Two utterances Tom Swift made when the fact of the disappearance of the tank became known to him were characteristic of the young inventor. The first was:

"How did they get it away?"

And the second was:

"Come on, let's get after 'em!"

Then, for a few moments, no one said anything. Tom, Ned, and Mr. Damon, with Mrs. Baggert in the background, stood looking at the great empty machine shop.

"Well, they got her," went on Tom, with a sigh. "I was afraid of this as soon as they left me alone at the factory."

"Is anything wrong?" faltered the housekeeper. "Didn't you send for the tank, Tom?"

"No, Mrs. Baggert, I didn't," Tom answered.

"But I don't understand," the housekeeper said. "A man came with a note from you, Tom, and in it you said to have him take the tank, with Koku and the men who know how to run it. We were so glad to hear from you, and know that you were all right, that we didn't think of anything else, your father and I. So he went out and saw that the tank got off all right. Koku was glad, for it's the first chance he'd had to ride in it."

"Who was the man who brought the note?" asked Tom, and he was striving to be calm. "To think of poor old dad playing right into the hands of the plotters!" he added, in an aside to Ned.

"Well, I don't know who the man was," said Mrs. Baggert. "He seemed all right, and of course having a note from you--"

"Who has that note now?" asked Tom quickly.

"Your father."

"Come on," and Tom led the way back to the house. "I'll have a look at that document, which of course I never wrote, and then we'll get after the plotters and the tank."

"She ought to be easy to trace," observed Mr. Damon. "Bless my fountain pen, but she ought to be easy to trace! She will leave a track like a giant boa constrictor crawling along."

"Yes, I guess we can trace her, all right," assented Tom Swift; "but the point is, will there be anything left of her? What's what I'm afraid of now."

Mr. Swift was still excited, but his worry had subsided as soon as he knew Tom was safe.

"The whole thing is a forgery, but fairly well done," Tom said, as he looked at the paper his father gave him--a brief note stating that Tom was well, but detained on business, and that the tank was to be brought to him, just where the bearer of the note would indicate. Koku, the giant, and several of the machinists, who knew how to operate the big machine, were to go with it, the note said.

"That made me sure everything was all right," said Mr. Swift. "I knew, of course, Tom, that plotters might try to get hold of your war secret, but I didn't see how they could if Koku and some of your own men were in possession."

"They couldn't--as long as they remained in possession," Tom said. "But that's the trouble. I'm afraid they haven't. What has probably happened is that under the direction of this man, who brought the forged note from me, Koku and the others took the tank where he directed them, thinking to meet me. Then, reaching the place where the rest of the plotters were concealed, they overpowered Koku and the others and took possession of the machine."

"They'd have trouble with Koku," suggested Ned.

"Yes, but even a giant can't fight too big a crowd, especially if he is taken by surprise, and that's probably what happened," remarked Tom. "Now the question is where is the tank, and how can we get her back? Every minute counts. If those German spies and their helpers remain in possession long, they'll find out enough of my secrets to enable them to duplicate the machine, and especially some of the most exclusive features. We've got to get after 'em!"

"They imitated your writing pretty well, Tom," Observed Ned, as he looked at the forged note.

"Yes; that's why they took all my papers away from me--to get specimens of my handwriting. I half suspected that, but I didn't quite figure out what their game was. Well, we know the worst now, and that's better than working in the dark. Now I'm going to have a bath and get into some decent clothes, and we'll see what we can do."

"Count on me, Tom!" exclaimed Ned. "I'll go the limit with you!"

"I knew you would, old man!"

"And me, too!" cried Mr. Damon. "Bless my open fireplace, but I'll send word to my wife that I'm not coming home to- night, and we can start the first thing in the morning, Tom."

"Yes; there isn't much use in going now, as it will soon be dark."

"How are you going to trace the tank, Tom?" asked Ned, when his chum had bathed and gotten into fresh clothes.

"I'm going to tour the country around here in an auto. The tank can make ten miles an hour, but that's nothing to what an auto can do. And we oughtn't to have much trouble in tracing her. No one whose house she passed would forget her in a hurry."

"That's so," agreed Ned. "But if they took her across country--"

"A different story," agreed Tom. "Come to think of it, maybe we'd better start to-night, Ned. We can make inquiries after dark as well as by daylight and get ready for an early morning hunt"

"Let's do it, then!" suggested his chum. "I'm ready. I'll send word that I'll not be home to-night."

"Good!" cried the young inventor. "We'll have an old- fashioned hunt after our enemies, Ned!"

"And don't leave me out!" begged Mr. Damon. Hurried preparations were made for the night trip. Tom ordered out one of his speediest, though not largest, automobiles, and told his helper to get the Hawk ready, to have her so she could start at a moment's notice if needed.

"You're not going in her, are you, Tom?" asked Ned.

"I may need her to-morrow for daylight hunting. If the tank's hidden somewhere, I can spot her from above more easily than from the ground. So if we get any trace of my machine, I can phone in and have the aeroplane brought to me."

"That's a good idea!"

