rules of the road, Matt slowed down to make sure the horse did not take fright at the automobile. The horse was going at a run, and the men seemed to be excited.

The one who was driving drew rein as the rig came alongside the car.

"Say," shouted the men, "did you boys see an air-ship anywhere in this vicinity?"

"Yes," answered Matt. "It was going south."

"Then we're on the right track?"

"So far as we know; but the air-ship was unmanageable and——"

The men in the buggy did not wait to hear any more. The driver began plying his whip and the horse again leaped onward.

"Who were those two men?" yelled Matt, anxious for a little information.

"Thieves!" came the answer, as rig and passengers once more vanished in a cloud of dust.

CHAPTER III.

THE BALLOON HOUSE.

"Yah!" shouted Carl. "Vat I dell you, Matt? I knew dere vas somet'ing der madder! Dem two fellers vas t'ieves, und dey haf shtole der air-ship. Py shinks, dey haf got demselufs indo drouple, und it vas goot enough for dem. Vat you going to do?"

Matt had begun turning the machine in the road. When he had pointed it the other way, he started off at a swift pace on the trail of the two men in the buggy.

"We'll try and overhaul those two fellows," answered Matt, "and tell them what we know. The information we've picked up may be valuable to them."

"Dey don't vas endidled to it," averred Carl. "Vy ditn't dey shtop und ask us somet'ings? Anyvay, how can dey ketch a flying machine mit a horse und puggy? You mighdt as vell dry to ketch a sky rocket mit a papy carriage."

"The Hawk will have to come down," said Matt, "and if those men are anywhere near it when it hits the earth they'll be able to recover the machine and catch the thieves."

"Oof der machine hits der eart' so hardt as vat I t'ink, it von't be vort' nodding, nor der t'ieves neider."

"There's a chance that the rascals will come down safely. If those men in the buggy had had their wits about them, they'd have hitched their rig to the fence and have jumped into the automobile. We could have hustled them over the ground four times as fast as they were going."

A few moments later the boys reached a place where the road branched. The horse and buggy were not in sight along either road.

"Vich vay now?" queried Carl.

"It's all guesswork," answered Matt, "but it's always a pretty good plan to keep to the right," and, with that, he drove the car along the right-hand branch.

After five minutes of fast running, they had not overtaken the rig and it was still not to be seen anywhere ahead. The boys knew they had been traveling three or four times as fast as the two men were going, and that, if they were on the right track, the men should have been overtaken long before.

Disappointedly, Matt halted the car and turned it in the other direction.

"No use, Carl," said he. "Those men must have taken the left-hand fork instead of the right. They're too far away, now, for us to think of finding them. We'll hike for South Chicago."

"Dot's der pest t'ing dot ve can do," returned Carl. "Ve'll find der owner oof der Hawk und gif him der trag-rope und der bapers."

"We won't find him. He must have been one of those two men in the buggy. Probably we can find where he lives, though, and turn the rope and the papers over to some one who will give them to him."

"Meppy ve pedder take der shtuff to der bolice, hey? Oof der fellers vas t'ieves, dot enfellup mighdt gif der bolice a line on dem."

"There's something in that, too," muttered Matt. "We'll try to find the owner of the Hawk, though, before we call on the police."

An hour later, the boys came into South Chicago along a turnpike that passed the rolling mills. A man on a motor-cycle was just coming out of a fenced enclosure near one of the mills, and Matt halted him for the purpose of making a few inquiries. From his looks, the man was of some consequence in the steel rail plant, and probably was well-informed as to affairs in South Chicago.

"Do you know of any one around here that has an air-ship?" asked Matt.

The question was something of a novelty, and the man laughed as he rested one foot on the ground and balanced his motor-cycle upright.

"I suppose air-ships will be thicker'n hops, one of these days," said he, "but just now they're about as seldom as hen's teeth. I understand there are a couple of men here who are working at air-ships—one of them came to the mills to see if he couldn't get some aluminum castings. He's got a balloon house about a quarter of a mile down the road, on the left. Drop in there and maybe you'll find the man—and the ship, too."

Matt thanked the man and followed him slowly as he sputtered off into town.

The balloon house, which was plainly visible from the road, was a long, high shed, and occupied a solitary position in the midst of a marshy field. The doors in one end of the shed, arranged in a series and reaching from ground to roof peak, were open.

Leaving the automobile at the roadside, the boys climbed a fence and made their way across the flat ground to the big house. On reaching the opened doors, one glance showed them that there was no air-ship in the shed.

On the earth floor, along one side of the great room, were two or three work benches and a litter of wood and metal scraps. There was also, in the farther end of the chamber, a number of small tanks, presumably used for the manufacture of hydrogen gas. As the boys stood in the doorway, two brawny men showed themselves from behind these tanks. They wore greasy overclothes and their sleeves were rolled up.

"Get out of here!" yelled one of the men. "We don't allow any reporters around this shebang."

"We're not reporters," answered Matt, standing his ground. "Do you keep an air-ship here?"

"Well, that's what this big shed is for."

The two men came closer to the boys, one of them filling and lighting a cob pipe as he approached.

"Is the name of it the 'Hawk?'" went on Matt.

"Right again," said the man who had been doing the talking.

His eyes were like gimlets, and bored their way into Matt through narrow slits.

"Who's the owner of the Hawk?" asked Matt.

"I'm the owner, and my name's Hector Brady. If Jerrold has sent you here——"

"I don't know any one by the name of Jerrold. Who is he, and why should he send me here?"

The sharp little eyes continued to study Matt.

"Before I say anything more," answered Brady, "you'd better tell me a little about yourself."

"I don't know as that's necessary, or——"

"You'd know how necessary it is if you were inventing machines and trying to keep your appliances a secret. I'm not the only man in South Chicago that's perfecting an air-ship. A fellow named Jerrold has cut into the same game, and he has some one nosing around here a good share of the time, trying to get wise to something. If Jerrold has sent you here——"

"He hasn't," broke in Matt. "I don't know Jerrold from Adam."

"What's your name?"

"King, Matt King."

Brady gave a jump.

"You don't mean to say you're the young Western phenomenon the Lestrange people have brought to Chicago to run in that five-day automobile race that's turned on at the Coliseum to-morrow?"

"I'm one of their racers," answered Matt. "They have four more in the race besides me."

"Well, by thunder!" Brady stood off and regarded Matt as though he was a natural curiosity. "Why, you're no more than a kid! They had your picture in the paper, after that Kansas race, but you're a heap younger than I thought. I guess you've forgotten more about gasolene-motors than a whole lot of people ever knew."

"Oh, it isn't so bad as that. I came here to do you a good turn, Mr. Brady, and I can't see the sense of raking up my past