had been spent on the _Porpoise_, until now it was
almost ready for a trial. The professor had discovered a new method of
propulsion. Instead of propellers or paddle-wheels, he intended to send
his craft ahead or to the rear, by means of a water cable.

Through the entire length of the ship ran a round hole or shaft, one
foot in diameter. Within this was an endless screw worked by powerful
engines. With a working model the professor had demonstrated that when
the endless screw was revolved it acted on the water just as another
sort of screw does in wood. The water coming in through the shaft served
as a rope, so to speak, and the screw, acting on it, pulled the craft
ahead or to the rear, according to the direction in which the screw was
revolved.

The submarine was a wonderful craft. It contained a powerful engine,
electric motors and dynamos, and machinery of all kinds. The engine was
a turbine, and steam was generated from heat furnished by the burning of
a powerful gas, manufactured from sea water and chemicals. So there was
no need to carry a supply of coal on the ship.

The interior of the vessel was divided into an engine-room, a kitchen,
combination dining-room and parlor, bunk rooms, and a conning tower, or
place for the steersman.

While the boys had been shooting at the target the professor and
Washington had been putting the finishing touches to the engine,
tightening nuts here and screwed up bolts there.

"I guess that will do," remarked the old inventor. "Call the boys,
Washington."

The colored man went to the door and gave three blasts on a battered
horn that hung from a string.

"Coming!" called Mark, as he and Jack ceased their marksmanship contest
and approached the shed.

"Now boys, we'll see if she works so far," said the professor. "If she
does, we'll give her a trial under water."

At the inventor's directions the boys started the gas to generating from
the chemicals. Soon the hissing of steam told them that there was power
in the boiler.

The professor entered the engine-room of the submarine. He looked over
the various wheels, levers, handles, gages and attachments, satisfying
himself that all were in proper shape and position.

"Three hundred pounds pressure," he muttered, glancing at the steam
indicator. "That ought to be enough. Are you all ready, boys?"

"All ready!" cried Jack.

Of course the test was only one to see if the engine worked, for the
boat could not move until