"What do you think it's all about, Mr. Damon?"

"I'm sure I don't know, Ned."

The two were at the home of the young bank clerk, preparing to start for the Swift place, it being nearly nine o'clock on the evening named by the youthful inventor.

"Bless my hat-rack!" went on the eccentric man, "but Tom isn't at all like himself of late. He's working on some invention, I know that, but it's all I do know. He hasn't given me a hint of it."

"Nor me, nor any of his friends," added Ned. "And he acts so oddly about enlisting--doesn't want even to speak of it. How he got exempted I don't know, but I do know one thing, and that is Tom Swift is for Uncle Sam first, last and always!"

"Oh, of course!" agreed Mr. Damon. "Well, we'll soon know, I guess. We'd better start, Ned."

"It's useless to try to guess what it is Tom is up to. He has kept his secret well. The nearest any one has come to it was when Harry figured out that Tom had a band of giant elephants which he was fitting with coats of steel armor to go against the Germans," observed Ned, when be and Mr. Damon were on their way.

"Well, that mightn't be so bad," agreed Mr. Damon. "But-- um--elephants--and wild giant ones, too! Bless my circus ticket, Ned! do you think we'd better go in that case?"

"Oh, Tom hasn't anything like that!" laughed Ned. "That was only Harry's crazy notion after he saw something big and ungainly careening about the enclosed yard of Shop Thirteen. Hello, there go Mary Nestor and her father!" and Ned pointed to the opposite side of the street where the girl and Mr. Nestor could be seen in the light of a street lamp.

"They're going out to see Tom's secret," said Mr. Damon. "There's plenty of room in my car. Let's ask them to go with us."

"Surely," agreed Ned, and a moment later he and Mary were in the rear seat while Mr. Damon and Mr. Nestor were in the front, Mr. Damon at the wheel, and they were soon speeding down the road.

"I do hope everything will go all right," observed Mary.

"What do you mean?" asked Ned.

"I mean Tom is a little bit anxious about this test."

"Did he tell you what it was to be?"

"No; but when he called to invite father and me to be present he seemed worried. I guess it's a big thing, for he never has acted this way before--not talking about his work."

"That's right," assented Ned. "But the secret will soon be disclosed, I fancy. But how is it you aren't going to the dance with Lieutenant Martin? He told me you had half accepted for to-night."

"I had." And if it had been light enough Ned would have seen Mary blushing. "I was going with him. It's a dance for the benefit of the Red Cross to get money for comfort kits for the soldiers. But when Tom sent word that he'd like to have me present to-night, why--"

"Oh, I see!" broke in Ned, with a little laugh. "'Nough said!"

Mary's blushes were deeper, but the kindly night hid them.

Then they conversed on matters connected with the big war- -the selling of Liberty Bonds, the Red Cross work and the Surgical Dressings Committee, in which Mary was the head of a junior league.

"Everybody in Shopton seems to be doing something to help win the war," said Mary, and as there was just then a lull in the talk between her father and Mr. Damon her words sounded clearly.

"Yes, everybody--that is, all but a few," said Mr. Nestor, "and they ought to get busy. There are some young fellows in this town that ought to be wearing khaki, and I don't mean you, Ned Newton. You're doing your bit, all right."

"And so is Tom Swift!" exclaimed Mr. Damon, as if there had been an implied accusation against the young inventor. "I heard, only to-day, that one of his inventions--a gas helmet that he planned--is in use on the Western front in Europe. Tom gave his patents to the government, and even made a lot of the helmets free to show other factories how to turn them out to advantage."

"He did?" cried Mr. Nestor.

"That's what he did. Talk about doing your bit--"

"I didn't know that," observed Mary's father slowly. "Do you suppose it's a test of another gas helmet that Tom has asked us out to see to-night?"

"I hardly think so," said Ned. "He wouldn't wait until after dark for that This is something big, and Tom must intend to have it out in the open. He probably waited until after sunset so the neighbors wouldn't come out in flocks. There's been a lot of talk about what is going on in Shop Thirteen, especially since the arrest of the German spies, and the least hint that a test is under way would bring out a big crowd."

"I suppose so," agreed Mr. Nestor. "Well, I'm glad to know that Tom is doing something for Uncle Sam, even if it's only helping with gas helmets. Those Germans are barbarians, if ever there were any, and we've got to fight them the same way they fight us! That's the only way to end the war! Now if I had my way, I'd take every German I could lay my hands on--"

"Father, pretzels!" exclaimed Mary.

"Eh? What's that, my dear?"

"I said pretzels!"

"Oh!" and Mr. Nestor's voice lost its sharpness.

"That's my way of quieting father down when he gets too strenuous in his talk about the war," explained Mary. "We agreed that whenever he got excited I was to say 'pretzels' to him, and that would make him remember. We made up our little scheme after he got into an argument with a man on the train and was carried past his station."

"That's right," admitted Mr. Nestor, with a laugh. "But that fellow was the most obstinate, pig-headed Dutchman that ever tackled a plate of pig's knuckles and sauerkraut, and if he had the least grain of common sense he'd--"

"Pretzels!" cried Mary.

"Eh? Oh, yes, my dear. I was forgetting again."

There was a moment of merriment, and then, after the talk had run for a while in other and safer channels, Mr. Damon made the announcement:

"I think we're about there. We'll be at Tom's place when we make the turn and--"

He was interrupted by a low, heavy rumbling.

"What's that?" asked Mr. Nestor.

"It's getting louder--the noise," remarked Mary. "It sounds as if some big body were approaching down the road-- the tramp of many feet. Can it be that troops are marching away?"

"Bless my spark plug!" suddenly cried Mn Damon. "Look!"

They gazed ahead, and there, seen in the glare of the automobile headlights, was an immense, dark body approaching them from across a level field. The rumble and roar became more pronounced and the ground shook as though from an earthquake.

A glaring light shone out from the ponderous moving body, and above the roar and rattle a voice called:

"Out out of the way! We've lost control! Look out!"

"Bless my steering wheel!" gasped Mr. Damon, "that was Tom Swift's voice! But what is he doing in that--thing?"

"It must be his new invention!" exclaimed Ned.

"What is it?" asked Mr. Nestor.

"A giant," ventured Ned. "It's a giant machine of some sort and --"

"And it's running away!" cried Mr. Damon, as he quickly steered his car to one side--and not a moment too soon! An instant later in a cloud of dust, and with a rumble and a roar as of a dozen express trains fused into one, the runaway giant--of what nature they could only guess--flashed and lumbered by, Tom Swift leaning from an opening in the thick' steel side, and shouting something to his friends.