In a few minutes the tank was empty and the divers stepped out into the ship, freed from the oppressive weight of water. The ship continued to vibrate and seemed to be in motion. "What is it?" asked Mark as soon as he had his helmet off. "De ship am free! We've got off de mud bank!" exclaimed Washington, running in from the engine room. It was true. Enough had been dug so that, with the power of the screw working backward, there was sufficient force to pull the _Porpoise_ from her perilous position. "Empty the tanks and rise to the surface," said the professor. In a few minutes the ship was on top of the waves, the adventurers freed from the double danger of the mud and sharks. They congratulated each other on the good outcome of the fight with the monster fishes. Wearied with their labors and the battle under the waves, the travelers sat up on the deck breathing in the fresh breeze. Then, after a while, supper was made ready and eaten with good appetites. It was decided hereafter to sail along near the surface at night, and not to submerge the ship deeply save during daylight, when it was easier to distinguish objects under the water. Following this plan the _Porpoise_ steamed along just awash that night, and the next day was sent down about fifty feet below the surface. One afternoon, when the travelers were resting, having partaken of a fine meal, the professor went on deck to make some observations, the ship having been raised for that purpose. He came down, somewhat excited. "Well, we're half way to the pole," he announced. "How can you tell?" asked Mark. "Because we have just crossed the equator. We went over the imaginary line three minutes ago." "I was wondering what made it so warm," said Jack. "I guess you dreamed it was hotter," spoke the professor. "It has been just as hot as this for the last few days. Crossing the line makes no difference." "Then we are really in the southern hemisphere now," said Mark. "That's where we are," replied Mr. Henderson. He put his instruments away. "Well, we may as well go below the surface again," he remarked. "Come quick! Hurry up!" yelled Washington from the deck. "Dar's a shipwreck up heah! Somebody's on it!" CHAPTER XV THE STRANGE SHIPWRECK The professor, followed by the boys and Tom, Bill and Andy hastened on deck. They saw Washington pointing excitedly off to the west. There, rising and falling on the easy swell, was the wreck of a large vessel. She had been a three masted schooner, but now only the stumps of the masts remained and the craft was rolling to and fro. It had settled low in the water, and was quite deep by the head, so that, at times, the waves broke over the bow in a shower of spray. But what attracted the attention of the adventurers more than anything else was the sight of two figures near what had been the after cabin of the ship.