"How do you mean

"Why not call it the M. N. 1.? Isn't that a good name?"

"The M. N. 1." mused Tom. "Not so bad. If the N. C. 4 flew over
the ocean the M. N. 1 ought to be able 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."