The professor thought a moment.
“We shall have to pull the escape valve and let out some gas,” he said. “The rising sun has warmed the air till the expansion of gas has made the bag too buoyant for the planes to have any effect on it.”
“Won’t that waste the gas?” asked Joe.
“Yes, but we will have to do it. Mr. Tubbs, pull the escape valve, please,” said the professor, whose nose was red and whose teeth were beginning to chatter.
“It’s snowing!” cried Nat suddenly.
The air was filled with flying flakes, and the Discoverer seemed to be soaring through a wonderful white void. But it was no time for admiring such effects.
Reaching above his head, Mr. Tubbs gave the cord that worked the escape valve situated on the top of the big bag, a sharp tug.
Then he gave it another and another, with no results.
“It’s stuck fast!” he said, the words coming out shrilly from his blue, frozen lips.
A look of dismay spread over the professor’s face.