“I guess it was rats,” said Jack.  “They are always in ships.”

“Old wooden ships, yes,” admitted Mark.  “But I’ll bet there’s not a rat aboard the Mermaid.”

“Then you were dreaming,” said Jack, as if that settled it.

Mark did not speak further of the noise, but he did considerable thinking.  However, the next night there was no further disturbance.

The fourth day out, when everything had passed off well, the engines doing their best, the professor decided to speed them up a bit, since he was satisfied they had “found” themselves as mechanics term it.

“We’ll see how fast we can go through the water,” said Mr. Henderson, “and then I think we can safely turn our course south.  We are well beyond the ordinary lines of travel now.”

Having oiled the bearings well, and seen that everything was in place and properly adjusted, the professor and the boys took their places in the conning tower, while Washington, Tom and Bill remained in the engine room.  Andy stayed on deck with his gun.

“I might see a big fish, and we could vary our bill of fare,” he said with a laugh.

“Here we go!” exclaimed the professor as he shifted the levers and turned some wheels and valves.  “Now we’ll see how fast we can travel.”

As he spoke the Mermaid responded to the added impulse of the compressed air and shot through the water at a terrific speed.  The sudden increase in momentum almost threw the boys from their feet, and they would have fallen had they not grasped some projecting levers.

“I guess that will do,” Said the scientist.  “I think we have speed enough for almost any emergency.  I’ll let her run at this rate for a while, and then we’ll slack up.”

Looking ahead, the boys could see the green waters parting in front of the bow of the Mermaid, as if to make room for her.  Two huge waves were thrown upon either side.

Suddenly, dead ahead, there loomed up a big black object.

“Look out you’ll hit the rock!” cried Mark to the professor, who was steering.

With a turn of his wrist Mr. Henderson moved the wheel which controlled the tube.  It was deflected and sent the boat to larboard.

At that instant from the rock two small fountains of water rose in the air, falling back in a shower of spray through which the sun gleamed.

“That’s not a rock!  It’s a big whale!” cried Jack.  “And we’re going to hit him!”

The professor had miscalculated the speed of the craft, or else had not thrown her far enough to larboard, for, a second later, the Mermaid was almost upon the big leviathan.

With a desperate twirling of the steering wheel the professor veered the craft as far as possible.  But all he could do did not suffice, for the craft hit the whale a glancing blow on the side, and the ship careened as if she would turn turtle.

At the same time there rang out from upon deck the sound of a rifle shot.  Old Andy had taken a chance at the enormous creature of the deep.