his submarine, while his son and Mr. Sharp were
sailing above the clouds.

On their return, however, and after the bank mystery had been cleared
up, Tom and Mr. Sharp, aided Mr. Swift in completing the submarine,
until, when the present story opens, it needed but little additional
work to make the craft ready for the water.

Of course it had to be built near the sea, as it would have been
impossible to transport it overland from Shopton. So, before the keel
was laid, Mr. Swift rented a large cottage at a seaside place on the
New Jersey coast and there, after, erecting a large shed, the work on
the Advance, as the under-water ship was called, was begun.

It was soon to be launched in a large creek that extended in from the
ocean and had plenty of water at high tide. Tom and Mr. Sharp made
several trips back and forth from Shopton in their airship, to see that
all was safe at home and occasionally to get needed tools and supplies
from the shops, for not all the apparatus could be moved from Shopton
to the coast.

It was when returning from one of these trips that Tom brought with him
the paper containing an account of the wreck of the Boldero and the
sinking of the treasure she carried.

Until late that night the three fortune-hunters discussed various
matters.

"We'll hurry work on the ship," said Mr. Swift it length. "Tom, I
wonder if your friend, Mr. Damon, would care to try how it seems under
Water? He stood the air trip fairly well."

"I'll write and ask him," answered the lad. "I'm sure he'll go."

Securing, a few days later, the assistance of two mechanics, whom he
knew he could trust, for as yet the construction of the Advance was a
secret, Mr. Swift prepared to rush work on the submarine, and for the
next three weeks there were busy times in the shed next to the seaside
cottage. So busy, in fact, were Tom and Mr. Sharp, that they only found
opportunity for one trip in the airship, and that was to get some
supplies from the shops at home.

"Well," remarked Mr. Swift one night, at the close of a hard day's
work, "another week will see our craft completed. Then we will put it
in the water and see how it floats, and whether it submerges as I hope
it does. But come on, Tom. I want to lock up. I'm very tired to-night."

"All right, dad," answered the young inventor coming from the darkened
rear of the shop. "I just want to--"

Ne paused suddenly, and appeared to be listening. Then he moved softly
back