"Then we will have to sail for the nearest land in the small boat,"

replied the captain. "I will provision it and we will take all the precautions we can."

It was with anxious hearts that the little party embarked in the little craft that was carried on the _Porpoise_. It was barely large enough to hold them. The professor was the last in, and he lighted a long fuse that led to the dynamite before taking his seat. Then with Tom and Bill at the oars the little craft moved away.

"How long before the explosion will take place?" asked Jack.

"I timed the fuse for ten minutes," answered Mr. Henderson. "That will enable us to get far enough away so we will not be swamped by a wave."

Five minutes later Mark, who was intently watching the volcanic rock, gave a loud cry.

"There she goes!" he shouted.

They all looked to behold a wonderful thing. As easily as though it was some conjuring trick the rock began to settle down in the water. Lower and lower it went until only the tallest jagged points showed above the waves, whose crested tops the keel of the ship now almost touched.

"That isn't the explosion!" exclaimed the professor. "The fuse has not had time to get to the powder yet."

"But the rock is disappearing!" yelled Andy.

As he spoke the big pile of volcanic stone vanished completely and the _Porpoise_ floated easily on the surface of the sea.

"Hurrah!" cried Mark.

"It am de most kloslostrous occurranceness dat eber transpositioned itself!" exploded Washington.

"Let's row back to the ship now!" cried Mark.

"Not yet!" said the professor quickly. "The dynamite has not gone off yet."

"There it goes now," remarked Jack.

At that instant a big column of water shot upward and a dull rumbling could be heard. A few seconds later the little boat rocked violently from the effects of the waves. Then the sea became calm, and the _Porpoise_ could be seen dancing up and down on the heaving billows.

"Now we can go back in safety," spoke Mr. Henderson, and Tom and Bill bent to the oars.

"What happened?" asked Mrs. Johnson. "The rock seemed to disappear before the explosion occurred."

"That's exactly what it was," explained the inventor. "By some strange freak of nature the volcanic ma.s.s dropped back into the ocean a little before I was ready to blow it to pieces. In settling down it lowered the ship. Then the explosion occurred beneath the waves. If I had waited a little while I need not have risked the dynamite."

"Well, there was no guarantee that the rock would go back where it came from," remarked Jack.

"No, we had to act," agreed the professor. "But now let us go aboard."

They rowed back to the _Porpoise_, which they found had sustained no damage from the queer experience it had been through.

The motors were set in motion and in a little while the craft was again moving through the water out of the dangerous vicinity of the volcanic area.

"Who has the two watches to-night?" asked Mr. Henderson after supper had been served.

"Washington and Andy," answered Jack, who kept track of the matter.

"I guess we'll change it, and have you and Mark take them," went on the captain. He gave Jack a peculiar look, and made a sign to him not to say anything. Wondering what was to come Jack went up on deck to watch the sunset.

CHAPTER XXI

CAUGHT IN A WHIRLPOOL

The boy was joined a little later by the captain, who, coming close to him whispered:

"I am going to try to discover the ghost to-night. That is why I wanted you boys on hand to help me. This thing must be stopped if it is a joke, and, if it isn't--"

"Do you think it is some one playing tricks?" asked Jack.

"I don't know what to think," answered Mr. Henderson. "We will see what happens to-night."

Mark came on deck a little while, and the three talked of the strange appearances of the mysterious white object. The boys told of their experiences, and Jack related more fully his on the night the ship went up on the big rock upheaval.

"I don't believe in ghosts," said Mr. Henderson, "I'm going to lay this one," and he smiled grimly.

Night settled down. Jack, who had the first tour under the new arrangement, had made himself comfortable in the conning tower, and Mr.

Henderson had hidden himself in the companionway. His idea was to thus guard both openings into the ship and ascertain whether the ghost came from within or without the craft.

Up to a short time before twelve o'clock nothing out of the ordinary happened. The only sound was the lapping of the waves on the steel sides of the _Porpoise_, and now and then a splash as a big fish leaped out of the water. There was only the slightest breeze.

Jack who, somehow or other, felt much sleepier than usual, caught himself nodding several times. Once he awoke with a start and realized that he had been dozing.

"Come, come," he remarked to himself, "this will not do at all. This is a fine way to watch for a ghost."

He remained wide awake for perhaps five minutes. Then he was off to the land of nod again. He was just dreaming that he was skating on a pond and was playing snap the whip with a lot of boys, when he awoke with a start.

He felt something pressing on his chest and to his horror, as he looked up, he saw a big towering white object standing over him. A second glance showed him it was a man, or the semblance of one, and the thing's foot was on his chest.

With a terrified scream Jack sprang up, upsetting the ghost, which, the boy thought at the time, seemed rather heavy for an unearthly spirit.

"Did you catch it?" cried the professor.

"No! Yes! I don't know!" yelled Jack, struggling to his feet in time to see the white object glide down the stairs that led from the conning tower into the forward cabin.

"Run after it! We must solve the mystery!" cried Mr. Henderson springing from the companionway up on deck.

But at that moment the ship began to whirl about in a circle slowly at first, but with increasing speed until Jack and the professor felt sick and dizzy. All about the water seemed to be bubbling and boiling, while, at the same time, there arose on the air a mournful howl.

The professor jumped to the rail and looked over the side. What he saw made him recoil with horror.

"Quick! Close the man-hole hatch!" he cried. "Shut the door of the conning tower!"