generator would flow into the bag. At
the same time he tried to shut off the generating apparatus, but
that had become jammed in consequence of the pipe clogging, and the
powerful vapor continued to manufacture itself automatically in
spite of all that Tom could do.

The only safe way out of the danger, unless he could remove the
obstruction, was to descend to earth, and, as Ned had said, open
every outlet. But to have done that in mid-air would have been
dangerous, as the large volume of gas, suddenly liberated, would
have hung about the airship in a cloud, smothering all on board. If
they were on the earth they could run away from it, and remain away
until the vapor had blown off.

"Is Mr. Damon keeping her on the course, Ned?" asked Tom, pausing a
moment to get his breath after a series of frantic efforts.

"Yes, and I think we're closing in on them a little."

"That's good. Are they still headed for the border?"

"Yes, I guess they're going to take no chances to-night. They're
going right back to Canada where they came from."

"Well, we'll be hot after 'em. Whistle through the tube, and tell
Koku to come here and give me a hand. He's with Mr. Damon in the
pilot house."

Ned sent the message, and then gave his whole attention to the
light. This was necessary, as the smugglers