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?"