"We have solved every problem that we met," said the professor while he, with Jack and Mark, were in the conning tower, as Washington was preparing a meal.

"Except two," said Jack.

"What are they?"

"The ghost of the submarine, and the ident.i.ty of the anarchists who blew up the Easton hotel."

"Perhaps both riddles may be solved before we get back to Maine,"

answered Mr. Henderson.

They both were, sooner, and in stranger ways than either of the boys expected. That night it was Jack's first watch on deck. The ship was speeding on, and by the air the boy knew they were approaching icebergs.

At midnight a strange and sudden chill in the air made him look up.

Almost dead ahead was a big berg. He quickly shut off the engine, and narrowly avoided a collision. Then happening to glance back he saw, standing near the companionway leading down into the man-hole a ghostly white shape.

"I'll find out what you are this time, or go overboard with you," said Jack to himself, clenching his teeth. He crawled along the deck until he thought he was within leaping distance of the weird white thing. Then he made a leap.

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.

And later on he did take them on a trip, a thousand miles underground,--but that is another story to tell.

It was about a week later that the voyagers came within sight of Key West.

"Off there lies the United States," said Mr. Henderson.

"Hurrah for home!" cried Mark.

Three days later they landed at a small Florida town. The sight of the _Porpoise_ attracted throngs of people to the dock where she tied up.

Among them was a newsboy.

"Get me all the papers for the past month," said Jack. "I want to see what the news is."

"Same here," put in Mark, and the papers were soon brought.

"Hurrah!" exclaimed Jack, as soon as he had looked at several of the sheets.

"What is it?" asked Mark, who was unfolding a paper.

"Anarchists Confess," read Jack. "Two Englishmen Admit They Blew Up Hotel Where Lord Peckham Was Stopping. No Suspicion Attaches to Two Youths Who So Mysteriously Disappeared!"

"Hurrah!" joined in Mark.

"Those are only the head lines," went on Jack. "There's a long story, and I guess it lets us out."

The two boys were completely cleared of the slightest shade of suspicion of the outrage, and there was even an interview with the English detective in which he admitted that he was wrong.

A week later the _Porpoise_ tied up at her own dock, whence she was launched.

"Back again," remarked the professor as he stepped ash.o.r.e. "I've been to the south pole, and to the north pole. I wonder where I shall go next?"

"To the big hole and underground," suggested Jack.

"We shall see," said Mr. Henderson with a twinkle in his eyes.