experiment which meant much to them. The
hissing grew louder.

"Be ready to jump," advised Mr. Sharp.

"I will," answered the lad. "But the pressure is going up very slowly.
Maybe you'd better turn on more gas."

"I will. Here she goes! Look out now. You can't tell what is going to
happen."

With a sudden hiss, as the powerful gas, under pressure, passed from
the tank, through the pipes, and into the aluminum container, the hand
on the gauge swept past figure after figure on the dial.

"Shut it off!" cried Tom quickly. "It's coming too fast! Shut her off!"

The man sprang to obey the command, and, with nervous fingers, sought
to fit the wrench over the nipple of the controlling valve. Then his
face seemed to turn white with fear.

"I can't move it!" Mr. Sharp yelled. "It's jammed! I can't shut off the
gas! Run! Look out! She'll explode!"

Tom Swift, the young inventor, whose acquaintance some of you have
previously made, gave one look at the gauge, and seeing that the
pressure was steadily mounting, endeavored to reach, and open, a
stop-cock, that he might relieve the strain. One trial showed him that
the valve there had jammed too, and catching up a roll of blue prints
the lad made a dash for the door of the shop. He was not a second
behind his companion, and hardly had they passed out of the structure
before there was a loud explosion which shook the building, and
shattered all the windows in it.

Pieces of wood, bits of metal, and a cloud of sawdust and shavings flew
out of the door after the man and the youth, and this was followed by a
cloud of yellowish smoke.

"Are you hurt, Tom?" cried Mr. Sharp, as he swung around to look back
at the place where the hazardous experiment had been conducted.

"Not a bit! How about you?"

"I'm all right. But it was touch and go! Good thing you had the gauge
on or we'd never have known when to run. Well, we've made another
failure of it," and the man spoke somewhat bitterly.

"Never mind, Mr. Sharp," went on Tom Swift. "I think it will be the
last mistake. I see what the trouble is now; and know how to remedy it.
Come on back, and we'll try it again; that is if the tank hasn't blown
up."

"No, I guess that's all right. It was the aluminum container that went
up, and that's so light it didn't do much damage. But we'd better wait
until some of those fumes escape. They're not healthy to breathe."

The cloud of yellowish smoke was slowly rolling away, and the man and