to navigate under it. I think I'll
do that, Ned."

So the Advance, rebuilt and refitted in many ways, was christened the
M. N. 1, and a wonderful craft she proved to be. Mary Nestor was quite
pleased when Tom told her what he had done. She appreciated the
delicate compliment he had paid her.

Busy and more busy were the days that passed. As the M. N. 1 had to be
refitted some miles from Tom's home, where it was feasible to launch
her for the trip, he had to make the journey between the drydock and
his shop either by automobile or aeroplane. Often he choose the latter,
since he had a number of small, speedy craft in his hangars. Sometimes
Ned or Mr. Damon went with him, but Mr. Hardley could never be induced
to ride in an airship.

"I'll travel on the ocean or under it," he said, "but I'm not going to
take a chance in the air. I'm too afraid of falling."

"Tom, what's this?" asked Ned one day, when he and Tom had come to see
how the work of remodeling the submarine was getting along. "It looks
like something you used when you dug your big tunnel."

"That's a new kind of diving bell," Tom answered. "You know it isn't
easy to get treasure out of a sunken ship. It isn't like picking it off
the bottom of the ocean. We've got to get it out from inside--perhaps
from inside a strong box or a safe. This bell may come in useful."

"Can't you use the special diving suits that you always used to carry?"
the financial manager wanted to know.

"We might, if the water isn't too deep," replied Tom. "But you know
there is a limit to how far down a man in even my kind of diving dress
can go. With this diving bell a much greater depth can be reached. And
this diving bell is not like any you have ever seen or read about. My
father gave me the idea for it. I'll demonstrate it to you some day."

A diving bell is shaped like its name. A common glass tumbler thrust
down into a pail of water, with the open side down, will show exactly
the principle on which a diving bell works. It illustrates the fact
that two things cannot occupy the same place at the same time.

Pushing the tumbler, open end down, into the pail of water, leaves a
space in the upper end of the tumbler which the water cannot fill,
because it is already occupied with air. Imagine a big tumbler, made of
thick steel, lowered into the water. Air pumped into the upper part not
only keeps the water from entering, but also enables a man inside to
breathe and to move about