ot. 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.

And so the ninety-mile journey down Long Island Sound, partly under
water, partly on the surface, to New York, was completed. The greater
voyage to the Delaware Capes followed, and at all times the little
sixty-three-foot boat that was but eleven feet in diameter at her
greatest girth carried her crew and equipment with perfect safety and
without the least inconvenience.

Such a vessel, small in size but great in destructive power, is a force
to be reckoned with by the most powerful battle-ship. No defense has yet
been devised that will ward off the deadly sting of the submarine's
torpedo, delivered as it is from beneath, out of the sight and hearing
of the doomed ships' crews, and exploded against a portion of the hull
that cannot be adequately protected by armour.

Though the conning-dome of a submarine presents a very small target,
its appearance above water shows her position and gives warning of her
approach. To avoid this tell-tale an instrument called a periscope has
been invented, which looks like a bo