because rudders force it down, and water taken into tanks in its
interior hold it to a certain depth. It can rise by ejecting this
extra water and by setting the rudders in the proper position.

A submarine moves under water by means of electric motors, the
current of which is supplied by storage batteries. 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.

"Well, we are as deep down as it is safe to go in the river,"
announced Tom, as the gauge showed a distance below the surface
of a little less than twenty-nine feet. "Now we'll move into the