"No use trying that," concluded the lad, after a tiring
attempt to force back the sliding door with his hands. "I've
got to call for help."

He shouted until the vibrations in the confined space made
his ears ring, and the mere exertion of raising his voice to
the highest pitch made his heart beat quickly. Yet there
came no response. He hardly expected that there would be
any, for with his father and Mr. Sharp away, the engineer
absent on an errand, and Mrs. Baggert in the house some
distance off, there was no one to hear his calls for help,
even if they had been capable of penetrating farther than
the extent of the shed, where the under-water craft had been
constructed.

"I've got to wait until some of them come out here,"
thought Tom. "They'll be sure to release me and make a
search. Then it will be easy enough to call to them and tell
them where I am, once they are inside the shed. But--" He
paused, for a horrible fear came over him. "Suppose they
should come--too late?" The tank was airtight. There was
enough air in it to last for some time, but, sooner or
later, it would no longer support life. Already, Tom
thought, it seemed oppressive, though probably that was his
imagination.

"I must get out!" he repeated frantically. "I'll die in
here soon."

Again he tried to shove back the steel door. Then he