On the surface
when the hatches can be opened, oil or gasolene engines are used.
These engines cannot be used under water because they depend on a
supply of air, or oxygen, and when the submarine is tightly
sealed all the air possible is needed for her crew to breathe.
While cruising on the surface a submarine recharges her storage
batteries to give her motive power when she is submerged.
There are many types of submarines, some comparatively simple
and small, and others large and complex. In some it is possible
for the crew to live many days without coming to the surface.
Tom Swift's reconstructed craft compared favorably with the
best and largest ever made, though she was not of exceptional
size. She was very strong, however, to allow her to go to a great
depth, for the farther down one goes below the surface of the
sea, the greater the pressure until, at, say, six miles, the
greatest known depth of the ocean, the pressure is beyond belief.
And yet is possible that marine monsters may live in that
pressure which would flatten out a block of solid steel into a
sheet as thin as paper.
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