Mark and Jack were orphans, who had been adopted by Professor Henderson, who spent all his time making wonderful machines for transportation, or conducting strange experiments. The two boys had been rescued by Professor Henderson and Washington White from a train wreck. Although both boys were badly hurt, they were nursed back to health by the eminent scientist, who soon learned to care for the lads as though they had been his own sons. They aided the professor, as soon as they were able, in constructing an airship, called the _Electric Monarch_, in which Professor Henderson hoped to be able to reach the North Pole. The boys thoroughly enjoyed the trip through the air, and had many thrills fighting the savage Eskimos. Finally, they succeeded in passing over the exact spot of the North Pole during a violent snowstorm. Not satisfied with their experiences after conquering the North, the adventurers set out for the Antarctic regions in a submarine boat. This trip, even more remarkable than the first, took them to many strange places in the South Atlantic. They were trapped for a time in the Sargasso Sea, and they walked on the ocean floor in new diving suits, one of the professor's marvelous inventions. It was on the voyage to the south that, coming to the surface one day, the adventurers saw a strange island in the Atlantic Ocean, far from the coast of South America. On it was a great whirlpool, into which the _Porpoise_, their submarine boat, was nearly drawn by the powerful suction. The chasm might lead to the center of the earth, it was suggested, and, after thinking the matter over, on their return from the Antarctic, Professor Henderson decided to build a craft in which they might solve the mystery. The details of the voyage they took in the _Flying Mermaid_, are told of in the third volume, entitled "Five Thousand Miles Underground." The _Mermaid_ could sail on the water, or float in the air like a balloon. In this craft the travellers descended into the centre of the earth, and had many wonderful adventures. They nearly lost their lives, and had to escape, after running through danger of the spouting water, leaving their craft behind. For some time they undertook no further voyages, and the two boys, who lived with Professor Henderson in a small town on the coast of Maine, were sent to attend the Universal Electrical and Chemical College. Washington remained at home to minister to the wants of the old professor, and Andy Sudds went off on occasional hunting trips. But the spirit of adventure was still strong in the hearts of the boys and the professor. One day, in the midst of some risky experiments at college, Jack and Mark, as related in "Through Space to Mars," received a telegram from Professor Henderson, calling them home. There they found their friend entertaining as a guest Professor Santell Roumann, who was almost as celebrated as was Mr. Henderson, in the matter of inventions.