could glance back and see the foremost of the other crowd dipping down into the dry channel.
"The steps! The steps!" suddenly cried Ned, when they had run a long distance, as a faint gleam of daylight beyond showed the opening beneath the stone altar. "We're safe now."
"Hardly, but a few minutes will tell," said Tom. "The balloon is in shape for a quick rise, and then we'll leave this horrible place behind."
"And all the gold, too," murmured Ned regretfully.
"We've got some," said Mr. Damon, "and I wouldn't take a chance with those head-hunters for all the gold in the underground city."
"Same here!" panted Tom. Then they were at the steps and ran up them.
Out into the big auditorium they emerged, weak and faint, and toward the hidden dirigible balloon they rushed.
"Quick!" cried Tom, as he climbed into the car, followed by Mr. Damon and Eradicate. "Shove it right under the broken dome, Ned, and I'll turn on the gas machine. It's partly inflated."
A moment later the balloon was right below the big opening. The blue sky showed through it—a welcome sight to our friends. The hiss of
