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.