"Well, Ned, are you ready?"

"Oh, I suppose so, Tom. As ready as I ever shall be."

"Why, Ned Newton, you're not getting afraid; are you? And after you've been on so many trips with me?"

"No, it isn't exactly that, Tom. I'd go in a minute if you didn't have this new fangled thing on your airship. But how do you know how it's going to work--or whether it will work at all? We may come a cropper."

"Bless my insurance policy!" exclaimed a man who was standing near the two lads who were conversing. "You'd better keep near the ground, Tom."

"Oh, that's all right, Mr. Damon," answered Tom Swift. "There isn't any more danger than there ever was, but I guess Ned is nervous since our trip to the underground city of gold."

"I am not!" indignantly exclaimed the other lad, with a look at the young inventor. "But you know yourself, Tom, that putting this new propeller on your airship, changing the wing tips, and re-gearing the motor has made an altogether different sort of a craft of it. You, yourself, said it wasn't as reliable as before, even though it does go faster."

"Now look here, Ned!" burst out Tom. "That was last week that I said it wasn't reliable. It is now, for I've tried it out several times, and yet, when I ask you to take a trip with me, to act as ballast--"

"Is that all you want me for, Tom, to act as ballast? Then you'd better take a bag of sand--or Mr. Damon here!"

"Me? I guess not! Bless my diamond ring! My wife hasn't forgiven me for going off on that last trip with you, Tom, and I'm not going to take any more right away. But I don't blame Ned--"

"Say, look here!" cried Tom, a little out of patience, "you know me better than that, Ned. Of course your more than ballast--I want you to help me manage the craft since I made the changes on her. Now if you don't want to come, why say so, and I'll get Eradicate. I don't believe he'll be afraid, even if he--"

"Hold on dar now, Massa Tom!" exclaimed an aged colored man, who was an all around helper at the Swift homestead, "was yo' referencin' t' me when yo' spoke?"

"Yes, Rad, I was saying that if Ned wouldn't go up in the airship with me you would."

"Well, now, Masa Tom, I shorely would laik t' 'blige yo', I shore would. But de fack ob de mattah am dat I has a mos' particular job ob white washin' t' do dish mornin', an' I 'spects I'd better be gittin' at it. It's a mos' particiilar job, an', only fo' dat, I'd be mos' pleased t' go up in de airship. But as it am, I mus' ax yo' t' 'scuse me, I really mus'," and the colored man shuffled off at a faster gait than he was in the habit of using.

"Well, of all things!" gasped Tom. "I believe you're all afraid of the old airship, just because I wade some changes in her. I'll go up alone, that's what I will."

"No, I'll go with you," interposed Ned Newton who was Tom's most particular chum. "I only wanted to be sure it was all right, that was all."

"Well, if you've fully made up your mind," went on the young inventor, a little mollified, "lend me a hand to get her in shape for a run. I expect to make faster time than I ever did before, and I'm going to head out Waterford way. You'd better come along, Mr. Damon, and I'll drop you off at your house."

"Bless my feather bed!" gasped the man. "Drop me off! I like that, Tom Swift!"

"Oh, I didn't mean it exactly that way," laughed Tom. "But will you come."

"No, thanks, I'm going home by trolley," and then as the odd man went in the house to speak to Tom's father, the two lads busied themselves about the airship.

This was a large aeroplane, one of the largest Tom Swift had ever constructed, and he was a lad who had invented many kinds of machinery besides crafts for navigating the upper regions. It was not as large as his combined aeroplane and dirigible balloon of which I have told you in other books, but it was of sufficient size to carry three persons besides other weight.

Tom had built it some years before, and it had seemed good enough then. Later he constructed some of different models, besides the big combination affair, and he had gone on several trips in that.

He and his chum Ned, together with Eradicate Sampson, the colored man, and Mr. Damon, had been to a wonderful underground city of gold in Mexico, and it was soon after their return from this perilous trip that Tom had begun the work of changing his old aeroplane into a speedier craft.

This had occupied him most of the Winter, and now that Spring had come he had a chance to try what a re-built motor, changed propellers, and different wing tips would do for the machine.

The time had come for the test and, as we have seen, Tom had some difficulty in persuading anyone to go along with him? But Ned finally got over his feeling of nervousness.

