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!"

"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.

Once more Ned glanced at the pressure gage. It showed seven hundred pounds now, and there was only a margin of safety of one hundred pounds more, ere a terrific explosion would occur. Still Tom had not given the order to descend to earth.

"Are you going to make it, Tom?" asked the government agent, anxiously, as he stood over the young inventor.

"I--I think so," panted Tom. "Are we near the Dominion line,"

"Pretty close," was the discouraging answer. "I'm afraid we can't get 'em before they cross. Can you use any more speed?"

"I don't know. Ned, see if you can get another notch out of her."

With one hand Ned reached for the accelerator lever on the wall near him, and pulled it to the last notch. The Falcon shot ahead with increased speed, but, at the same instant there came a gasp from Koku, and the sound of something breaking.

"There! He's done it!" cried Tom in despair. "I was afraid you'd be too strong for that wrench, Koku. You've broken off the handle. Now we'll never be able to loosen that valve."

Ned gave one more glance at the pressure gage. It showed seven hundred and fifty pounds, and the needle was slowly moving onward.

"Hadn't we better descend," asked Mr. Whitford in a low voice.

"I--I guess so," answered Tom, despairingly. "Where are we?"

Ned flashed the light downward for an instant.

"Just crossing over the St. Regis Indian reservation again," he replied. "We'll be in Canada in a few minutes more."

"Where are the smugglers?"

"Still ahead, and they're bearing off to the right."

"Going toward Montford," commented the government man. "We've lost 'em for to-night, anyhow, but they didn't get their goods landed, at any rate."

"Send her down, Ned!" exclaimed Tom, and it was high time, for the pressure was now within twenty-five pounds of the exploding point.

Down shot the Falcon, while her rival passed onward triumphantly in the darkness. Ned held the light on the smugglers as long as he dared, and then he flashed it to earth to enable Mr. Damon to pick out a good landing place.

In a few moments Tom's silent airship came to rest on a little clearing in the forest, and Tom, with Ned's help, at once opened every outlet of the gas machine, a thing they had not dared do while up in the air.

"Come on, now, run, everybody!" cried Tom. "Otherwise you'll he smothered!"

They leaped from the craft, about which gathered the fumes of the powerful gas, as it hissed from the pipes. Running a hundred yards away they were safe, and could return in a few minutes.

"We're in Canada," remarked Mr. Whitford, as they came to a halt, watching the airship.

"How do you know?" asked Ned.

"As we landed I saw one of the stone boundary posts," was the answer. "We're on English territory, and we can't touch the smugglers if we should see them now."

"Well, we'll soon be back in Uncle Sam's land," declared Tom. "We can go back on board the Falcon to sleep shortly. Jove! I wish I could have caught those fellows!"

"Never mind, we'll get 'em yet," counseled Mr. Whitford.

Waiting until he was sure all the vapor had disappeared, Tom led the way back to the Falcon. No great harm had been done, save to lose considerable gas, and this could be remedied. Tired and disappointed from the chase, they sought their bunks, and were soon asleep. In the morning Tom and Ned began work on the clogged pipes.

This work was nearly accomplished by noon, when Mr. Damon, coming back from a stroll, announced that they were but fifteen minutes walk from the St. Lawrence River, as he had seen the sparkling waters from a neighboring hill.

"Let's go over and have a look at it," proposed Ned. "We can easily finish this when we get back. Besides, Tom, we don't want to get to our regular camp until after dark, anyhow."

The young inventor was willing, and the two lads, with Mr. Whitford, strolled toward the historic stream. As they drew near the bank, they saw, anchored a little distance out, a small steamer. Approaching it, as if she had just left the shore at a point near where our friends stood, was a gasolene launch, containing several men, while on shore, in front of a small shanty, stood another man.

This latter individual, at the sight of Tom, Ned and Mr. Whitford, blew a shrill whistle. Those in the launch looked back. The man on shore waved a red flag in a peculiar way, almost as the soldiers in the army wig-wag signals.

In another moment the launch turned about, and put for shore, while the lone man hurried back into the hut.

"Hum!" remarked Tom. "Those are queer actions."

"Suspicious actions, I should say," said Mr. Whitford. "I'm going to see what this means."