he wondered what was happening as the battle progressed.
Fiercely the fight was kept up. Now the red imps would be driven back, and again they would swarm about the airship, until it seemed as if they must overwhelm it. Then the fire of the white adventurers was redoubled. The electric rifle did great work, and Tom did not have to stop and refill the magazine, as did the others.
Suddenly, above the noise of the conflict, Tom Swift heard an ominous sound. It was a hissing in the air, and well he knew what it was.
"The gas bag!" he cried. "They've punctured, it! The vapor is escaping. If they put too many holes in the bag it will be all up with us!"
"What's to be done?" asked Mr. Durban.
"If we can't drive them back we must retreat ourselves!" declared Tom desperately. "Our only hope is to keep the airship safe from harm."
Once more came a rush of the savages. They had discovered that the gas bag was vulnerable, and were directing their arrows against that. It was punctured in several more places. The gas was rapidly escaping.
"We've got to retreat!" yelled Tom. He hurried to the engine-room, and turned on the power. The great propellers revolved, and sent the Black Hawk scudding across the level plain. With
