Could they live to reach the surface? was the thought in the mind of every one. The heat was terrific. They were breathing in gasps. Professor Henderson went to the water tank, thinking to throw some of the fluid over himself and his companions, but he found it so warm that it almost burned his hand.

"Keep up your courage!" exclaimed the inventor. "We will soon be at the top!"

Almost as he spoke the Porpoise bounded from the waves, and fell back in a splash of foam on the surface of the billows. They were at the surface.

The professor rushed for the manhole and soon opened it. He crawled out on the deck, followed by the others. They breathed in deep breaths of the fresh air.

The submarine continued to sail on. Every minute the sea seemed to boil more violently, until at last the waves were covered with a cloud of steam, through which it was difficult to observe where they were going.

"Hadn't we better turn back," suggested Mark.

"Our only hope is to press on," replied Mr. Henderson. "We may cross this zone of boiling water soon."

He went into the conning tower to make an observation. He came on the deck the next minute, very much excited.

"What's the matter? Are we sinking?" asked Andy.

"We are directly over the south pole!" exclaimed the professor. "We have reached the goal! We have come to the spot hundreds of men have tried to reach! It has been left for us to succeed. Look at the deflecting needle!"

They crowded into the conning tower to note it. The slender hand of steel stood straight up and down, indicating that the ship was over the south pole, one of the two chief centres of magnetism of the earth.

"If we only dared stop to make some scientific notes and observations," said the professor, "we could render much valuable aid to the seekers after truth. But it would be sure death to stay in the boiling water!"

"I guess we'd better be getting out of this if we want to reach home alive," spoke Andy.

Indeed they were all suffering very much, for the heat from the water was awful.

"Speed her up, Washington!" called the professor. "We must get out of here!"

"Which way shall I steer?" asked the colored man.

"Straight ahead. We are now bound north!"

"Bound north!" cried Jack.

"Certainly," answered the professor. "We have passed over the exact spot where the south pole is. The deflecting needle is beginning to tilt again. The compass is indicating a northerly direction. You know that after you go as far south as you can, you have to begin to go back north. Well, we have gone as far south as we can. Now we are going north. We have turned the southern end of the globe, and are on our way back."

For several hours the Porpoise continued along on top of the water. By degrees, as they left the vicinity of the boiling ocean, it became cooler. The water ceased to seethe and bubble, and Jack found, on experiment, that he could bear his hand in it.

"Hurrah!" he cried, "we are safe now."

"Next we'll have to prepare to freeze to death," spoke Mark. "It's either one extreme or the other this trip. But we've had lots of fun and excitement."

"Plenty of the last," agreed Jack.

On and on went the submarine. Once it was out of the range of the terrible heated zone, the atmosphere rapidly cooled, until the adventurers were glad to don their heavy garments again.

"This marks the ending of the first half of the voyage," announced the professor. "Now we are going back. We have accomplished something no other living man has done and I am proud of it. Proud of all of you, and proud of the ship!"

Several hours later, when it was deemed safe, the Porpoise was sunk beneath the waves, and once more she speeded along through the water at a fast speed. The ship seemed to know she was going home, for never had she made better time.

"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 identity 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 ashore. "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.