ht to be plenty of fresh air," went on the inventor. "It
is renewed automatically."

Tom jumped up and looked at an indicator. He uttered a startled cry.

"The air hasn't been changed in the last hour!" he exclaimed. "It is
bad. There's not enough oxygen in it. I notice it, now that I've
stopped working. The gage indicates it, too. The automatic air-changer
must have stopped working. I'll fix it."

He hurried to the machine which was depended on to supply fresh air to
the submarine.

"Why, the air tanks are empty!" the young inventor cried. "We haven't
any more air except what is in the ship now!"

"And we're rapidly breathing that up," added Captain Weston solemnly.

"Can't you make more?" cried Mr. Damon. "I thought you said you could
make oxygen aboard the ship."

"We can," answered Mr. Swift, "but I did not bring along a supply of
the necessary chemicals. I did not think we would be submerged long
enough for that. But there should have been enough in the reserve tank
to last several days. How about it, Tom?"

"It's all leaked out, or else it wasn't filled," was the despairing
answer. "All the air we have is what's in the ship, and we can't make
more."

The treasure-seekers looked at each other. It was an awful situation.

"Then the only thing to do is to fix the machinery and rise to the
surface," said Mr. Sharp simply. "We can have all the air we want,
then."

"Yes, but the machinery doesn't seem possible of being fixed," spoke
Tom in a low voice.

"We must do it!" cried his father.

They set to work again with fierce energy, laboring for their very
lives. They all knew that they could not long remain in the ship
without oxygen. Nor could they desert it to go to the surface, for the
moment they left the protection of the thick steel sides the terrible
pressure of the water would kill them. Nor were the diving suits
available. They must stay in the craft and die a miserable death-unless
the machinery could be repaired and the Advance sent to the surface.
The emergency expanding lifting tank was not yet in working order.

More frantically they toiled, trying every device that was suggested to
the mechanical minds of Tom, his father, Mr. Sharp or Mr. Jackson, to
make the pumps work. But something was wrong. More and more foul grew
the air. They were fairly gasping now. It was difficult to breathe, to
say nothing of working, in that atmosphere. The thought of their
terrible position was in the minds