Inquiry at the shop where the tank had been built and kept disclosed the fact that, in addition to Koku, three of Tom's men had gone in her to help manage the machine under the direction of the man who bore the forged note. That he was one of the plotters not hitherto observed by either Ned or Tom seemed certain.

"And they took Koku and some of the men merely to make it look natural and as if it were all right," Tom said. "Naturally that deceived my father, who thought, of course, that I was waiting for the machine. Well, it was a slick trick, Ned, but we may fool them yet."

"I hope so, Tom."

Night had fully fallen when Tom, Ned, and Mr. Damon started away in the touring car.

Out onto the road rolled the automobile. During the little daylight that had remained after his arrival at home and following the discovery of the loss of the tank Tom and Ned had traced it, by the marks of the big steel caterpillar belts, to the main road. It had gone along that some distance, just how far could not be said.

"But by using the searchlight of the auto we can trace her as long as they keep her on the road," said Tom. "After that we'll have to trust to luck, and to what inquiries we can make."

The touring car carried a powerful lamp, and by its gleams it was easy to trace for a time the progress of the ponderous tank. There was no need to make inquiries of persons living along the way, though once or twice Tom did get out to ask, confirming the fact that the big machine had rumbled past in a direction away from the Swift home.

"I had an idea they might have doubled on their tracks for a time, and backed her up just to fool us," Tom said. "They might do that, keeping her in the same tracks."

But this, evidently, had not been done, and the tank was making good speed away from the Swift Louse. They kept up the search until about midnight, and then a heavy rain began just before they reached a point where several roads branched.

"Luck's with them!" exclaimed Tom. "This will wash away the marks, and we'll have to go it blind. Might as well put up here for the night," he added, as they came to a village hotel.

It was evident that little more could be done in the rain and darkness, and there was danger of over-running the trail of the tank if they kept on. So they turned in at the hotel and got what little rest they could in their anxious state of minds.

Tom tried to be cheerful and to look for the best, but it was hard work. The tank was his pet invention, and, moreover, that her secrets should fall into the hands of the enemy and be used for Germany and against the United States eventually, made the young inventor feel that everything was going wrong.

The rain kept up all night, and this would make it correspondingly hard for them to pick up the trail in the morning.

"The only thing we can do is to make inquiries," decided Tom. "Fortunately, the tank can't easily be hidden."

They started off after an early breakfast. The roads were so muddy and wet that traveling was difficult and dangerous for the automobile, and they were disappointed in finding no one who had seen or heard the tank pass up to a point not far from the hotel where they had stayed overnight. From then on the big machine seemed to have disappeared.

"I know what they've done," Tom said, when noon came and they had found no trace of the ponderous war machine. "They've left the road and taken her cross country, and we can't find the spot where they did this because the rain has washed out the marks. Well, there's only one thing left to do."

"What's that?" asked Ned.

"Get the Hawk! In that we can look down and over a big extent of country. That's what I'll do--I'll phone for the airship. The rain is stopping, I think."

The rain did cease by the time one of Tom's men brought the speedy aircraft to the place named by the young inventor in his telephone message. There were still several hours of daylight left, and Tom counted on them to allow him to rise in the air and look down on the tanks possible hiding place.

"One thing's sure," he told Ned: "I know the limit of her speed, and she can't be farther off than at some place within a circle of about one hundred and twenty-five miles from my house. And it's in the direction we're in. So if I circle around up above, I may spot her."

"I hope so," murmured Ned.

It was arranged that Mr. Damon should take the automobile back, with Tom's mechanician in it, and Tom and Ned would scout around in the aircraft, which carried only two.

"You ought to have a machine gun with you, Tom, if you plan to attack those fellows to get back the tank," Ned said.

"Oh, I don't imagine I'll need it," he said. "Anyhow, a machine gun wouldn't be of much effect against the tank. And they can't fire on us, for there wasn't any ammunition for the guns in Tank A, unless they got some of their own, and I hardly believe they'd do that. I'll take a chance, anyhow."

And so the search from the air began. It was disappointing at first. Around and around circled Tom and Ned, their eyes peering eagerly down from the heights for a sight of the tank, possibly hidden in some little-known ravine or gully.

Back and forth, like a speck in the sky, Tom guided the Hawk, while Ned took observation after observation with the binoculars.

At last, when the low-sinking sun gave warning that night would soon be upon them, Ned's glasses picked up something on the ground far below that made him sit suddenly straighter in his seat.

"What is it?" asked Tom through the speaking apparatus, feeling the movement on the part of his chum.

"I see something down there, Tom," was the answer. "It doesn't look like the tank, and yet it doesn't look as a clump of trees and bushes ought to look. Have a peep yourself. It's just beyond that river, against the side of the hill--a lonesome place, too."

Tom took the glasses while Ned assumed control of the Hawk, there being a dual system for operating and steering her.

No sooner had the young inventor got the focus on what Ned had indicated than he gave a cry.

"What is it?" asked the young bank clerk.

"Camouflaged!" cried Tom, and without stopping to explain what he meant, he handed the binoculars back to Ned and began to guide the Hawk down toward the earth at high speed.