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!"
"What is it?" Jack managed to ask.
"We are caught in a whirlpool!" Mr. Henderson yelled as he leaped down the companionway and pulled the heavy steel cover after him.
Stricken with a nameless dread, Jack closed the water-tight door of the conning tower and made his way to the cabin. He could hardly get down the stairs, so swiftly was the ship whirling about.
He found the captain busy in the engine room and, in response to calls, Washington and Mark came hurrying in. They had been awakened by the commotion and the strange movements of the Porpoise.
"Turn on all the lights," ordered the inventor. "We must prepare for the worst."
The incandescents were soon glowing and in the glare the frightened adventurers gathered about Mr. Henderson, wondering what new terror had befallen them.
"See!" exclaimed the inventor. "We are going comparatively slow now, but we are on the outer edge. Wait until we reach the centre."