In the tube which opened at the very tip end of the nose of the craft lay a Whitehead (or automobile) torpedo, which when properly set and ejected by compressed air propelled itself at a predetermined depth at a speed of thirty knots an hour until it struck the object it was aimed at or its compressed air power gave out.

The seven Holland boats built for the United States Navy, of which the Fulton is a prototype, carry five of these torpedoes, one in the tube and two on either side of the hold, and each boat is also provided with one compensating tank for each torpedo, so that when one or all are fired their weight may be compensated by filling the tanks with water so that the trim of the vessel will be kept the same and her stability retained.

The Fulton, however, was bent on a peaceful errand, and carried dummy torpedoes instead of the deadly engines of destruction that the man-o’-war’s man dreads.

“Dive thirty,” ordered the captain, at the same time giving his wheel a twist to direct the vessel’s course according to the pointing finger of the compass.

“Dive thirty, sir,” repeated the steersman below, and with a slight twist of his gear the horizontal rudders turned and the submarine inclined downward; the level-indicator showed a slight slant and the depth-gauge hand turned slowly round—­twenty-two, twenty-five, twenty-eight, then thirty feet, when the helmsman turned his wheel back a little and the vessel forged ahead on a level keel.

At thirty feet below the surface the little craft, built like a cigar on purpose to stand a tremendous squeeze, was subjected to a pressure of 2,160 pounds to the square foot.  To realise this pressure it will be necessary to think of a slab of iron a foot square and weighing 2,160 pounds pressing on every foot of the outer surface of the craft.  Of course, the squeeze is exerted on all sides of the submarine boats when fully submerged, just as every one is subjected to an atmospheric pressure of fifteen pounds to the square inch on every inch of his body.

The Fulton and other submarine boats are so strongly built and thoroughly braced that they could stand an even greater pressure without damage.

When the commander of the Fulton ordered his vessel to the surface, the diving-steersman simply reversed his rudders so that they turned upward, and the propeller, aided by the natural buoyancy of the boat, simply pushed her to the surface.  The Holland boats have a reserve buoyancy, so that if anything should happen to the machinery they would rise unaided to the surface.

Compressed air was turned into the ballast tanks, the water forced out so that the boat’s buoyancy was increased, and she floated in a semi-awash, or light, condition.  The engineer turned off the current from the storage batteries, threw off the motor from the propeller shaft, and connected the gasoline engine, started it up, and inside of five minutes from the time the Fulton was navigating the waters of the Sound at a depth of thirty feet she was sailing along on the surface like any other gasoline craft.