se from the interior of the submarine, and the
heavy door, built to withstand the pressure of tons of water, could not
be forced except by the proper means.

"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 repeated his cries
until he was weary. No one answered him. He fancied once he could hear
footsteps in the shed, and thought, perhaps, it was Andy, come back to
gloat over him. Then Tom knew the red-haired coward would not dare
venture back. We must do Andy the justice to say that he never realized
that he was endangering Tom's life. The bully had no idea the tank was
airtight when he closed it. He had seen Tom enter and a sudden whim
came to him to revenge himself.

But that did not help the young inventor any. There was no doubt about
it now--the air was becoming close. Tom had been imprisoned nearly two
hours, and as he was a healthy, strong lad, he required plenty of
oxygen. There was certainly less than there had been in the tank. His
head began to buzz, and there was a ringing in his ears.

Once more he fell upon his knees, and his fingers sought the small
projections of the gear on the inside of the door He could no more
budge the mechanism than a c