The man nodded, and, with a small wrench in his hand, went to one end
of the tank. The youth, looking anxiously at him, turned his gaze now
and then toward a gauge, somewhat like those on steam boilers, which
gauge was attached to an aluminum, cigar-shaped affair, about five
feet long.

Presently there was a hissing sound in the small frame building where
the two were conducting an 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.