"And now you had better report for your geometry lesson," went on the professor. "I need the laboratory now for a class in physics. Just tell the janitor to come here and sweep up the broken glass. I am very glad neither of you boys was seriously injured. You must be more careful next time." "Oh, Mark was careful enough," said Jack. "It was all my fault. I didn't think the gas was quite so powerful." "All right," answered the professor with a smile as Jack and Mark passed out on their way to another classroom. The two lads, whom some of my readers have met before in the previous books of this series, were friends who had become acquainted under peculiar circumstances. They were orphans, and, after having had many trying experiences, each of them had left his cruel employers, and, unknown to each other previously, had met in a certain village, where they were obliged to beg for food. They decided to cast their lots together, and, boarding a freight train, started West. The train, as told in the first volume to this series, called "Through the Air to the North Pole," was wrecked near a place where a certain Professor Amos Henderson, and his colored helper, Washington White, lived. Mr. Henderson was a learned scientist who was constantly building new wonderful machines. He was working on an airship, in which to set out and locate the North Pole, when he discovered Jack and Mark, injured in the freight wreck. He and Washington White carried the lads to the inventor's workshop, and there the boys recovered. When they were well enough, the professor invited them to live with him, and, more than that, to take a trip with him North Pole. They went, in company with Washington and an old hunter, named Andy Sudds, and some other men, whom the professor took along to help him. Many adventures befell the party. They had battles with wild beasts in the far north, and were attacked by savage Esquimaux. Once they were caught in a terrible storm. They actually passed over the exact location of the North Pole, and Professor Henderson made some interesting scientific observations. In the second volume of this series, entitled "Under the Ocean to the South Pole," Professor Henderson, Jack, Mark, Washington and old Andy Sudds, made even a more remarkable trip. The professor had a theory that there was an open sea at the South Pole, and he wanted to prove it. He decided that the best way to get there was to go under the ocean in a submarine boat, and he and the boys built a very fine, craft, called the Porpoise, which was capable of being propelled under water at a great depth. The voyagers had rather a hard time of it. They were caught in a great sea of Sargasso grass, monstrous suckers held the boat in immense arms, and it required hard fighting to get free. The boys and the others had the novel experience of walking about on the bottom of the sea in new kinds of diving suits invented by the professor.