t was made immensely
strong to withstand powerful pressure at the bottom of the ocean. The
submarine could penetrate to a depth of about three miles. Below that
it was dangerous to go, as the awful force would crush the plates,
powerful as they were.

"Well, we'll rush things to-morrow and the next day," observed Tom as
he prepared to leave the building. "Then we'll soon see if it works."

For the next week there were busy times in the shop near the ocean.
Great secrecy was maintained, and though curiosity seekers did stroll
along now and then, they received little satisfaction. At first Mr.
Swift thought that the visit of Mr. Berg would have unpleasant results,
for he feared that the agent would talk about the craft, of which he
had so unexpectedly gotten a sight. But nothing seemed to follow from
his chance inspection, and it was forgotten.

It was one evening, about a week later, that Tom was alone in the shop.
The two mechanics that had been hired to help out in the rush had been
let go, and the ship needed but a few adjustments to make it ready for
the sea.

"I think I'll just take another look at the water tank valves," said
Tom to himself as he prepared to enter the big compartments which
received the water ballast. "I want to be sure they work properly and
quickly. We've got to depend on them to make us sink when we want to,
and, what's more important, to rise to the surface in a hurry. I've got
time enough to look them over before dad and Mr. Sharp get back."

Tom entered the starboard tank by means of an emergency sliding door
between the big compartments and the main part of the ship. This was
closed by a worm and screw gear, and once the ship was in the water
would seldom be used.

The young inventor proceeded with his task, carefully inspecting the
valves by the light of a lantern he carried. The apparatus seemed to
be all right, and Tom was about to leave when a peculiar noise
attracted his attention. It was the sound of metal scraping on metal,
and the lad's quick and well-trained ear told him it was somewhere
about the ship.

He turned to leave the tank, but as he wheeled around his light flashed
on a solid wall of steel back of him. The emergency outlet had been
closed! He was a prisoner in the water compartment, and he knew, from
past experience, that shout as he would, his voice could not be heard
ten feet away. His father and Mr. Sharp, as he was aware, had gone to a
nearby city for some to