he watches the gauge."

"Well, I think you'll find Mr. Hardley all right," said Mr. Damon, who
seemed to have taken a strong liking to his new friend.

Certainly the latter showed no signs of nervousness as the craft slowly
settled to the proper depth. He asked numberless questions, showing his
interest in the operation of the M. N. 1, but he showed not the least
sign of fear. However, as Tom said, that might come later.

"We are going down now," Tom explained, as he pointed out to Mr.
Hardley the various controlling wheels and levers, "by filling our
ballast tanks with water. We can rise, when needful, by forcing out
this water by means of compressed air. When we are on the ocean we can
go down by using our diving rudders, and in much quicker time than by
filling our tanks."

"How is that?" asked the seeker after the Pandora's gold.

"Filling the tanks is slow work in itself," replied Tom, "and they have
to be filled very carefully and evenly, so we don't stand on our stern
or bow in going down. We want to sink on an even keel, and sometimes
this is hard to accomplish. But we are doing it now," and he called
attention to an indicator which told how much the M. N. 1 might be
listing to one side or to one end or the other.

A submarine, as everyone knows, is essentially a water-tight tank,
shaped like a cigar, with a propeller on one end. It can sink below the
surface and move along under water. It sinks 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