While Tom and his chum are in the house of the Russian, who so strangely produced the platinum just when it was most needed, I am going to take just a little time to tell you something about the hero of this story. Those who have read the previous books of this series need no introduction to him, but in justice to my new readers I must make a little explanation.

Tom Swift was an inventor, as was his father before him. But Mr. Swift was getting too old, now, to do much, though he had a pet invention--that of a gyroscope--on which he worked from time to time. Tom lived with his father in the village of Shopton, in New York state. His mother was dead, but a housekeeper, named Mrs. Baggert, looked after the wants of the inventors, young and old.

The first book of the series was called "Tom Swift and His Motor-Cycle," and in that I related how Tom bought the machine from a Mr. Wakefield Damon, of Waterford, after the odd gentleman had unintentionally started to climb a tree with it. That disgusted Mr. Damon with motor-cycling, and Tom had lots of fun on the machine, and not a few daring adventures.

He and Mr. Damon became firm friends, and the oddity of the gentleman--mainly that of blessing everything he could think of--was no objection in Tom's mind. The young inventor and Ned Newton went on many trips together, Mr. Damon being one of the party.

In Shopton lived Andy Foger, a bullying sort of a chap, who acted very meanly toward Tom at times. Another resident of the town was a Mr. Nestor, but Tom was more interested in his daughter Mary than in the head of the household. Add Eradicate Sampson, an eccentric colored man who said he got his name because he "eradicated" dirt, and his mule, Boomerang, and I think you have met the principal characters of these stories.

After Tom had much enjoyment out of his motor-cycle, he got a motor boat, and one of his rivals on Lake Carlopa was this same Andy Foger, but our hero vanquished him. Then Tom built an airship, which had been the height of his ambition for some years. He had a stirring cruise in the Red Cloud, and then, deserting the air for the water, Tom and his father built a submarine, in which they went after sunken treasure. In the book, "Tom Swift and His Electric Runabout," I told how, in the speediest car on the road, Tom saved his father's bank from ruin, and in the book dealing with Tom's wireless message I related how he saved the Castaways of Earthquake Island.

When Tom went among the diamond makers, at the request of Mr. Barco Jenks, and discovered the secret of phantom mountain the lad fancied that might be the end of his adventures, but there were more to follow. Going to the caves of ice, his airship was wrecked, but he and his friends managed to get back home, and then it was that the young inventor perfected his sky racer, in which he made the quickest flight on record.

Most startling were his adventures in elephant land whither he went with his electric rifle, and he was the means of saving a missionary, Mr. Illingway and his wife, from the red pygmies.

Tom had not been home from Africa long before he got a letter from this missionary, telling about an underground City in Mexico that was said to be filled with gold. Tom went there, and in the book, entitled, "Tom Swift in the City of Gold," I related his adventures.

How he and his friends were followed by the Fogers, how they eluded them, made their way to the ruined temple in a small dirigible balloon, descended to the secret tunnel, managed to turn aside the underground river, and reach the city of gold with its wonderful gold statues--all this is told in the volume.

Then, after pulling down, in the centre of the underground city, the big golden statue, the door of rock descended, and made our friends prisoners. They almost died, but Andy Foger and his father, in league with some rascally Mexicans and a tribe of head-hunters, finally made their way to the tunnel, and most unexpectedly, released Tom and his friends.

There was a fight, but our hero's party escaped with considerable gold and safely reached Shopton. Now, after a winter spent in work, fixing over an old aeroplane, we again meet Tom.

"Would you mind telling me something about where this platinum comes from, and if you can get any more of it?" asked Tom, after a pause, following the strange statement made by the Russian.

"I will gladly tell you the story," spoke Mr. Petrofsky, "for I am much interested in inventions, and I formerly did something in that line myself, and I have even made a small aeroplane, so you see I know the need of platinum in a high power magneto."

"But where did you get such pure metal?" asked Tom. "I have never seen it's equal."

"There is none like it in all the world," went on the Russian, "and perhaps there never can be any more. I have only a small supply. But in Siberia --in the lost mine-- there is a large quantity of it, as pure as this, needing only a little refining.

"Can't we get some from there?" asked the young inventor eagerly. "I should think the Russian government would mine it, and export it."

"They would--if they could find it," said Ivan Petrofsky dryly, "but they can't--no one can find it--and I have tried very hard--so hard, in fact, that it is the reason for my coming to this country--that and the desire to find and aid my brother, who is a Siberian exile."

"This is getting interesting," remarked Ned to Tom in a low voice, and the young inventor nodded.

"My brother Peter, who is younger than I by a few years, and I, are the last of our family," began Mr. Petrofsky, motioning Tom and Ned to take chairs. "We lived in St. Petersburg, and early in life, though we were of the nobility, we took up the cause of the common people."

"Nihilists?" asked Ned eagerly, for he had read something of these desperate men.

"No, and not anarchists," said Mr. Petrofsky with a sad smile. "Our party was opposed to violence, and we depended on education to aid our cause. Then, too, we did all we could in a quiet way to help the poor. My brother and I invented several life-saving and labor-saving machines and in this way we incurred the enmity of the rich contractors and government officials, who made more money the more people they could have working for them, for they made the people buy their food and supplies from them.

"But my brother, and I persisted, with the result that we were both arrested, and, with a number of others were sent to Siberia.