"Understand, Tom," spoke Ned, "it isn't because I don't think you know how to work an aeroplane that I hesitated. I've been up in the air with you enough times to know that you're there with the goods, but I don't believe even you know what this machine is going to do."

"I can pretty nearly tell. I'm sure my theory is right."

"I don't doubt that. But will it work out in practice?"

"She may not make all the speed I hope she will, and I may not be able to push her high into the air quicker than I used to before I made the changes," admitted Tom, "but I'm sure of one thing. She'll fly, and she won't come down until I'm ready to let her. So you needn't worry about getting hurt."

"All right--if you say so. Now what do you want me to do, Tom?"

"Go over the wire guys and stays for the first thing. There's going to be lots of vibration, with the re-built motor, and I want everything tight."

"Aye, aye, sir!" answered Ned with a laugh.

Then he set at his task, tightening the small nuts, and screwing up the turn-buckles, while Tom busied himself over the motor. There was some small trouble with the carburetor that needed eliminating before it would feed properly.

"How about the tires?" asked Ned, when he had finished the wires.

"You might pump them up. There, the motor is all right. I'm going to try it now, while you attend to the tires."

Ned had pumped up one of the rubber circlets of the small bicycle wheels on which the aeroplane rested, and was beginning on the second, when a noise like a battery of machine guns going off next to his ear startled him so that he jumped, tripped over a stone and went down, the air pump thumping him in the back.

"What in the world happened, Tom?" he yelled, for he had to use all his lung power to be heard above that racket. "Did it explode?"

"Explode nothing!" shouted Tom. "That's the re-built motor in action."

"In action! I should say it was in action. Is it always going to roar like that?"

Indeed the motor was roaring away, spitting fire and burnt gases from the exhaust pipe, and enveloping the aeroplane in a whitish haze of choking smoke.

No, I have the muffler cut out, and that's why she barks so. But she runs easier that way, and I want to get her smoothed out a bit.

"Whew! That smoke!" gasped his chum. "Why don't you--whew- -this is more than I can stand," and holding his hands to his smarting eyes, Ned, gasping and choking, staggered away to where the air was better.

"It is sort of thick," admitted Tom. "But that's only because she's getting too much oil. She'll clear in a few minutes. Stick around and we'll go up."

Despite the choking vapor, the young inventor stuck to his task of regulating the motor, and in a short while the smoke became less, while the big propeller blades whirled about more evenly. Then Tom adjusted the muffler, and most of the noise stopped.

"Come on back, and finish pumping up the tires," he shouted to Ned. "I'm going to stop her now, and then I'll give her the pressure test, and we'll take a trip."

Having cleared his eyes of smoke, Ned came back to his task, and this having been finished, Tom attached a heavy spring balance, or scales, to the rope that held the airship back from moving when her propellers were whirling about.

"How much pressure do you want?" asked Ned.

"I ought to get above twelve hundred With the way the motor is geared, but I'll go up with ten. Watch the needle for me."

It may be explained that when aeroplanes are tested on the earth the propellers are set in motion. This of course would send a craft whizzing over the ground, eventually to rise in the air, but for the fact that a rope, attached to the craft, and to some stationary object, holds it back.

Now if this rope is hooked to a spring balance, which in turn is made fast to the stationary object, the "thrust" of the propellers will be registered in pounds on the scale of the balance. Anywhere from five hundred to nine hundred pounds of thrust will take a monoplane or biplane up. But Tom wanted more than this.

Once more the motor coughed and spluttered, and the big blades whirled about so fast that they seemed like solid pieces of wood. Tom stood on the ground near the levers which controlled the speed, and Ned watched the scale.

"How much?" yelled the young inventor.

"Eight hundred."

Tom turned on a little more gasolene.

"How much?" he cried again.

"Ten hundred. That'll do!"

"No, I'm going to try for more.

Again he advanced the spark and gasolene levers, and the comparatively frail craft vibrated so that it seemed as if she would fly apart.

"Now?" yelled Tom.

"Eleven hundred and fifty!" cried Ned.

"Good! That'll do it. She'll give more after she's been running a while. We'll go up."

Ned scrambled to his seat, and Tom followed. He had an arrangement so that he could slip loose the retaining rope from his perch whenever he was ready.

Waiting until the motor had run another minute, the young inventor pulled the rope that released them. Over the smooth starting ground that formed a part of the Swift homestead darted the aeroplane. Faster and faster she moved, Ned gripping the sides of his seat.

