Their adventures on that journey are told of in the first volume of this series, entitled, "Through the Air to the North Pole, or, The Wonderful Cruise of the _Electric Monarch_." The two boys, Mark then being fifteen and Jack a year older, had met the professor under peculiar circumstances. They were orphans, and, after knocking about the world a bit, had chanced to meet each other. They agreed to seek together such fortune as might chance to come to them. While in the town of Freeport, N. Y., they were driven away by a constable, who said tramps were not allowed in the village. The boys jumped on a freight train, which broke in two and ran away down the mountain, and the lads were knocked senseless in the wreck that followed. As it chanced Professor Henderson had erected nearby a big shop, where he was building his airship. He and Washington were on hand when the wreck occurred and they took the senseless boys to the airship shed. The boys, after their recovery, accepted the invitation of the professor to go on a search for the north pole. As the airship was about to start Andy Sudds, an old hunter, and two men, Tom Smith and Bill Jones, who had been called in to assist at the flight, held on too long and were carried aloft. Somewhat against their will the three latter made the trip, for the professor did not want to return to earth with them. The party had many adventures on the voyage, having to fight savage animals and more savage Esquimaux. They reached the north pole, but in the midst of such a violent storm that the ship was overturned, and the discovery of the long-sought goal availed little. After many hardships, and a fierce fight to recover the possession of the ship, which had been seized by natives, the adventurers reached home. Since then a little over a year had passed. The professor, having found he could successfully navigate the air, turned his attention to the water, and began to plan a craft that would sail beneath the ocean. To this end he had moved his machine shop to this lonely spot on the Maine coast. The two boys, who had grown no less fond of the old man than he of them, went with him, as did Washington White, the negro, who was a genius in his way, though somewhat inclined to use big words, of the meaning of which he knew little and cared less. Andy Sudds, the old hunter, had also been induced to accompany the professor. "I hunted game up north and in the air," said Andy, "and if there's a chance to shoot something under the water I'm the one to do it." Needing more assistance than either the boys, Andy or Washington could give, the professor had engaged two young machinists, who, under a strict promise never to divulge any of the secrets of the submarine, had labored in its building. Now the queer craft was almost finished. As it rested on the ways in the shed, it looked exactly like a big cigar, excepting that the top part was level, forming a platform.