"What plan have you in mind?" asked Tom of Mr. Whitford, when some of the Indians had gone back to their shanties, leaving a few staring curiously at the airship, as she rested on the ground, bathed in the glow of her electric lights.

"Well, I think the best thing we can do is just to stay right here, Tom; all night if need be. As Big Foot says, there have been airships passing overhead at frequent intervals. Of course that is not saying that they were the smugglers, but I don't see who else they could be. There's no meet going on, and no continental race. They must be the smugglers."

"I think so," put in Ned.

"Bless my diamond ring!" exclaimed Mr. Damon. "But what are you going to do when you see them overhead?"

"Take after them, of course!" exclaimed Tom. "That's what we're here for; isn't it Mr. Whitford?"

"Yes. Do you think you can rise from the ground, and take after them in time to stand a chance of overhauling them, Tom? You know they may go very fast."

"I know, but I don't believe they can beat the Falcon. I'd rather wait down here than hover in the air. It isn't as dark as it was the other night, and they might see us with their glasses. Then they would turn back, and we'd have our trouble for nothing. They've actually got to cross the border with smuggled goods before the law can touch them; haven't they?"

"Yes, I couldn't arrest them on Canadian territory, or over it. I've got to get them on this side of the border. So perhaps it will be as well to lie here. But do you suppose you can hear them or see them, as they fly over?"

"I'm pretty sure I can. The sound of their motor and the whizz of the propellers carries for some distance. And then, too, I'm going to set the searchlight to play a beam up in the air. If that gets focused on 'em, we'll spot 'em all right."

"But suppose they see it, and turn back?"

"I don't believe they will. The beam will come from the ground straight upward you know, and they won't connect it with my ship."

"But that fellow who was sneaking up when Koku caught him, may find some way to warn them that you have come here," suggested Ned.

"He won't get much chance to communicate with his friends, while my men have him," said Mr. Whitford significantly. "I guess we'll take a chance here, Tom."

So it was arranged. Everything on the airship was gotten ready for a quick flight, and then Tom set his great searchlight aglow once more. Its powerful beams cut upward to the clouds, making a wonderful illumination.

"Now all we have to do is to wait and watch," remarked Tom, as he came hack from a last inspection of the apparatus in the motor room.

"And that is sometimes the hardest kind of work," said Mr. Whitford. "Many a time I have been watching for smugglers for days and nights at a stretch, and it was very wearying. When I got through, and caught my man, I was more tired than if I had traveled hundreds of miles. Just sitting around, and waiting is tiresome work."

The others agreed with him, and then the custom officer told many stories of his experiences, of the odd places smugglers would hit upon to conceal the contrabrand goods, and of fights he had taken part in.

"Diamonds and jewels, from their smallness, and from the great value, and the high duty on them when brought into the United States, form the chief articles of the high class smugglers," he said. "In fact the ones we are after have been doing more in diamonds than anything else, though they have, of late, brought much valuable hand-made lace. That can be bought comparatively cheap abroad, and if they can evade paying Uncle Sam the duty on it, they can sell it in the United States at a large profit."

"But the government has received so many complaints from legitimate dealers, who can not stand this unfair competition, that we have been ordered to get the smugglers at any cost."

"They are sharp rascals," commented Mr. Damon. "They seem to be making more efforts since Tom Swift got on their trail."

"But, just the same, they are afraid of him, and his searchlight," declared Mr. Whitford. "I guess they fancied that when they took to airships to get goods across the border that they would not be disturbed. But two can play at that game."

The talk became general, with pauses now and then while Tom swept the sky with the great searchlight, the others straining their eyes for a sight of the smugglers' airships. But they saw nothing.

The young inventor had just paid a visit to the pilot house, to see that his wheels and guiding levers were all right, and was walking back toward the stern of the ship, when he heard a noise there, and the fall of a heavy body.

"Who's that?" he cried sharply. "Is that you, Koku?"

A grunt was the only answer, and, as Tom called the giant's name the big man came out.

"What you want, Mr. Tom?" he asked.

"I thought you were at the stern," spoke Tom. "Someone is there. Ned, throw the light on the stern!" he called sharply.

In a moment that part of the ship was in a bright glare and there, in the rays of the big lantern, was stretched out Big Foot, the Indian, comfortably sleeping.

"Here! What are you doing?" demanded Mr. Whitford, giving him a vigorous shake.

"Me sleep!" murmured Big Foot. "Lemme be! Me sleep, and take ride to Happy Hunting Grounds in air-bird. Go 'way!"

"You'll have to sleep somewhere else, Big Foot," spoke the agent with a laugh. "Koku, put him down under one of the trees over there. He can finish his nap in the open, it's warm."

The Indian only protested sleepily, as the giant carried him off the ship, and soon Big Foot was snoring under the trees.

"He's a queer chap," the custom officer said. "Sometimes I think he's a little off in his head. But he's good natured."

Once more they resumed their watching. It was growing more and more wearisome, and Tom was getting sleepy, in spite of himself.

Suddenly the silence of the night was broken by a distant humming and throbbing sound.

"Hark!" cried Ned.

They all listened intently.

"That's an airship, sure enough!" cried Tom.

He sprang to the lever that moved the lantern, which had been shut off temporarily. An instant later a beam of light cut the darkness. The throbbing sounded nearer.

"There they are!" cried Ned, pointing from a window toward the sky. A moment later, right into the glare of the light, there shot a powerful biplane.

"After 'em, Tom!" shouted Mr. Whitford.

Like a bird the Falcon shot upward in pursuit noiselessly and resistlessly, the beam of the great searchlight playing on the other craft, which dodged to one side in an endeavor to escape.

"On the trail at last!" cried Tom, as he shoved over the accelerator lever, sending his airship forward on an upward slant, right at the stern of the smugglers' biplane.