water, from the dock in the cove to a point off the Massachusetts coast and return.

Early the next morning all were aboard. To each one was assigned a particular station. Washington, with Mark as an assistant, was in the engine-room. Jack was to watch the various gages and registers to give warning of any danger. The professor, of course, would be in the conning tower and operate the craft. Andy was to be with him, to watch out, with his sharp eyes, for any danger that might loom up in the path of the searchlight. Tom and Bill were to be ready to help where needed.

With a hissing sound the water filled the tanks and the Porpoise sunk beneath the waves. The engine that worked the endless screw was started, and the threads, working on the water cable, shot the boat ahead.

"We're off!" yelled Washington.

About sixty feet below the surface the craft was sent along. Mile after mile was covered as shown by the patent log. The lights were turned off, and through the thick plate glass windows the strange inhabitants of the sea were observed.

"I think I'll go a little nearer the surface," said the professor to Andy. The inventor started the pumps that emptied the tanks. The craft rose slightly.

"Quick! Stop her!" shouted the old hunter, grasping the captain's arm.

Something black, like a grim shadow, loomed up in the dull glare of the searchlight.

"What is it?" cried the professor.

"We're goin' to hit somethin' hard!" yelled Andy.

"It's the hull of a ship!" exclaimed the inventor as he jammed the reversing lever hard over.

It was too late. The next instant the Porpoise, with a shock that made her shiver from stem to stern, collided with the steel side of a small warship.

CHAPTER IV

IN THE MIDST OF FIRE

"Pull the secondary emergency lever!" cried the professor through the speaking tube to Washington. "We must reach the surface at once!"

"Are we damaged?" asked Andy, scrambling to his feet, for the shock had knocked him down. The professor had not fallen because he clung to the steering wheel.

The ship gave a sudden lurch.

"We're sinking!" cried Bill, rushing to the conning tower from the engine-room.

"That's only the action of one of the emergency levers," said the professor calmly. "It forces compressed air into the tanks the more quickly to empty them of water. I think we are safe."

"What is it?" asked Mark, as, followed by Jack, he came forward.

"We tried to do the torpedo act to one of Uncle Sam's ships," explained Andy.

The electric lights had been switched on, and, with the Porpoise flooded with the bright beams, those on board waited anxiously for what was to happen next.

Suddenly an upward motion was experienced. The next instant the craft bounced out of the water and fell back in a smother of foam, shaking and shivering, alongside a small armored warship that was anchored about two miles and a half from shore.

"Open the manhole," commanded Mr. Henderson.