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."