Page:Tom Swift and His Airship.djvu/12

2 if it goes too high, I'll warn you, and you can shut it off."

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-chaped 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.