"Here we go!" cried Tom, and the next instant they shot up into the air.

Ned Newton had ridden many times with his chum Tom, and the sensation of gliding through the upper regions was not new to him. But this time there was something different. The propellers seemed to take hold of the air with a firmer grip. There was more power, and certainly the speed was terrific.

"We're going fast!" yelled Ned into Tom's ear.

"That's right," agreed the young inventor. "She'll beat anything but my Sky Racer, and she'd do that if she was the same size." Tom referred to a very small aeroplane he had made some time before. It was like some big bird, and very swift.

Up and onward went the remodeled airship, faster and faster, until, when several miles had been covered, Ned realized that the young inventor had achieved another triumph.

"It's great, Tom! Great!" he yelled.

"Yes, I guess it will do, Ned. I'm satisfied. If there was an international meet now I'd capture some of the prizes. As it is--"

Tom stopped suddenly. His voice which had been raised to overcome the noise of even the muffled motor, sounded unnaturally loud, and no wonder, for the engine had ceased working!

"What's the matter?" gasped Ned.

"I don't know--a breakdown of some kind."

"Can you get it going again?"

"I'm going to try."

Tom was manipulating various levers, but with no effect. The aeroplane was shooting downward with frightful rapidity.

"No use!" exclaimed the young inventor. "Something has broken."

"But We're falling, Tom!"

"I know it. We've done it before. I'm going to volplane to earth."

This, it may be explained, is gliding downward from a height with the engine shut off. Aeroplanists often do it, and Tom was no novice at the art.

They shot downward with less speed now, for the young inventor had thrown up his headplanes to act as a sort of brake. Then, a little later they made a good landing in a field near a small house, in a rather lonely stretch of country, about ten miles from Shopton, where Tom lived.

"Now to see what the trouble is," remarked our hero, as he climbed out of his seat and began looking over the engine. He poked in among the numerous cogs, wheels and levers, and finally uttered an exclamation.

"Find it?" asked Ned.

"Yes, it's in the magneto. All the platinum bearings and contact surfaces have fused and crystallized. I never saw such poor platinum as I've been getting lately, and I pay the highest prices for it, too. The trouble is that the supply of platinum is giving out, and they'll have to find a substitute I guess."

"Can't we go home in her?" asked Ned.

"I'm afraid not. I've got to put in new platinum bearings and contacts before she'll spark. I only wish I could get hold of some of the better kind of metal."

The magneto of an aeroplane performs a service similar to one in an automobile. It provides the spark that explodes the charge of gas in the cylinders, and platinum is a metal, more valuable now than gold, much used in the delicate parts of the magneto.

"Well, I guess it's walk for ours," said Ned ruefully.

"I'm afraid so," went on Tom. "If I only had some platinum, I could--"

"Perhaps I could be of service to you," suddenly spoke a voice behind them, and turning, the youths saw a tall, bearded man, who had evidently come from the lonely house. "Did I hear you say you needed some platinum?" he asked. He spoke with a foreign accent, and Tom at once put him down for a Russian.

"Yes, I need some for my magneto," began the young inventor.

"If you will kindly step up to my house, perhaps I can give you what you want," went on the man. "My name is Ivan Petrofsky, and I have only lately come to live here."

"I'm Tom Swift, of Shopton, and this is my chum, Ned Newton," replied the young inventor, completing the introductions. He was wondering why the man, who seemed a cultured gentleman, should live in such a lonely place, and he was wondering too how he happened to have some platinum.

"Will that answer?" asked Mr. Petrofsky, when they had reached his house, and he had handed Tom several strips of the precious silverlike metal.

"Do? I should say it would! My, but that is the best platinum I've seen in a long while!" exclaimed Tom, who was an expert judge of this metal. "Where did you get it, if I may ask?"

"It came from a lost mine in Siberia," was the unexpected answer.

"A lost mine?" gasped Tom.

"In Siberia?" added Ned.

Mr. Petrofsky slowly nodded his head, and smiled, but rather sadly.

"A lost mine," he said slowly, "and if it could be found I would be the happiest man on earth for I would then be able to locate and save my brother, who is one of the Czar's exiles," and he seemed shaken by emotion.

Tom and Ned stood looking at the bearded man, and then the young inventor glanced at the platinum strips in his hand while a strange and daring thought came to him.