“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 were resorting to dodging tactics, in an endeavor to escape.  Now they would shoot upward, and again toward the earth, varying the performance by steering to the right or left.  Ned had constantly to shift the light to keep them in focus, so that Mr. Damon could see where to steer, but, with all this handicap, the eccentric man did very well, and he was never far out in his judgment.

“By Jove!” suddenly murmured Tom, as he tried once more in vain to open a clogged valve.  “I’m afraid we can’t do it.  Koku, lend a hand here!” he exclaimed as the giant entered.  “See if you can twist this wrench around, but don’t break off the handle, whatever you do.”

“Me shove,” replied the giant simply, as he grasped the big wrench.