"Of the horrors we endured there I will say nothing. However, you have probably read much. In the country near which we were quartered there were many mines, some of salt and some of sulphur. Oh, the horrors of those mines! Many a poor exile has been lost in the windings of a salt mine, there to die miserably. And in the sulphur mines many die also, not from being lost so much as being overcome by stifling gases. It is terrible! And sometimes they are purposely abandoned by their guides, for the government wants to get rid of certain exiles.

"But you are interested in platinum. One day my brother and I who had been sent to work in the salt mines, mistook a turning and wandered on and on for several miles, finally losing our way. We had food and water with us, or we would have perished, and, as it was, we nearly died before we finally found our way out of an abandoned opening.

"We came out in the midst of a terrible snowstorm, and wandered about almost frozen. At last we were found by a serf who, in his sled, took us to his poor cottage. There we were warmed and fed back to life.

"We knew we would be searched for, as naturally, our absence would lead to the suspicion that we had tried to escape. So as soon as we were able, we started back to the town where we were quartered. The serf wanted to take us in his sled, but we knew he might be suspected of having tried to aid us to get away, and he might be arrested. So we went alone.

"As might have been expected, we became lost again, and wandered about for several days. But we had enough food to keep us alive. And it was during this wandering that I came upon the platinum mine. It was down in a valley, in the midst of a country densely wooded and very desolate. There was an outcropping of the ore, and rather idly I put some of it in my pockets. Then we wandered on, and finally after awful suffering in terrific storms, were found by a searching party and brought back to the barracks."

"Did they think you had escaped?" asked Tom.

"They did," replied the Russian, "and they punished us severely for it, in spite of our denials. In time I managed secretly to smelt the platinum ore, and I found I had some of the purest metal I had ever seen. I was wishing I could find the mine, or tell some of my friends about it, when one of the officers discovered the metal in my bed.

"He demanded to know where I had gotten it, and knowing that refusal would only make it the worse for me I told him. There was considerable excitement, for the value of the discovery was recognized, and a search was at once made for the mine.

"But, even with the aid we were able to give, it could not be located. Many expeditions went out to hunt for it but came back baffled. They could not penetrate that wild country."

"They should have used an aeroplane," suggested Tom.

"They did," replied the Russian quickly, "but it was of no use."

"Why not?" the young inventor wanted to know.

"Because of the terrific winds that almost continually sweep over that part of Siberia. They never seem to cease, and there are treacherous air currents and 'pockets' that engulfed more than one luckless aviator. Oh, you may be sure the Russian government spared no means of finding the lost platinum mine, but they could not locate it, or even get near the place where they supposed it to be.

"Then, perhaps thinking that my brother and I were concealing something, they separated us. Where they sent him I do not know, but I was doomed to the sulphur mines. I was heartbroken, and I scarcely cared whether I lived or died. But an opportunity of escape came, and I took it. I wanted to save my brother, but I did not know where he was, and I thought if I could make my way to some civilized country, or to free America, I might later be able to save my brother.

"I went to England, taking some of my precious platinum with me, and stayed there for two years. I learned your language, but my efforts to organize an expedition to search for the lost mine, and for my brother, failed. Then I came here, and--well, I am still trying."

"My! That is certainly interesting!" exclaimed Ned, who had been all attention during the telling of the story.

"And you certainly had a hard time," declared Tom. "I am much obliged for this platinum. Have you set a price on it? It is worth much more than the ordinary kind."

"The price is nothing to you," replied the Russian, with a smile. "I am only too glad to help you fix your aeroplane. Will it take long? I should like to watch you."

"Come along," invited Tom. "I can soon have it going again, and I'll give you a ride, if you like."

"No, thank you, I'm hardly up to that yet, though I may be some day. The machine I made never flew well and I had several bad falls."

Tom and Ned worked rapidly on the magneto, and soon had replaced the defective bits of platinum.

"If the Russians had such a machine as this maybe they could have gotten to that mine," suggested Ned, who was very proud of Tom's craft.

"It would be useless in the terrific winds, I fear," answered Ivan Petrofsky. "But now I care little for the mine. It is my brother whom I want to save. He must be in some of the Siberian mines, and if I had such a craft as this I might be able to rescue him."

Tom Swift dropped the file he was using. A bright light sparkled in his eyes. He seemed strangely excited.

"Mr. Petrofsky!" he cried, "would you let me have a try at finding your brother, and would you come with me?"

"Would I?" asked the Russian eagerly. "I would be your debtor for life, and I would always pray for you, if you could help me to save my brother Peter."

"Then we'll have a try at it!" cried Tom. "I've got a different airship than this--one in which I can travel three thousand miles without coming down. I haven't had any excitement since I got back from the city of gold. I'm going to Russia to help you rescue your brother from exile, and I'm also going to have a try for that lost platinum treasure!"

"Thank heaven, there is some hope for poor Peter at last," murmured Mr. Petrofsky earnestly.

"You never can get to the platinum mine," said Ned. The winds will tear your airship to pieces."

"Not the kind I'm going to make," declared Tom. "It's going to be an air glider, that will fairly live on high winds. Ho! for Siberia and the platinum mines. Will you come?"

"I don't know what you mean by an air glider, Tom Swift, but I'll go to help rescue my brother," was the quick answer, and then, with the light of a daring resolve shining in his eyes, the young inventor proceeded to get his aeroplane in shape for the trip back to Shopton.