"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.