He landed on something soft, which, the moment he struck it, let out a yell that sounded loud on the quiet night. Then the thing began to fight. But Jack fought back and held on bravely. "Here! What are you tryin' to do?" exclaimed a voice in his ear. "What are you trying to do?" asked Jack indignantly, finding that the words came from the "ghost." "Nice way to treat a man! Half kill him!" the white thing went on. "Just when I'm trying to get a little sleep you come along and pull me out of bed!" "Why, it's Bill Jones," exclaimed Jack, as the light from the conning tower lamp fell on the face of the "ghost." "Of course it is; who did you think it was?" asked Bill. "What are you doing on deck in your night shirt?" asked the boy, letting the helper rise. "Me? On deck? Ain't I in my bunk?" "I should say not," replied Jack. "What are you doing on deck?" "Well! well!" remarked Bill, rubbing his eyes. "I've gone and done it again." "Done what?" "Walked in my sleep. I'm a great sleep walker. Greatest you ever knew. Once I climbed to the top of our barn when I was asleep." "So you're the ghost of the submarine," exclaimed Jack. "That explains it." "I guess you're right," admitted Bill, as the others came on deck to see what all the row was about. "I never thought of it when I heard about the ghost, but I can account for it now. I'd get out of my bunk, wander out on deck, and then crawl back again. Of course, being barefoot, or in fur slippers, I made no sounds. I don't wonder you thought I was a spirit. Queer I didn't wake up after some of the things I went through." "And you always managed to get back to your bunk in time so that we never caught you at it," said Jack. "However, it's all over now." And so it was, for after that Bill tied a chair in front of his bunk, and if he did get out in his sleep he stumbled against it and awoke before he had gone far. Northward the _Porpoise_ continued on her journey. She entered a vast field of ice, and only her ability to sink below the surface enabled her to get through it unharmed. There were few adventures going home. Once a big whale rammed the ship, as had happened on the going voyage, and several times they were surrounded by hordes of wild polar fish and walrusses, but there were no accidents, and in a couple of weeks the ship entered the temperate zone. Then came lazy happy days of sailing through the tropical region. They landed at several islands and renewed their supply of food. "I'm coming back this way some day," observed Mr. Henderson one afternoon as the ship was sailing along on top of the waves. "What for?" asked Jack. "To investigate that strange island with a big hole in the middle that seems to lead to the centre of the earth," was the answer. "I have a fancy we can explore that by means of a balloon. I'm going to try." "Will you take us along?" asked the two boys. "I'll see," replied the professor.